zotero/storage/H7UFQGL9/.zotero-ft-cache

On this page: [Introduction] [Concrete ring & circular rail track]
[Central-support building] [Cabin] [Corner-blocks] [Locomotive system]
[Rotational speed] [Antenna system] [Transmitter modulation & spectrum]
[FuSAn724 transmitters] [FuSAn725 transmitters] [Optical encoder disk]
[Command uplink] [Electrical & signal distribution] [Electrical power]
[Monitoring antenna mast & receiver] [Unknown/unclear aspects] [Patents]
[References]
Directly related pages pages: ["Bernhard" ground station locations]
[The "Bernhard/Bernhardine" Luftwaffe radio-navigation system]
[The "FuG120 "Bernhardine" airborne Hellschreiber printer system]
For other Radio Air Navigation systems: see the Radio Air Nav tabs of the menu at the top of this page.
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©2004-2019 F. Dörenberg, unless stated otherwise. All rights reserved worldwide. No part of this publication may be used without permission from the author.
Latest page update: June 2024 (replaced Fig. 168, expanded text around it). Previous updates: February 2024 (added Fig. 5, Fig. 50and text); January 2023 (replaced Fig. 57 and expanded associated text); July 2022 (expanded the section on Hein, Lehman & Co); May 2022 (split the transmitter section into FuSAn 724 &725; added associated figures, added ref. 10 & 164, updated optical disk section, made separate section for transmitter modulation & spectrum); July 2020 (added ref. 265 about rail traction); February 2020 (added ref. 253B); October-November 2019 (added ref. 253A and associated text, added highres version of ref. 15); August 2019 (split into two pages, added ref. 208C, 243).
INTRODUCTION
"Bernhard" is the German codename for the ground-station ("Stellung", "Anlage") of the "Bernhard/Bernhardine" radio-navigation system that was used by the Luftwaffe during part of WW2. The beacon ground-station is a complete radio transmitter installation - including the antennas. A complete installation is a "Funk Sende-Anlage", abbreviated "FuSAn". The FuSAn developed for the "Bernhard" system was FuSAn 724. Its two transmitters had a maximum output power of 500 watt each. On-board the aircraft, the "Bernhard" beacon had a counterpart radio system ("Funk Gerät") called "Bernhardine": the FuG 120. A FuG includes the receiver (or transmitter, as the case may be), antenna (s), and installation rack(s), power supply, control
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boxes, etc. At the heart of the FuG 120 was a Hellschreiber-printer. It printed the bearing data transmitted by the selected "Bernhard" beacon.
The numbers 724 and 120 are entries in a multi-category running numbering system. The range FuSAn 700-799 was reserved for "Bodengeräte, Navigationssendeanlagen" (ground equipment, navigation transmitter stations) and FuG 100-150 for "Navigations- und Kommandoübertragungsgeräte" (navigation & command-uplink equipment). Ref. 2, 185.
Table-1: FuG120 and FuSAn724/725 within the equipment numbering system (source: ref. 2D)
Note that over a dozen different fixed and mobile FuSAn types were developed for various radar and radio-navigation systems of the Luftwaffe, primarily by Telefunken, Lorenz, and the DVL (Deutsche Versuchsanstalt für Luftfahrt), ref. 141.
The "Bernhard" FuSAn comprised a large rotating antenna system (ca. 25 x 35 meter). The main sub-systems of this rotating navigation beacon are (see Figure 1):
The rotating upper structure ("Gerüst"), consisting of: a large antenna system that comprises three antenna arrays,
a cabin ("mitdrehender Geräteraum" = co-rotating equipment room) with the two transmitters, a small square block near each of the four corners of the cabin. A large concrete ring, with a circular rail track, and four electric locomotives for rotating the upper structure.
A small round central-support and equipment building ("feststehender Geräteraum" stationary equipment room) in the middle of the ring.
A remote antenna mast and receiver, for monitoring the signals transmitted by the beacon. Sources of electrical power.
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The Telefunken company had gained good experience with the steel construction of the Large and Small "Knickebein" beacon systems, with their large antenna arrays, rotable on a circular track with central support. This gave them confidence for applying the same approach to the continuously rotating Bernhard system. Ref. 181 (p. 71, §3; 1942).
Fig. 1: The "Bernhard" ground-station of the "Bernhard/Bernhardine" radio-navigation system
(click here to get full size)
Basic characteristics of the "Bernhard/Bernhardine" system are: Frequency: 30 - 33.1 MHz. Transmitter power: 2 × 500 watt (FuSAn 724)
The available literature often refers to "Bernhard" as FuSAn 724/725. The 725version was intended to have more powerful transmitters: 5000 W each. However, there is no evidence that these transmitters ever entered into service, were even developed or were available off-the-shelf. Ref. 20 and 2C4 state that they were planned only. The exact reasons for the power increase is unknown, but would typically be extended range and improved immunity against interference and jamming.
The wiring list of the "Bernhard" station contains several items with two gauge specifications: one for the 500 W transmitters, an a much heavier gauge for 4000 W transmitters (i.e., not 5000 W, see cables nr. 6-8, 33, 34, in ref. 189). Antenna system dimensions: ≈28 x 35 m (HxW, 92x115 ft). Antenna system track diameter: 22.5 m (≈74 ft).
The specified average track radius (mid-point between the inner and outer rail)
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was 10.55 m.
The track was laid on top of a concrete ring width a width of 1.5 m, hence an outer diameter of 22.6 m.
Antenna system weight: 120 metric tons (265000 lbs), ref. 2C4. Only ref. 183 (p. 19) states 120 metric tons as the weight of the "Gerüst", i.e., of the super-structure without the locomotives. Some literature states the weight as 102 tons (ca. 256000 lbs), which may be a typographical error, or 100 tons (ref. 20) - without being specific about what is included in the weight.
Antenna rotational speed: 12 degrees per second (2 revolutions per minute, see the "rotational speed" section below).
This means that the small locomotives that turned this enormous antenna installation, moved at a respectable linear speed of about 8 km per hour (5 mph).
The speed had to very accurate: to within ±0.2-0.3 % of the nominal speed (ref. 181, p. 80). Note that by design, the printer in the aircraft can not work with a beacon that turns at a different speed.
System accuracy: initially ±1°, then improved to ±0.5°, finally reduced to ±4° by using a single-trace ( = simpler) printer system, a single transmitter, and a single antenna system; the latter was under development towards the end of the war, see the FuG 120 k section.
Operational range: 150-500 km (80-270 nm), depending on aircraft altitude with respect to the "Bernhard" antenna (p. 22 in ref. 15).
THE CONCRETE RING AND CIRCULAR RAIL TRACK
The base of the entire rotating "Bernhard" superstructure is a large concrete ring. On top of the ring lies a circular rail track, for the four locomotives that actually rotated the system. The superstructure ( = equipment cabin + antenna systems) is basically a large turntable. Standard width of the ring is 1.5 m (5 ft), as measured at several of the still-existing rings. The specified diameter of the center of the circular rail track ( = midway between the two rails) was 2x10.55 = 21.1 m (ref. 193). Hence, the outside diameter of the ring was about 21.1 + 1.5 = 22.6 m.
Fig.2: Concrete ring with the circular rail track
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Fig. 3: Satellite image of the "Bernhard" site at Arcachon/France - overhead view (ca. 2013)
(source: http://www.geoportail.gouv.fr/)
A rail track comprises two main elements: The track structure: Steel rails
A rail fastening system. This typically takes the form of ties (crossties; UK: sleeper, D: Querschwelle), and fasteners to fix the rails to the ties. Their purpose is to maintain the correct distance between the rails ( = gauge), hold the rails upright, and transfer the loads from the rails to the underlying track bed and ground. Fasteners take the form of rail anchors, tie-plates, base-plates, soleplates, rail-chairs (D: "Schienenstuhl"), bolts and nuts.
The track bed or foundation: typically layers of ballast, sub-ballast, and subgrade (layers of crushed rock, gravel, and sand) on top of the natural ground. For applications with very high loading ( = large "weight on wheels"), the track bed may be "ballastless": a continuous slab of reinforced concrete on top of a subgrade. The latter is the case for the "Bernhard" track. This has a major advantage compared to the traditional track structure: no need for regular heavy maintenance to restore the desired track geometry and smoothness (e.g., by tamping the ballast and associated re-aligning of the rails).
The next Figure shows the curved "Bernhard" rails, fastened to I-beam ties (UK: "sleepers") with standard clamps. This Figure also shows a "rail gauge rod" (a.k.a. "gauge tie rod" and "gauge tie bar") between the rails. There were 80 such "Spurstangen", evenly distributed between the 120 cross-ties. A rail gauge rod is a member bar that is specially designed to join two steel rails at the rail bottom. Their purpose is to protect the rails from tilting, and to hold the track to gauge ( = keep the gauge constant around the track). A distinction is made between single-ended and double-end rods, depending on one or both ends being adjustable. Here, a simple steel rod is used, with both ends threaded.
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Fig. 4: The "Bernhard" track
(source "Bernhard" track image: adapted from Fig. 1 in ref. 193)
The standard rail profile ("Regelprofil") of the Deutsche Reichsbahn (and of its successor, the Deutsche Bundesbahn, until 1963) was Schiene 49 (S49), where "49" refers to its weight in kg/ m. It was, and still is, also used for narrow gauge tracks, tramway and subway tracks. It would have made sense to use a readily available national standard profile for the "Bernhard" track. A section of rail found near the ring of Be-0 confirms that profile S49 was used:
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Fig. 5: Dimensions of Deutsche Reichsbahn standard rail profile S49 and a section of "Bernhard" rail
(source profile dimensions: de.wikipedia.org; source rail segment: ©2014 A. Ladenthin/H. Bergmann; used with permisison; )
The "Bernhard" track is "narrow gauge": the distance between the rail-heads is less than local standard gauge.Standard gauge in Germany is 1435 mm ( = 561⁄2 inches). It is the same as in North America, most of Europe and the Middle East, in China and Australia. For this standard gauge, narrow gauge is defined as 500-1435 mm (18-561⁄2 inch). To ensure the accuracy of the "Bernhard/Bernhardine" system, stable and accurate rotation of the antenna system was required. This translated to very tight tolerances for the rail track. They were specified by Telefunken (Dept. V/Mo in Berlin-Zehlendorf) and the Hein, Lehmann & Co. company Telefunken's standard manufacturer of the antenna installations.
Per the 1942 adjustment and verification instructions for the "Bernhard" rail track, the nominal dimensions and tolerances are as follows (ref. 193, see Fig. 5 below):
Gauge ("Spurweite") is the distance between the inside of the rail heads). Here: 842 mm, with an acceptable tolerance of ±1 mm ( = 5/128 inch).
On-center distance between the rail heads: 900 mm (≈ 3 ft), with an acceptable tolerance of ±1 mm.
The top of the inside rail is higher than the outside rail by 24 mm (nearly 1 inch). Allowed tolerance: ±1 mm.
The top of the rails must be at the same height, all the way around the track. This is verified for 20 evenly distributed points around inside rail of the track, the first point being the one at which the height difference between inside and outside rail is checked. The height of these 20 points must be within ±2 mm ( = 5/64 inch) of each other.
Average radius of the track (mid-point between inside and outside rail): 10548 mm (10.55 m = 34 ft 7 inch). Allowable tolerance: ±15 mm (0.6 inch).
There is a large ball bearing at the center of the roof of the round building in the middle of the concrete ring. The top flange of the ball bearing raceway must be higher than inside rail by 1297 mm ( = 4 ft 3 inch). Allowable tolerance: ±30 mm (1.2 inch). This is
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not an extremely critical dimension: it was compensated when installing the locomotives, by placing shims between superstructure frame and the ball-joint ("Kugelzapfen") on top of the locomotives.
After installing the rails, all specified dimensions had to be checked against the associated tolerances. All measured values were recorded on a special form, and sent to department V/Mo of Telefunken in Berlin-Zehlendorf. This enabled determining trends during regular reverification. For some measurements, a reference point had to be defined. It had to be marked clearly and permanently. Ref. 193.
Fig. 6: cross section with nominal dimensions and tolerances (source of dimensions and tolerances: ref. 193)
The documented "Bernhard" installations have at least six different fastening systems. In all cases, the rails are fixed in place either with 120 ties, or with 120 sets of anchors in the concrete.
A: the concrete ring has 120 pairs of rectangular vertical holes, with a square dimple half way between them. Example: Be-15 Szymbark/Bytów. Possibly, pre-fab ties were used that had a rectangular vertical post at both ends, and those ends were inserted into the vertical holes. The purpose of the dimples is unknown.
B: the concrete ring has 120 pairs of rectangular vertical holes, but there is only a dimple for every third pair of those holes. Example: Be-14 Aidlingen/Venusberg. There, the holes measure about 11x22 cm and are about 50 cm deep; the dimples are 17x17 cm. A pair of steel rods is anchored in the bottom of each rectangular hole. The part of the rods that sticks out above the ring is threaded (M20). The holes are not placed very accurately. However, as the upper part of the steel rods can be moved around, this is not an issue when installing the rail fasteners.
C: the concrete ring has 120 pairs of rectangular vertical holes, but there are no dimples. There are no steel rods anchored in the holes. Examples: Be-12 Nevid/Plzň, and Be-16 Sonnenberg/Hornstein.
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Fig. 7: The various forms of rail fastening and associated features in the concrete ring
D: the concrete ring has no holes, but there are 120 sets of 2+2 threaded rods sticking out where there are vertical holes in versions A-C. This requires more accurate placement of the rods than in cases A-C. Examples: Be-4 La Pernelle, Be-11 Trzebnica/ Trebnitz.
E: the ties are sections of I-beam that are embedded into the top of the concrete ring; 2+2 vertical bolts are welded onto each end of the I-beams. Variations:
Concrete ring with a flat top. Examples: Be-10 Hundborg, Be-3 Le-Bois-Julien, Be-6 Marlemont, Be-2 Mt.-St.-Michel-de-Brasparts, Be-7 Arcachon. At Archachon, the I-beams are 151⁄2 cm wide and 17 cm tall ( = standard I-beam per DIN 1025), and are embedded in a concrete layer that was poured separately.
Concrete ring with a rounded top. Example: Be-8 Schoorl/Bergen, where the ties are 1.3 meter long and the bolts are placed at 13 and 27 cm from each end of the ties; here too, the ties have the standard width and height per DIN 1025. Rounding the concrete top must have required additional effort. It is unclear why it was done.
F: two sets of 120 ties, one set is narrower than the ties at all other sites. Example: Be-0 Trebbin which was also a "Bernhard" test site. The narrow ties have a width of 7 cm, and may have been from an initial version of the track. However, both sets of ties are embedded into the same layer of concrete, i.e., both sets were already installed when the concrete of the top layer was poured. The bolts on the narrow ties appear to have been removed with a grinding tool, suggesting that they predate the wide ties.
Fig. 8: The various forms of rail fastening and associated features in the concrete ring
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Fig. 9: Pairs of threaded rods
I visited the remains of the "Bernhard" ring of Buke in April of 2015. This ring was destroyed by the British after the war, exposing the cross-section of the concrete. It suggests that the concrete was cast with as many as six distinct parts, see the photo below. The top layer is about 7 cm (≈3 inch) thick.
Fig. 10: Cross-section of the ring at Buke
A number of the "Bernhard" rings have rail-ties (sections of I-beam) that are embedded into a separate layer of concrete on top of the ring:
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Fig. 11: Top layer of concrete, with embedded I-beam rail-tie at Arcachon
The on-center inside rail of the track has a radius of 10.55 - 0.45 = 10.1 meter. For the outside rail, this is 10.1 + 0.9 = 11 meters. This is very small for a rail track, and causes a large difference (≈4.3 %) in the speed between the inside and outside wheels of the locomotive bogies. This "slippage" causes problems with normal bogies (US: trucks) that have rigid axles, with wheels that cannot turn independently. It also causes problems with traction, required locomotive tractive effort, and wear of the wheel flanges and the rails. One way to solve this, is to use wheels with a smaller diameter (here: 4.3%) on the inside rail of the track. In turn, this requires that the inside rail be raised slightly (here: 2.4 cm):
Fig. 12: Raising the inside rail with a block on the ties
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The "Bernhard" installations at, e.g., Be-3 Le-Bois-Julien, Be-6 Marlemont, and Be-10 Hundborg, had a block on all ties. At Be-5 Mt.-St.-Michel-Mt.-Mercure, there is only a block on every fourth tie.
Fig. 13: A block on the inside end of all ties of Be-6 at Marlemont (source: unknown)
Fig. 14: A block on the mounting plate on the inside end of all ties of Be-10 at Hundborg (source: Hundborg Lokalhistoriske Arkiv; used with permission)
A jig was used to check the gauge and the height difference between the inside and the outside rail. Gauge and height verification is repeated at each of the 80 rail gauge rods:
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Fig. 15: Measuring the gauge and the height difference between inside & outside rail height
(source: adapted from Fig. 2 in ref. 193)
The bottom of the jig has a notch at each end. A spirit level (a.k.a. mason's level; D: "Wasserwaage", F: "niveau à bulle") is placed on top of the jig. The notches are sized such that for nominal dimensions, the spirit level shows "horizontal" and the gap between the jig and the rail heads is 3 mm. The latter gap is verified with calibrated shims. The shims are also used to raise the jig on the side of the inside or the outside rail, until the spirit level shows "horizontal":
Fig. 16: Measuring the height difference between inside & outside rail height (source: adapted from Fig. 4 in ref. 193)
Generally, the aim is for trains to run without any contact between the flange of the wheels and
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the rail head. Such contact causes stress on the frame of the bogie, and wear on the wheels and rails. This is why wheel treads normally have a conical shape that widens towards the flange of the wheel. On a circular track with a very small radius, it may be necessary to align the bogie axles radially with the curvature of the track. I.e., the axles always point at the center of the circular track, perpendicular to the rails. No information is available about the bogie design of the Bernhard locomotives. So it is unknown if this approach was actually used. Note that the wheel-pairs of the Small Knickebein rotable beacon - a predecessor of the Bernhard beacon were angled. See here on the Knickebein page. The track diameter of the Small Knickebein was about 50% larger than that of the Bernhard!
Fig. 17: Bogie axles angled towards the center of the circular track
The relative height of points around the track, and the relative height of the top of the round building is verified with a two-tube water level (D: "Schlauchwaage", F: "niveau à eau"). This level works on the principle of "communicating vessels". It consists of a sufficiently long flexible tube that is filled with water. There is a glass tube at both ends of the tube. The tubes have an adjustable scale. First, a reference point ("datum", D: "Normalpunkt") is chosen. As many points around the track must be measured, it is most convenient to use a reference point at the round building in the middle of the circular track. This will also be used to measure the height of the building. A two meter long surveyor's pole is pounded into the soil, and against the overhanging roof of the round building. With a sprit level, the height of the top of the ball bearing in the roof is marked on the pole:
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Fig. 18: Measuring relative height around the track and of the top of the round building (source: adapted from Fig. 6 in ref. 193)
With a two-tube water level, the height of the top of the inside rail is measured at rail gauge rod number 1. This height is also marked on the pole. The relative height between top of the rail and ball bearing flange can now be determined with a ruler. The height of the inside rail is then measured at every fourth rail gauge rod (i.e., at 20 points in total).
Obviously the circular rail track was not transported to the "Bernhard" sites in one piece. Rather, they were delivered as sections of curved rail. It is unknown which type of rail joints were used:
Conventional joints with a small expansion gap (D: "Stoßlücke"). The rail sections are connected with so-called "fishplate" joint-bars (D: "Schienenlasche") and bolts through the rail. The "fishing" is the vertical web between rail head and rail base. The gaps cause the familiar clickety-clack noise when the train wheels bump over them. There is a tie underneath both of the joining rail ends. None of the "Bernhard" tracks have pairs of ties that are placed right next to each other. So it is unlikely that this type of joint was used.
Expansion joints (a.k.a. "breather switch", "adjustment switch", D: "Dehnungsfuge", "Schienenauszug"). The ends of the mating rail sections are tapered diagonally, and are not bolted together with fishplates. This type of joint allows for smoother transitions ( = reduced noise and vibration) than conventional joints.
Fusion-welded seamless joints. The welding is done on-site with the "Thermit" process that is described below. The result is a continuous welded rail (CWR, D: "durchgehen geschweißtes Gleis", "lückenloses Gleis"). Such rails need very solid anchoring, in order to to avoid warping of the tracks due to thermal contraction/expansion (e.g., "sun kink").
The "Bernhard" track has a length of π x track diameter = π x 21.5 ≈ 67.5 m. The expansion of steel rails is ca. 12 mm per °C per km. Concrete has a expansion coefficient of 10 mm per °C per km. Assuming a very moderate summer-winter difference in rail temperature of 50 °C, the relative expansion would have been ca. 6.8 mm (≈1/4 inch). Note that this is a steady-state value, as steel and concrete have different thermal inertia.
For CWR, the Deutsche Bahn installs a heavy concrete rail-tie every 60 cm. To minimize expansion/contraction forces, the rails are installed when the
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temperature is around 20 °C. The ties of the "Bernhard" track are spaced by... 60 cm!
Fig. 19: Three standard types of rail joints
(left-to-right: conventional joint, expansion joint, seamless joint)
The "Thermit" process was discovered by the German chemist Hans Goldschmidt around 1890. He was a student of the German chemist Prof. Robert Bunsen (yes, of the famous "Bunsen burner" - actually the "Bunsen & Desaga" burner). Goldschmidt patented the process (originally intended for purifying metals) in Germany 1895, and named it "Thermit". It is a simple but intense exothermal reaction: a mixture of powdered iron oxide (commonly known as "rust") and aluminum is converted into iron and aluminum oxide, plus enough heat - over 2400 °C (4300 °F) - to fully melt the mixture in a matter of seconds! The reaction is typically started by igniting a magnesium "sparkler" stick or ribbon that is put into the mixture. Unlike blast furnace smelters, basically no external heat needs to be applied. The aluminum slugs will float on top of the melted iron. The mix normally contains additives, to obtain the desired steel alloy. The process can be used for welding large steel parts, such as shafts, cables, pipes, and rails. It can also be used for casting of parts, and for under water welding. The first commercial welding application were tramway rail projects in Essen/Germany in 1899 and Berlin in 1901. The deutsche Reichsbahn followed in 1928. The process was patented in the USA in 1928, by the Metal & Thermit Company (named Goldschmidt Detinning Company until the first world war). Ever since the 1920s, it is the standard process world-wide for welding rail tracks. The intense pyrotechnic process has also found military applications (grenades, incendiary devices, etc.). The aluminothermic Thermit rail-welding process is as follows:
The ends of two rail sections that are to be welded, are brought together with 2-3 cm spacing (1 inch) and are aligned.
The gap is clamped with a two-part ceramic shell that has the same shape as the crosssection of the rail. This is needed to avoid the molten steel from running off. The sides of the clamp are sealed with special molding sand. The ends of the two rail sections are pre-heated to ca. 1000 °C with a blow torch.
A ceramic "funnel" pot is placed on top of the molding clamp, and is filled with Thermitmixture.
The mixture is ignited with magnesium ribbon, and then boils. The entire process only takes about 25 sec. After cooling off, the pot, clamp, and slugs are removed. The weld is ground, to get a smooth rail head.
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Fig. 20: Thermit-welding of tramway tracks in the city center of Bremen/Germany around 1900
(source unknown)
Rails that are dirty (grease, decomposing leaves (finally explained in 2020, see ref. 265), rain, snow, and ice) significantly reduce traction of locomotives. Traction heavily depends on the adhesion coefficient μ between the steel wheels and the steel rails (ref. 158):
μ ≈ 30% for clean, dry rails. μ ≈ 20% for rails that are clean but wet or icy, and for greasy rails. μ ≈ 5-10% for rails covered with (decomposing) plant leaves.
This is why the final version of the "Bernhard" stations had a track-cover that moved with the rotating installation (p. 20 in ref. 183). Not only to maintain traction and avoid disturbance of the constant speed ("Gleichlauf"), but also to avoid accidents.
Based on available photos, such a cover was installed at least at the "Bernhard" stations Be-4 at La Pernelle (though not yet in March of 1943), Be-8 at Schoorl/Bergen, Be-9 at Bredstedt, and Be-10 Hundborg. The "Bernhard" station Be-0 at Trebbin did not have such a cover.
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Fig. 21: A steel track-cover moves with the rotating platform (station Be-4 at La Pernelle, France, July 1944)
Fig. 22: Cross-section of the track cover - made of sheet metal and support braces
Fig. 23: The cover has a "box" around each pair of support wheels (Station Be-10 at Hundborg/Denmark)
Fig. 24: Side-view of the track cover - "box" around each pair of support wheels
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Fig. 25: Cross-section of the track cover - "box" around each pair of support wheels
The small round building below the rotating cabin contained electronic equipment. Operators had to get to an from that building, even when the station was rotating. So they had to cross the rail track. To facilitate this, two sets of stairs were integrated into the moving rail cover, placed at opposite sides of cabin (p. 20 item 6 in ref. 183, though some photo material suggest only one such set of stairs...).
Fig. 26: One of the two sets of stairs for crossing the covered track (Station Be-4 at La Pernelle/France)
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Fig. 27: The destroyed "Bernhard" Be-8 at Bergen/Schoorl - upside-down section of track cover
(source: ref. 127)
Fig. 28: Staircase section of the track cover (photo - turned right-side-up: ref. 127)
The wooden cabin on the turntable has an entry door at each end. Stairs are attached to the turntable, to get in and out of the cabin, while rotating or stationary (p. 20 item 4 in ref. 183).
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Fig. 29: The stairs attached to the turntable, providing access to both ends of the cabin (Station Be-9 at Bredstedt)
THE ROUND CENTRAL-SUPPORT BUILDING
At the center of the concrete ring of the "Bernhard" beacon, there is round brick building with a flat concrete roof:
Fig. 30: Concrete ring with central- support building
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Fig. 31: Left-to-right - the round building of Be-7 at Arcachon, Be-14 at Aidlingen, and Be-3 at Le-Bois-Julien
(photo Le-Bois-Julien: ©2006 T. Oliviers, used with permission)
Fig. 32: The round building of Be-8 at Bergen/Schoorl (left), and Be-12 at Nevid/Plzeň (sources: photo Be-8: ref. 127; Be-12: © Jacek Durych, used with permission)
This small building has two functions:
Central support for the heavy rotating superstructure ( = cabin and antenna systems) of the beacon. Stationary equipment room (D: "feststehender Geräteraum").
EQUIPMENT ROOM
The following items were installed in this equipment room below the rotating superstructure (see Figure 33):
A 15-ring slip-ring assembly, suspended from the ceiling of the equipment room. Sliprings allow electrical lines to traverse continuously rotating mechanical joints. The rotor of the assembly was driven by a shaft that descended through the ceiling of the equipment room (see Fig. 43) and rotated with the superstructure. The slip-rings passed electrical power and signals between the stationary equipment room and the rotating cabin above it.
Optical encoder disk assembly, suspended from the slip-ring assembly and driven by the shaft of that assembly. Three audio tone modulators:
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a constant 1800 Hz audio tone for the pointer beam transmitter
2600 Hz audio tone pulses, representing the compass scale in Hellschreiber format, for the compass scale transmitter.
2600 Hz audio tone pulses, representing the command-message text string in Hellschreiber format, for the compass scale transmitter (when replacing transmission of the compass scale with the command-message).
Two Hellschreiber printers (the same HS 120 printers as used in the aircraft), for printing: the signals transmitted by the beacon, as received by a remote receiver.
the command-message text string, to verify it before actually transmitting it (only installed at a few beacons).
A patch board and associated patch cord, for composing the up to 10 characters of the command-message (only installed at a few beacons).
A power distribution and control panel. The panel also indicated the exact rotational speed of the optical disk (and, hence, of the beacon), as measured by a tachometer track on that disk. A switch for selecting the forward/reverse rotational direction of the beacon. An emergency shutdown button.
Fig. 33: The round building below the rotating superstructure - equipment and interconnections
(source: derived from ref. 189 and 190)
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The photo below is the only one that I have of the inside of the equipment room:
Fig. 34: Equipment inside the round brick equipment room of the Bernhard installation at Hundborg
(source: Figure 30 in ref. 93A)
The 15-ring slip-ring assembly passed the following electrical power to the rotating cabin:
DC, for the DC motors in the four locomotives. The associated fuses had a rating of 160 A / 500 V (ref. 189).
3-phase AC, constant frequency. Used for the synchronous AC motor in locomotive nr. 4. The associated fuses were rated at 80 A / 500 V.
3-phase AC, nominally 50 Hz (directly from the public power grid or a local backup generator). Used for the power supplies of the two transmitters, as well as general lighting and heating.
The slip-ring assembly passed the following other signals to and from the rotating cabin: Constant-tone audio modulation for the pointer beam transmitter. Hellschreiber tone-pulse audio modulation for the compass scale transmitter.
Quadrant-keying ("Sektortastung") to both of the transmitters, by a switch contact that is actuated by a notched disk on the shaft of the slip-ring assembly. Most likely, this was (or could be) used to not transmit during the entire 360° rotation, but only in a specific limited directional range. This could be used to avoid detection of transmissions by Allied monitoring stations, e.g., in Britain. Beacons operating this way were referred to as "Sektorfunkfeuer" (p. 16 in ref. 181). Switch closure of the emergency shutdown button on the superstructure.
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No photos are available of the slip-ring assembly. As an example, the photo below shows the slip-ring assembly of German "Panther" and "Tiger" tanks of the same era. Their slip-rings have a diameter of 12 cm (≈5 inch). There are four brass rings for electrical power (12 & 24 volt, 50 amps) and seven rings for communication and lighting.
Fig. 35 The slip-ring stack assembly of Panzer V "Panther" and VI "Tiger" tanks (source: ref. 145A)
The standard "Bernhard" equipment included a full set of 52 spare tubes (valves), per sheet 19 & 20 in ref. 189 (pdf pp. 22, 23): For the modulators and transmitter-keying units:
10x RV12P2000, 1x RG12D60, 1x AZ12, 6x RV275, 4x RV335, 4x RG62, all made by Telefunken. 1x STV100/25Z and 1x STV280/80 made by Stabilovolt. For the measurement/monitoring equipment:
3x RV12P2000, 6x LV1, 6x RG12D60, 1x RGN4004, 3x RV275, 2x RV335, 2x RG62, all made by Telefunken. 4x STV150/15 and 1x STV280/80, made by Stabilovolt.
CONSTRUCTION
The walls of the building are made of brick. There are four windows of 1.2x1.2 m (4x4 ft), and a door. Whatever equipment was installed inside this building, it must have fitted through the door or a window. Floor-to-ceiling height inside the building is about 3 m (10 ft), so the floor is well below the base of the concrete ring. This is why there is a small trench and steps that lead down to the door.
The three diagrams below show the cross-section (with measured dimensions) of the concrete ring and round building of the "Bernhard" installation at Aidlingen/Venusberg, Arcachon, and Hundborg:
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Fig. 36: Cross-section of the installation at Aidlingen/Venusberg (based on the measurements that I took in June of 2012)
Fig. 37: Cross-section of the installation at Arcachon (based on the measurements that I took in July of 2012)
Fig. 38: Cross-section of the Bernhard ring on Gåsbjerg hill at Hundborg (based on ref. 115)
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The concrete roof of the round building is supported by four columns, made of massive steel Ibeams (H-beams, D: "Doppel T Träger"): the flanges are 30 cm (1 ft) wide and 24 mm (1") thick! The web of the beams is 32 cm (121⁄2 inch) wide and 15 mm (0.6 inch) thick. So these columns have a cross-section of 30x37 cm. The columns are spaced evenly in the round wall. The roofjoists are made of the same heavy I-beams. Why would such a solid, heavy construction be necessary? The rotating superstructure weighed 120 metric tons (265 thousand lbs). Assuming the weight was distributed evenly between the four locomotives and the central support, the roof had to carry 120 / 5 = 24 metric tons (53 thousand lbs) statically!
The following diagrams show more details of the steel structure:
4 large steel I-beam columns, with end-plates. The flange of these vertical I-beams is 30 cm (1 ft) wide, and the height of the beams is 37 cm (1 ft 3")
4 large steel I-beam joists, with a joist-to-column brace and a triangular filler plate. The brace is a heavy steel plate (3 cm thick), as wide as the flanges of the columns and the joists. The joist and brace are mounted to the column with 8 bolts. The brace prevents the structure from racking ( = sideways swaying of the tops of the columns). The brace is mounted to the column at a 50° angle.
4 steel doubler-plates, to better transfer vertical forces on the joist to the column, and to distribute any bending force to both flanges of the columns. The doubler-plate is mounted onto the end-plate of the column with 8 bolts, and to the joist with eight bolts.
2 identical steel octagonal plates, to interconnect the four joists. Each joist is mounted to the two octagonal plates with 6 bolts. The plates have hole at the center.
4 small steel plates that form the sides of a box that is placed between the octagonal plates. The corners of the box are butted up against the web of the joists, but do not appear to be welded to them. The function of the box is unclear - possibly to prevent the center of the top octagonal plate from being pushed down, possibly they where used to pre-assemble the two octagonal plates.
4 small steel I-beams, connecting the joists above the braces. This makes the structure torsionally stiff. I found no sign that the columns are interconnected at the bottom.
Numerous steel concrete reinforcement rods/bars ("rebar"), placed radially inside the concrete of the roof. The ends of the rods are curled back.
Fig. 39: The major elements of the massive steel support structure of the round building (based on my measurements of the "Bernhard" Be-14 ground station)
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Fig. 40: Dimensions of the steel "skeleton" of the round building (based on my measurements of the "Bernhard" Be-14 ground station)
A doubler-plate reinforces the joint of each joist and the associated column:
Fig. 41: Dimensions of the column-to-joist doubler plate
(based on my measurements of the "Bernhard" Be-14 ground station)
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Fig. 42: Details of the steel support structure of the "Bernhard" Be-14 ground station
Two heavy octagonal plates interconnect the four large I-beams joists at the center of the roof. One plate on top, one from below. There are two brackets on the bottom plate, for suspending equipment. The space between the brackets is about 55 cm (22 inch).
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Fig. 43: The octagonal mounting plate against the ceiling - with mounting brackets to suspend equipment
Fig 44A: Dimensions of the octagonal mounting plates and associated brackets (based on my measurements of the "Bernhard" Be-14 ground station)
Four smaller I-beams make the top of the structure torsionally stiffer, by bracing the four main joists:
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Fig. 44B: Top view of the steel support structure
(based on my measurements of the "Bernhard" Be-14 ground station)
The next photo shows the remains of the steel structure (upside down), after removing the walls and collapsing the roof:
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Fig. 45A: The remains of the steel structure of Be-12, after destruction of the building in 2015
(source: © jdvlavicka)
Fig. 45B: The remains of the steel structure at Be-12, after destruction of the building in 2015
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(source: © jdvlavicka)
The baseline for the "Bernhard" ground station was the experience gained during 1939-1941 with the ring and the antenna system of the "Knickebein" rotatable beam system. It operated in the same 30-33 MHz frequency range as the "Bernhard/Bernhardine" system. At the center of the "Small Knickebein" ring, there also was a central support structure: a concrete block measuring 1.4 x 1.4 m, with a rotatable steel pivot on top:
Fig. 46B: The concrete central support block with pivot of "Small Knickebein" station Kn-13
(compare with Fig. 39 above; source photo of I-beam remnant & spacing measurements: ref. 230Q1)
...except for the last Small Knickebein that was built: Kn-13. It did not have a concrete block, but an I-beam structure with four corner-columns. It had to be rigid and hold the above pivot construction, to keep the rotatable antenna structure centered on the rail track. So, it must have had crossing joists - just like the "skeleton" of the "Berhard" central support, but significantly smaller and with a much lighter construction:
Fig. 46B: Remnant of a vertical I-beam of the central support structure of "Small Knickebein" station Kn-13
(compare to Fig. 39 & 45A/B above; source photo of I-beam remnant & spacing measurements: ref. 230Q1)
A very large ball bearing was installed at the center of the roof:
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Fig. 47: The round building with the raceway of a large ball bearing in the middle of the roof
(Be-12 at Nevid/Plzeň; source: © Jacek Durych, used with permission)
It had an outer diameter of about 40 cm (≈16 inch). The ball bearing held a cylindrical tube, that rotated with the antenna system and the cabin. The bottom of this tube has a flange, for connecting to equipment that rotated with the tube. The tube had a diameter of about 14 cm (51⁄2 inch).
Fig. 48: Tubular shaft descending through the roof of Be-6 at Marlemont and Be-10 at Hundborg
(source Hundborg photo: www.gyges.dk, used with permission; note the original wiring)
A small turntable is installed on top of the ball bearing, and the tube is attached to it from below:
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Fig. 49: small turntable on top of the shaft
(Be-6 at Marlemont; note the large mounting bolts inserted into the turntable from below)
THE CABIN
Below the antenna installation, and rotating with it, is a steel truss bridge. The rectangular bottom frame of the bridge is made of heavy I-beams. It is supported by the four locomotives and by the round building at the center of the concrete ring. The upper frame of the bridge was suspended from the large lattice truss-joist of the lower antenna system.
Figure 51: Cabin of Be-4 at La Pernelle/France
Inside the bridge is a long "co-rotating" wooden cabin ("mitdrehendes Holzhaus"). It measured about 20x4x3m (LxWxH, ≈66x13x10 ft), based on p. 20 in ref. 183, as confirmed by photometric analysis of available photos. Its length is close to the inside diameter of the ring (≈20.5 m). The cabin was made of heavy wooden planks. There is a entrance door and set of stairs at both ends of the cabin.
Ca. 1943, Telefunken contracted its standard antenna structure supplier, Hein, Lehmann & Co., to provide "Panzerung von Holzhäusern" for 12 "Bernhard" stations (purchase order nr.
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253/33163, ref. 177C). That is, for sheet-metal protection of the wooden cabin. "Panzerholz" is plywood that is covered with sheet metal armoring on one side or on both sides. The price was 4167 Reichsmark per BE-station. Based on general inflation data, this is equivalent to ca. US$21,900 or €20,250 (early 2017, ref. 177). The metal protection was only installed at five stations (Be-2, Be-3, Be-4, Be-8, and Be-10), before the course of the war intervened. The protective panels probably took the form of large panels that could be slid in front of the cabin windows, see Fig. 91 below. The wooden part of the panels involved Fa. Rostock in Trebbin (ref. 176A). This company operated three owned or leased sawmills in Trebbin (close to Be-0) since the early 1930s, and supplied wooden construction materials for a number of "radar" installations and other Wehrmacht constructions.
Fig. 52: The cabin at Be-10 Hundborg/Denmark - with protective siding panels
At some "Bernhard" sites, these sliding covers are on the outside of the bridge frame that suspends the cabin from the truss-joist. At other sites, these panels slide between the cabin and that frame.
The cabin contained the two transmitters, AC/DC electrical power distribution and controls for the locomotive motors, and for cabin heating & lighting. See the "Electrical & signal distribution" section. Ref. 13 (p. 4.09) suggests that the cabin was divided into three sections. The section on the right (looking at the front of the antenna system) contained the transmitter equipment. This is confirmed by the layout diagram of the Be-0 station. The center section housed the controls for the four electric locomotives. The section on the left was a workspace. A plaque at La Pernelle (Be-4) states that it had one or more beds in it.
The photo below shows the power distribution and control panel ("Schaltwandtafel") inside the rotating cabin of Be-10 at Hundborg. The spoked handwheel in the lower right-hand corner (sheet 15/20 in ref. 189) belongs to the circuitry for bringing the locomotives up to speed from standstill, and for slowing down to standstill ("Kontroller und Anlaßwiderstand"). Also see the "Electrical & signal distribution" section.
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Fig. 53: German engineer describing controls in the cabin of Be-10 at Bredstedt to a member of the RAF-ADW
(source: Australian War Memorial photo SUK14636, public domain; ca. August 1945)
THE FOUR CORNER-BLOCKS
At each of the four corners of the cabin, there is a "block" or square silo of about 11⁄2x11⁄2x2 m (WxDxH; ≈5x5x61⁄2 ft). They are mounted on a cantilever, away form the main cabin and above the rear bogie of the locomotive underneath. There are no openings for ventilation in the walls or the roof. No conduits appear to emanate from the bottom. From the available photos it is clear that there are no windows, and it appears that there is no door. Whatever was in there, apparently did not require access! So it was not electrical or mechanical equipment, nor a container with brake sand for the locomotives (for which there would have been a tube descending in front of the powered wheels).
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Fig. 54: One of the cantilevered corner sheds in a 1946 film clip of Be-4 at La Pernelle) (source: Cinémathèque de Normandie)
The next photo shows that the blocks were empty at some point in time (probably during construction, as the photo dates from March 1943):
Figure 55: Low-altitude oblique RAF aerial photo of the La Pernelle site (source: ref. 172A; photo by G.R. Crankenthorp, taken on 3 March 1943)
Most likely, they contained dead weight (e.g., stone, sand, concrete, lead), to get more weight on the traction wheels of the locomotives - even though the entire structure carried by the locomotives was already quite heavy by itself. However, the blocks must have had considerable weight: a triangular stabilizing arm was installed between the cantilever supporting each block,
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and the bottom frame of the bridge:
Figure 56: One of the four stabilizing arms of the turntable at Be-10 in Hundborg
At some sites, these blocks have a flat roof (e.g., Be-9 at Bredstedt, Be-10 at Hundborg). At other sites, the four-sided roof is pointed (e.g., Be-4 at La Pernelle, Be-7 at Arcachon).
Figure 57: Corner-sheds with a flat roof (Be-10 at Hundborg/Denmark)
Figure 58: Corner-sheds with a pointed roof (Be-4 at La Pernelle/France)
Unlike the main cabin, the walls are not made of wooden planks - they apear to have been made of sheet metal:
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Figure 59: Corner-block of Be-9 at Bredsted/Germany - note the construction details (unedited photo taken May 1945 by Flt. Lt. Herbert Bennet, RAF Mobile Signals Units of No. 72 Signals Wing; © David Bennet; used with permission)
In available post-war photos of "Bernhard" installations (La Pernelle, Arcachon), the main cabin is completely stripped of its materials - but not the four corner-blocks! Either the material was hard to remove and carry away, not valuable enough, or not usable for some other reason.
Fig. 60: Post-war photo of Be-7 at Arcachon - installation dismantled and stripped, except for the corner sheds
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Fig. 61: Post-war photo of Be-4 at La Pernelle - installation dismantled and stripped, except for the corner sheds (source: unknown)
The photo below (a post-war postcard) shows the site of Be-2 at Mont-St.-Michel-de-Brasparts. The installation was dismantled in 1946, but the (solid!) corner blocks were abandoned inside the concrete ring:
Fig. 62: Postcard of the Bernhard site Be-2 at Mont-Saint-Michel-de-Brasparts (late 1940s / early 1950s?) (source: unknown)
No such blocks have been found at the "Bernhard" sites were there still are visible remains these days.
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THE LOCOMOTIVE SYSTEM
The superstructure of the "Bernhard" (i.e., the antenna system and bridge with cabin) was rotated by four electrically powered locomotives on the circular rail track. Each locomotive had two bogies (US: trucks). The photos below shows that each bogie had two axle-boxes with leafspring suspension. Such axle-boxes typically have greased sliding bearings (a.k.a., journal bearings, not ball bearings). Each of the locomotives had 2 x (2+2) = 8 wheels, so the four locomotives had 32 wheels in total. The weight of the superstructure was carried by the four locomotives and the round central support building below it. Let's assume that the weight was evenly distributed among the four locomotives. So, the combined locomotives carried 4/5 x 120 = 96 metric tons. Hence, each wheel carried a weight of 96 / 32 = 3 metric tons. This is well below the standard railway limit of at least 11 tons/wheel, for the load at which both the rail head and full-size train wheels are damaged (about 6 tons/wheel for standard tramway ("Sraßenbahn") wheels).
Fig. 63 Each locomotive has two bogies (Be-10 at Hundborg) - 32 wheels in total (source: www.gyges.dk, used with permission)
Based on photometric analysis, the locomotives measured about 4x1.2 m (LxH, 13x4 ft). The wheels on the outside rail had a diameter of about 60 cm (24 inch), the ones on the inside rail about 55 cm (22 inch). This is based on the diameters of the inside and outside circular rail track being about 4.3% different, and the wheels on the inside rail having a diameter that is 2 x 2.4 = 4.8 cm smaller than the wheels on the outside rail. The wheels have about half the size of a standard rail wagon wheel, which is about the size of a tramway wheel.
Fig. 64: The side of the locomotive on the outside of the track - direction of motion is to the left
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The next photos show that the locomotives had large access holes, normally covered with a rectangular cover plate. The right-hand photo shows that the locomotives had external downgearing between one of the two motors and the forward bogie. The down-gearing ratio is small: about 1.6:1. The gearing is covered, so it is not sure if it was a chain or a belt. The width of the cover box suggests a belt.
Fig. 65: Close-up of two of the locomotives in a 1946 film clip of Be-4 at La Pernelle) (source: Cinémathèque de Normandie)
The photos in Fig. 65 and 66 show that the locomotives supported the weight of the superstructure at the point halfway between the front and rear bogies - which makes sense for weight distribution. There was ball socket ("Kugelpfanne") at this point on top of the locomotive (p. 8 in ref. 193). The socket held a large downward-pointing ball-stud ("Kugelzapfen") that was mounted underneath the I-beam frame of the superstructure. Between the superstructure and the locomotives, there are only conduits for electrical power cables to the motors (and a tachometer signal, but only at locomotive nr. 4, see Fig. 67).
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Fig. 66: Ball joint on a locomotive of Be-12 at Nevid and electrical conduits to the motors (magenta circle) (source: brdy.org)
As stated above, each locomotive had two double bogies. Each locomotive had two motors. Per ref. 10 (§7), each motor drove two wheels of a bogie. Presumably, this means that each bogie had one motor, and this motor only drove one axle of that bogie. Dividing motor power evenly between both axles of a bogie might have optimized traction, but would have complicated the construction.
Three of the locomotives had two DC motors. The fourth locomotive had one DC motor and one synchronous AC motor:
Fig. 67: Motorization of the four locomotives
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(source: derived from ref. 10, 189, 190)
There were three motor types used in the locomotives (ref. 10 (§7), ref. 190):
"Hauptantrieb": main drive, 220 volt DC-motors. Only locomotives nr. 1-3 had such a motor. They were used to smoothly accelerate the rotation from standstill to close to the nominal speed, and provide the majority of the drive power that was required during normal operation.
Per ref. 189, the specified gauge of the wiring to each of the motors was 25 mm2 (≈ 5.6 mm Ø, equivalent to about AWG 3). The gauge between the control panel
and the variable starter resistance was 95 mm2. I.e., a diameter of 11 mm (AWG 3/0-4/0). Note that aluminum wiring was specified for all cables. Aluminium has 61% of the conductivity of copper. Input fuses were rated 160 A / 500 V.
"Synchronantrieb": synchronous drive, 3-phase 380 volt AC 50 Hz synchronous motor drive. Only locomotive nr. 4 had such a motor. It was used to accurately maintain the nominal rotational speed - without an electrical or electromechanical control system! This motor was engaged once the DC main drive motors had brought the system to within 90-95% of the nominal speed. At that point, the synchronous motor would capture and lock on to the nominal speed, i.e., the excitation frequency of the 3-phase power (which is why that frequency had to be accurate).
The wire gauge for the 3-phase AC was 16 mm2 (≈ 4.5 mm Ø, equivalent to AWG 5). Modern 4-conductor insulated aluminium cable of this gauge has a current rating of 50 amps (e.g., cable type NAYY-J). The wiring for the DC field
excitation was 2.5 mm2 (≈ 1.8 mm Ø, ≈ AWG 13). Modern 2-conductor cable of this gauge has a current rating of about 18 amps. The associated fuses were rated 80 A / 500 V and 6 A / 500 V respectively.
"Nebenantrieb": auxiliary drive DC-motors, with a separately controlled field winding (a.k.a. "separately excited DC-motor"). All four locomotives had such a motor. The rotation direction of these motors was reversible. These motors were disengaged during normal operation. they were only used for positioning the system during tests and calibration/adjustment. These motors were significantly down-geared, to be able to (very) slowly rotate the system. Per ref. 189, the specified gauge of the wiring to each motor's field winding was
2.5 mm2 (≈ 1.8 mm Ø, ≈ AWG 13), and 6 mm2 (≈ 2.8 mm Ø, ≈ AWG 9-10) to each armature winding. The two input fuses were rated 35 A / 500 V.
At the nominal speed of the system (2 rpm), the locomotive wheels had to turn at about 72 rpm:
The diameter of the outer rail track was 21.95 m. Hence, the length of the outside track was close to 69 m.
The diameter of the outside wheels of the locomotive was about 60 cm = 0.6 m. Hence, the circumference of those wheels was close to 1.88 m. I.e., the wheels made about 35.8 revs per revolution of the system.
As the system turned at 2 rpm (30 sec/rev), the wheels ( = axles of the locomotive bogies) turned at about 71.6 rpm.
So, the synchronous drive motor definitely required some down-gearing. Its speed was fixed: it was determined by the 50 Hz electrical power and the (integer) number of rotor pole pairs. A
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direct drive would have required a prohibitively large number of rotor pole pairs: 50 Hz x 60 sec/ min / 71.6 rpm ≈ 42. Most likely, the auxiliary drive motors also required down-gearing. The gearings would have been integrated with drive axles of the locomotive bogies. Note that Fig. 114 above shows external gearing, but only with a small gear ratio. It is unknown which drive it was part of.
LOCOMOTIVE POWER
How powerful did the locomotives actually have to be? Let's do a simplistic reasonableness check, using the definition of "horsepower". On a level ( = horizontal) track, the required locomotive horsepower HPloc is (ref. 156, 157):
HPloc = W x T x S / 375
where
W = total gross train weight in tons (1000 lbs) T = total Train Resistance (a.k.a. Starting Resistance) per ton. A standard value used in the railway industry is 8 lbs/ton. Modern rail systems have a lower resistance. S = speed in mph 375 is a constant that assumes that no HP is used for driving accessories (gearing, compressor, alternator, ...)
Converting this to metric units, we get:
HPloc = W x T x S / 271
where
W = total gross train weight in metric tons (1000 kg) T = total Train Resistance per metric ton = 8 kg/ton S = speed in km/h = mph / 1.609
Total weight of the rotating superstructure was 120 metric tons, distributed among the four locomotives and the central support at the center of the concrete ring. The locomotives carried 4/5 x 120 = 96 metric tons. Wind load would increase this value. The specified diameter of the center of the circular rail track ( = midway between the two rails) was 2x10.55 = 21.1 m (ref. 193). Hence, the track length was π x 21.1 ≈ 66.3 m. This means that at 2 rpm = 120 rph, the small locomotives moved at a respectable speed of 120 x 66.3 = 8 km per hour (5 mph). Hence, the required total locomotive horse power is 96 x 8 x 8 / 271 = 22.7 HP at the traction wheels. There is down-gearing between the motor and the driven wheels. Let's assume a reasonable transmission efficiency of 85%. For the required total motor horsepower we now get:
HPmotor-total = HPloc / 0.85 = 22.7 / 0.85 = 26.7 HP
The "Bernhard" system used four locomotives. So, the required motor horsepower per locomotive would be:
HPmotor = HPmotor-total / 4 = 26.7 / 4 ≈ 6.7 HP
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Note that locomotive motors did not have to drive accessory loads such as generators and blowers. Hence, traction horsepower = brake horsepower. The above derivation does not take into account a requirements to accelerate to the nominal speed within a certain amount of time!
The rolling resistance of a railway vehicle (which is a science all by itself) is the sum of all forces acting through the wheels and the axles, that oppose motion of that vehicle. There are many sources of rolling resistance. Some vary only with weight (e.g., journal bearing resistance, rolling friction, track resistance), some are linearly proportional to speed (e.g., wheel-flange contact, wheel-rail interface, lateral and vertical movement), some depend on the square of the speed (e.g., aerodynamic), and some on the fourth time-derivative of displacement. Examples: Bearing friction.
Elastic deformation of the wheel-tread and of the rail, in the wheel-rail contact area. Note that the contact surface between a wheel and the rail is a very small elliptical area, called the "contact patch". It is typically only about 15 mm (0.6 inch) across! The weight of the wheel and the load that it carries, makes a "dent" in the surface of the rail head. This dent moves with the wheel, and is like a very small bow wave. So, trains actually always go uphill, even if the track is perfectly horizontal!
Losses due to wheel creep (during accelerations and decelerations, the elastic deformations cause the actual wheel displacement to be be different from its rolling distance).
Losses due to grinding of wheel flanges against the rail head, and "hunting" (horizontal back-and-forth waving movement of the bogies on the track). Wheel noise (vibration and resonances in the wheels).
Suspension "jounce" (the fourth time-derivative of displacement), due to impacts on rail joints (if any), and associated rebounds. Bumps and bounces convert horizontal momentum into vertical momentum; the associated energy is dissipated in the suspension. Track deformation (Rayleigh waves).
Aerodynamic drag that acts on exposed wheels and on the body of the locomotive. At low speed, this is normally quite small, if not negligible. However, in the case of "Bernhard", there is also drag of the large antenna system due to system movement and wind load. The antenna system is symmetrical with respect to the vertical axis of rotation. So there is a "push & pull" effect, depending on whether the movement is upwind or downwind. Wind will change the apparent weight on the locomotives.
Curve resistance, due to the radius of the curvature of the track. Note that regular "1 meter" gauge track typically has a minimum curve radius of 45 - 60 meters, about 4 - 6 times that of the circular "Bernhard" track! Without special measures, the curve resistance would have been quite high!
The "Bernhard" track had a gauge ("Spurweite", distance between the inside of the rail heads) of 842 mm. The on-center distance between the rail heads was 900 mm (≈ 3 ft). Ref. 193.
In normal rail applications, total resistance at low speed (less than about 15 km/h, ≈10 mph) is dominated by friction of the axle bearings (ref. 159). Note that a train with steel wheels on steel rails has a friction factor that is about 80% lower than that of a truck (UK: lorry) with rubber tires on pavement! Also note that the central load-bearing support below the rotating superstructure had a large ball bearing (diameter ≈40 cm ≈16 inch). Clearly, it too caused some rotational resistance.
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DC MOTOR DRIVE
Electrically powered rail vehicles (electric and diesel-electric train locomotives, streetcars/ tramways, subways) traditionally used DC traction motors. This remained the case until well after the advent of solid-state power electronics, in particular gate turn-off (GTO) thyristors, in the early 1960s. DC traction motors where primarily of the series-wound brushed type. I.e., with a commutator, and the field windings in series with the motor's armature windings. Note that brushless DC-motors only date back to the late 1950s, ref. 160.
Series DC-motors can produce their highest torque at low speed: as much as 3-8 times the fullload torque at nominal speed. This is ideal for traction applications. For a given field flux, DC motor speed is determined by the armature voltage, whereas the delivered torque is driven by the armature current.
Fig. 68: Basic types of wound Direct Current (DC) motors - classified by placement of the field winding
Fig. 69: Basic characteristics of series, shunt, and compound DC motors
As stated above, the speed of a DC motor depends on the voltage across the motor's armature and the field flux. Standard methods to vary the armature voltage of a series motor are: An adjustable resistance placed in series with the motor's armature. Ward-Leonard drive system. Rectified adjustable AC-voltage (ref. 161, 163G).
There are many other flavors of motor speed control (variable AC frequency, Pulse Width Modulation, ...). They are generally beyond the scope of this discussion, and of the technology available at the time.
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The adjustable series-resistance method (e.g., rheostat) has a major disadvantage: poor speed regulation ( = speed is highly load-dependent). There are also very high losses ( = heat dissipation) in the series resistance at low speed. That is, during speed-up from, and slow-down to stand-still. Some speed-control methods for series DC-motors are illustrated in Figure 70 and 71.
Fig. 70: Speed control of a series DC-motor via field or armature diverter, tapped fieldwinding, variable series resistance
Fig. 71: Speed control of a series DC-motor via series-parallel reconfiguration of split field-winding and multiple motors
The Ward-Leonard Drive System (D: "Leonardsatz", "Ward-Leonard-Umformer", "Leonard Doppel-Umformer") is basically an electro-mechanical way of generating a variable DC-voltage to control the speed of a DC-motor. Ref. 162A-162G. It was invented by Harry Ward Leonard in 1892. For about 75 years, there were few practical alternatives to this system - until the advent of solid-state power-electronics such as thyristors in the 1960s. Worldwide, this was the normal way to provide smooth, step-less control of the speed of high-power DC-motors, from zero to full speed. It has been - and still is - used in many applications, such as cannon/gun- and turretaiming, elevators, rolling mills, cranes, hoists, mining (colliery) winders, diesel-electric propulsion of locomotives and of special ships, strip-mining shovels, and heavy radar antennas. German WW2 radar antenna systems with a Ward-Leonard drive include the "Wassermann S" (FuMG 42; see figure 2 in ref. 162G; 36-60 m tall / 4 m diameter column, weight up to 60 tons) and the AEG-Telefunken "Würzburg Riese" (FuSE 65) with its large dish antenna (7.5 m diameter, 9.5 tons; ref. 162F). Allied radar system also used Ward-Leonard drives. E.g., the 20 ton antenna of the British "Marconi Type 7" was rotated with a 15 HP DC-motor, controlled with a Ward-Leonard set comprising a 24 HP 3-phase motor, a main DC-generator, and a small DC exciter generator. Ref. 162H.
The Ward-Leonard Drive System consists of a Ward-Leonard Drive Unit and a shunt-wound DC motor. The Drive Unit consists of a motor-generator. The motor (referred to as the "prime mover") has a near-constant speed. This can be a 3-phase or single-phase synchronous AC motor, or a combustion engine (diesel, gasoline/petrol) with a speed governor. The output shaft of the motor is coupled (direct-drive) to the input shaft of a DC-generator. The output voltage of this DC-generator is connected to the armature of the DC-motor that drives the load. The DCmotor need not be located near the motor-generator. The shunt-field of the DC-motor is connected to a constant voltage source; hence, the motor's excitation field (flux per motor-pole) is constant, and the torque only depends on the armature current - independent of the motor speed. The shunt-field of the DC-generator is connected to that same constant-voltage source, though via a rheostat (large variable resistor).
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Fig. 72: Ward-Leonard Drive System
The generator's output voltage is varied by changing the generator's field current with the rheostat. In turn, this changes the DC-motor's armature voltage, and hence its speed. The constant voltage source may be a rectified AC voltage (if the prime mover is an AC-motor). It can also be generated with a small DC exciter-generator ("selbsterregter Erregergenerator"), that is also driven by the prime mover, and has its shunt-field connected to its own armature (hence, "self-excited").
The Ward-Leonard drive unit is an electro-mechanical multi-kilowatt amplifier: a small change in the input current (generator field) results in a large change at the output (generator armature voltage and current). However, in its basic form, it is an open-loop control system: the rotational speed of the load is not measured and fed back in order to adjust the generator's field current. Hence, that speed is not regulated with the high precision required in the "Bernhard" application. Note that it is possible to expand the basic Ward-Leonard system with such a feedback loop.
Another drawback is that both the AC-motor (or the engine) and the DC-generator must be dimensioned for the full and peak power of the load-driving DC-motor(s) and system inefficiencies.
System efficiency is driven by the product of the efficiency of the three machines (AC-motor, DC-generator. DC-motor), and typically lower than that of rheostat control and field control methods. A single Ward Leonard drive unit can control multiple load-sharing DC motors in parallel ("group control"). Variations of the Ward-Leonard drive system are electro-mechanical amplifiers such as the Metadyne (1930s) and the Amplidyne (1940s).
As elegant and effective as the Ward-Leonard drive system may be, it was not what was used in the "Bernhard" to provide a variable DC-voltage to the locomotive drive system! The DC motors were regulated with series rheostat for speed up from standstill and down to standstill.
SYNCHRONOUS AC MOTORS
There are two basic types of AC motors: asynchronous motors and synchronous motors. Both have two main parts: a stator and a rotor. In a 3-phase motor (synchronous or asynchronous), the stator basically consists of triple pairs of formed coils of wire. Each coil is mounted in the
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slots of a laminated steel core. The coil pairs are spaced evenly around the stator. See Fig. 73. The two coils of each pair are connected in series.
Fig. 73: Simplified cut-away view of the stator of a 3-phase AC motor (synchronous or asynchronous)
Each stator coil-pair is energized by one phase of the 3-phase AC electrical power. Each energized coil-pair forms a pair of magnetic poles. The resulting magnetic field extends into the air gap between the stator and rotor, and into the rotor. The magnetic field strength and polarity of each pole-pair changes cyclically, as the AC excitation is sinusoidal. When all three phases are connected, the stator generates a rotating magnetic field (RMF). This field has a constant amplitude, and rotates with the same speed as the 3-phase excitation. E.g., for an excitation frequency of 50 Hz = 50 cycles/sec, the RMF rotates at 50/sec x 60 sec/min = 3000 rpm, and at 3600 rpm for 60 Hz excitation.
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Fig. 74: Concept of how RMF is generated by a 3-phase stator when excited with 3-phase AC power
We all know that unlike magnetic poles (North-South) attract each other, and like magnetic poles (North-North, South-South) repel each other. The motor's rotor can turn freely inside the stator. Let's take a rotor that consists of one or more magnetic pole-pairs. The stator generates a rotating magnetic field, so the magnetic rotor poles will try to remain aligned with that field: the rotor turns. There are several ways to make a stator pole-pair, see Fig. 74: A permanent magnet (bar magnet).
A coil with the ends short-circuited. By itself, such a coil does not generate a magnetic field. However, if a varying magnetic flux is induced in this coil, a current will circulate in the coil. The direction of this current is such that it opposes its cause (Lenz's Law). The cause is the varying induced flux. With the RMF of the stator, the induced flux in the rotor winding only varies if the rotor does not turn at the same speed as that RMF. This induced varying flux, combined with the induced current, generates an electro-magnetic force (EMF, Faraday's Law) that acts on the coil conductors. The magnitude of the torque ( = rotating force) is proportional to the relative rotational speed of the rotor, compared to the synchronous speed ( = the speed of the RMF). The speed difference is called "slip". The direction of the torque is such that torque is reduced (remember: Lenz' Law): in other words, such that the speed difference is reduced.
Important: there is no rotational force if the rotor turns at the synchronous speed! Hence, the rotor never turns at the synchronous speed, but always slower. The amount of slip depends on the mechanical load that is driven by the motor. The heavier the load, the larger the slip ( = lower motor speed). If the load varies, the speed varies. An AC motor with such a rotor is called an asynchronous motor. As it works on the principle of induction, it is also called an induction motor. Low-power asynchronous motors can have a slip of 5-10%, whereas asynchronous motors with a higher power rating have approx. 2-5% slip.
The rotor coils can be implemented as actual coiled wires (in which case the rotor windings are typically made accessible via slip rings), or simply be
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implemented as so-called "squirrel cage" (two parallel metal rings with a number of evenly spaced metal bars between them, often at a skew angle).
A coil that is energized by a DC voltage. This is equivalent to a permanent magnet. As the rotor has to turn, the DC power is supplied via slip rings. When the rotor is at standstill or at low speed, the alternating polarity of the stator's rotating magnetic field (RMF) sweeps by the poles of the rotor relatively fast. Each rotor pole is cyclically briefly pulled into one direction (without producing sufficient starting-torque), and than briefly in the opposite direction. The rotor may vibrate but will not turn!
Important: a motor with such a stator is inherently not self-starting! The motor needs a supplemental drive mechanism (e.g., a starter winding incorporated into the motor, or another motor) to first be accelerated to 90-95% of the synchronous speed (i.e., < 5-10% slip) - without energizing the rotor. At that point, the rotor is energized and automatically pulls into synchronism (a.k.a. "in step") with the RMF: the rotor poles are locked to the RMF and the rotor turns at synchronous speed. This is great for a constant speed drive application: no need for a closedloop speed control system! For obvious reasons, a motor with such a rotor is called a synchronous motor.
Important: a synchronous motor turns at the exact synchronous speed, from noload to full-load!
Contrary to the asynchronous motor, the motor torque is generated as a result of the physical angle between the stator and rotor. This phase angle is called the load angle, coupling angle, or torque angle. An increase in mechanical load causes this angle to also increase - but synchronous speed is maintained! If the mechanical load ever exceeds the motor's maximum torque, the rotor completely loses synchronism (drops "out of step") with the stator's RMF, and the motor comes to rest. The same happens if the rotor supply voltage or the stator supply voltage is reduced excessively.
For a constant load, the motor's EM torque is equal to the load torque and the torque angle is a non-zero constant. A sudden change in load will upset this steady state. The locking between the rotor and the RMF is not rigid! A sudden increase in load causes a temporary slow down of the rotor, which simultaneously increases the torque angle and the EM torque. This accelerates the rotor back to the synchronous speed. As the rotor reaches synchronous speed again, the torque angle is larger than needed and the rotor speed overshoots the synchronous speed. This reduces the torque angle, and the EMF torque drops simultaneously. The rotor decelerates and the rotor speed now undershoots the synchronous speed, etc. I.e., the rotor speed oscillates around the sync speed. This phenomenon is known as "hunting" and "phase swinging". Under certain conditions, these oscillation may diverge ( = exponentially increase in amplitude), even to destructive levels. The oscillation can be reduced by adding damping windings (a.k.a. amortisseur windings) to the rotor, and by large load inertia (e.g., a heavy flywheel, such as the rotating structure of the Bernhard system).
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Fig. 75: Possible configurations for a pole pair of an AC motor
The following graphs show the torque-versus-speed characteristics of an asynchronous and a synchronous AC motor:
Fig. 76: Torque-vs-speed characteristic of a typ. asynchronous AC motor (left) and of a synchronous AC motor
The synchronous rotor speed of an AC motor is clearly proportional to the RMF that is generated by the stator, i.e., to the frequency of the AC excitation of the stator. However, it also inversely proportional to the number pole-pairs of the rotor. The simple formula is given in the figure below. For instance, for a 50 Hz excitation, the synchronous rotor speed Ns is 3000, 1500, 1000, and 750 rpm, for 1, 2, 3, and 4 pole-pairs, respectively. The number of pole-pairs is an integer value, so it obviously cannot be chosen as freely as an excitation frequency. Compared to asynchronous motors of equal power and speed, synchronous motors are attractive for low-speed ( < 300 rpm) and ultra low-speed drive applications: their efficiency is high, their power factor can always be adjusted to 1 (via field current adjustment), and they are less costly.
Fig. 77: Synchronous motor rotors with 1, 2, & 3 pole-pairs and slip-rings for DC power (rotors shown with salient ( = protruding) poles, typ. for low speed applications, rather than cylindrical rotor with distributed windings)
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The characteristics of the synchronous motor of "Bernhard" locomotive nr. 4 are not known: neither the frequency of the 3-phase AC (other than "mid-frequency", which is typ. 400 - 2000 Hz), nor the number of rotor poles, nor the torque rating.
At least one of the locomotive motors of the Be-13 station at Buke was built by Ziehl-Abegg Elektrizitätsgesellschaft m.b.H., of Berlin-Weißensee. It was a 10 kW (13.6 metric horsepower, 13.4 US hp) "low rpm" motor (ref. 99). This suggests that it may have been the synchrounous AC motor. The motors at (some) other Bernhard stations may have been built by Siemens (e.g., ref. 103). This was probably Siemens-Schuckert, who also manufactured electric locomotives.
Ziehl-Abegg is a company specialized in electric motors. It was founded in 1910 by Emil Ziehl and the Swedish investor Eduard Abegg as Ziehl-Abegg Elektrizitäts-Gesellschaft m.b.H. Abegg dropped out of the partnership the same year, as he could not come up with the required funds, and the patent that he brought into the deal (ref. 214) proved useless. However, Abegg's initial "A" was retained (as a solid triangle) in the the "Z-A" company logo. Ref. 154. In 1897, Emil Ziehl invented the external rotor motor ("Außenläufermotor", outrunner motor: stator inside the rotor - very compact and excellent weight balancing). In 1904, he invented electrically powered gyroscopes with gimballed suspension. Prior to 1910, Emil Ziehl had developed electric motors and tested generators at AEG, and developed gyro-compasses at Berliner Maschinenbau AG (BEMAG, frmr. Eisengießerei und Maschinen-Fabrik von L. Schwartzkopff). BEMAG was a manufacturer of locomotives powered by steam, compressed air, and electricity. Ziehl-Abegg made DC-DC converters (DC-motor + generator) for Zeppelin airships and airplanes. Telefunken was a major customer. They also made electro-mechanical transverters ("Drehstrom-Gleichstrom-Umformer", i.e., AC-motor + DC-generator, as in Ward-Leonard Drive Systems) for elevators and generation of anode voltage of large transmitters, transformers for directional-gyros (e.g., SAM-LKu4), motor-generators such as the U 4a, and the motorgenerator-alternator of the U 120 of the "Bernhardine" system. After the war, the production facilities were carried off to the Soviet Union. In 1947, the company restarted, this time in the south of Germany (some 70 km northeast of Stuttgart). These days, Ziehl-Abegg AG is a manufacturer of electric motors for elevators, ventilation and air-conditioning systems.
Fig. 78: Company buildings of Ziehl-Abegg Elektrizitätsgesellschaft m.b.H in BerlinWeißensee
(source: www.ziehl-abegg.com, accessed 2023)
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Fig. 79: 1912 advertizing poster, relief on outside wall of the above building - before/after 2018 restoration
(sources: poster - ref. 155, plaques - www.pomm-restaurierung.de, accessed 2023)
Fig. 80: Ziehl-Abegg postmark on an envelope (1938) and listing in 1943 Berlin phonebook
(source: www.briefmarken12.de, accessed 2017)
THE LOCOMOTIVE MOTOR CONTROL SYSTEM
The task of the locomotive motor control system is to smoothly increase the rotational speed of the "Bernhard" beacon from stand-still to exactly 2 rpm, and to accurately maintain that speed. The signals transmitted by the "Bernhard" beacon were printed aboard the aircraft with the "Bernhardine" Hellschreiber-printer. The compass-scale channel of this printer was synchronized to pulses transmitted by the beacon. As explained in the "Optical Encoder Disk" section, to make this synchronization scheme work, the allowed tolerance on the 2 rpm beacon speed was only ±0.2-0.3 % (p. 80 in ref. 181 and p. 8 & 18 in ref. 183). This small tolerance had to be met, independent of variations in the motor load (e.g., rail resistance around the circular track, wind load), and independent of amplitude and frequency variations of the 3-phase 50 Hz primary AC power. Note that towards the end of the war, the minimum frequency of the 50 Hz power grid was reduced to 43.3 Hz in the Central German block, and to 41 Hz in the Western German block (ref. 14).
One standard way to control and regulate motor speed is with a closed-loop control system. This requires tachometer feedback of the momentary speed, for comparison against the speed
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set-point. The amount of speed error (and possibly one or more of its time derivatives, and its integral) is then used to command the motor to speed up, or slow down. If the control system is properly configured and dimensioned ( = control laws/algorithms), and it has sufficient control authority ( = "power"), then the torque-vs-speed curve of such a drive system can be made to approach that of a synchronous motor (see Fig. 125 above, ref. 215). So, when constant speed is required - as is the case here - then why not go straight to an inherently synchronous AC motor drive and use an AC power source that has a sufficiently constant frequency? Yes, indeed, why not! Doesn't this basically just move the control system from the motor to the AC generator? Yes, indeed. But there, it is easier to implement - as we shall see.
On the one hand, the locomotive drive system must operate with 3-phase primary AC power that has varying frequency and amplitude. On the other hand, DC power ( = rectified AC power) is required for several reasons. First of all, as explained in the "synchronous AC motors" section above, a synchronous motor is not self-starting. It must be brought close to synchronous speed by other means. Here: with DC traction motors. Also, the field winding of a synchronous AC motor is DC-powered. So, a 3-phase AC rectifier is required. And to complete the synchronous AC drive system, we need a "DC to fixed-frequency 3-phase AC" converter.
For low power applications, constant speed was often achieved with a "phonic motor" arrangement (ref. 235). Its concept was invented by Poul la Cour in Denmark in 1885 and patented by him in Britain in 1887. Ref. 235. It was used to synchronize telegraphy and teleprinter systems, as well as J.L. Baird's television system. In essence, it uses a stable electric oscillator to drive a synchronous AC motor. Since the 1920s, this was implented as the simple electronic audio tone generator. Its signal drives an electromagnet that is coupled to a mechanical tuning fork and continuously excites the fork. Tuning forks can only oscillate at a specific frequency. The resulting precise, constant fork vibration is captured via capacitive coupling. This signal is then amplifed to the required power level for the synchronous motor. This approach was also used in the antenna motor drive of some British 1920s rotating-beam beacons, and in the 1950's military Field Hellschreiber made by RFT.
The next diagram illustrates the three main blocks of the "Bernhard" locomotive drive system:
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Fig. 81: Top-level block diagram of the "Bernhard" motor drive system (source: derived from ref. 189, 190)
The "Electrical power control, conversion & distribution" block has the following functions and associated control panels:
Selection of the source of 3-phase 50 Hz (nominal) AC main power: the public power grid, or the local generator of the “Bernhard” station.
Protection against over-voltage / over-current conditions of the primary AC power, and emergency shutdown. For the latter, there was a shutdown button located in the rotating cabin, in the round building below it, and outside the concrete ring.
Conversion of the selected 3-phase 50 Hz AC power to DC power. This was done with a 6-anode Mercury Arc Rectifier described further below.
Conversion of DC power into 3-phase AC power that has a constant frequency (unlike the primary AC power). This conversion was done with an electro-mechanical DC-AC inverter; in this case, a so-called "Conz" converter as described further below.
Distribution of the AC and DC power. The selected 3-phase 50 Hz AC main power is distributed to the rectifier unit and to the rotating cabin (via a slip ring assembly in the round building below that cabin). The DC power from the rectifier unit is distributed to the DC-AC inverter and to the rotating cabin, Also see the "Electrical & signal distribution" section.
The motor drive control panel was located in the rotating cabin. It had separate controls for the three motor types (main drive DC, auxiliary drive DC with separate field winding, and synchronous AC). The block diagram in the next figure illustrates the power conversion and motor drive control functions with more detail:
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Fig. 82: Electrical power and signal interconnections of the locomotive system
(source: derived from ref. 93, 189, 190; 3-phase 380 Vac, 50 Hz from the backup Diesel generator could replace power from the publick grid)
The three main-drive DC motors were controlled via a heavy-duty variable resistor arrangement in series with the field and armature windings of each motor. The controller and resistor bank ware located in the rotating cabin (see the hand wheel on the "Main Drive" control panel in Fig. 83 below). The four auxiliary-drive DC motors had a separate field winding that was wired to the "Auxiliary Drive" control panel. The field current was also controlled with a variable resistor, but with a much lower power rating than that of the main-drive motors.
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Fig. 83: German engineer showing the locomotive control panel of Be-9 at Bredstedt to a member of the RAF-ADW
(source: Australian War Memorial photo SUK14636, public domain; ca. August 1945)
Since the synchronous AC motor provided inherent accurate speed control, there was no need for a closed-loop speed control system with a speed sensor. But there were two speed sensors: a tachometer in locomotive nr. 4 (with the synchronous AC motor), and a tachometer track on the optical encoder disk (measurement accuracy 0.1%) in the round building below the rotating cabin and superstructure. However, they were there for speed monitoring and alerting purposes only.
MERCURY ARC RECTIFIERS
A rectifier is an electrical device that converts alternating current (AC) to direct current (DC).
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The device achieves this by allowing electrical current to flow through it in one direction only. A half-wave rectifier only passes either the positive or the negative half of a full AC voltage cycle. A full-wave rectifier passes the positive half cycle directly, and the negative half cycle with inversed polarity:
Fig. 84: Half-wave and full-wave rectification of single-phase and 3-phase sinusoidal AC voltages
(assumes ideal rectifiers/diodes (no forward voltage drop, etc.), no source reactance (typ. inductive), no output smoothing, and no load)
By 1930, the Mercury Arc Rectifier (MAR, a.k.a. Mercury Vapor Rectifier; D: "Quecksilberdampfgleichrichter", ref. 163A-163R) had become the best method for rectifying high power AC voltage in industrial applications and electrification of light and heavy railroad. The discovery of the unidirectional current-flow of an atmospheric arc between a mercury pool and a carbon electrode, goes back to 1882 (Jules-Célestin Jamin and his co-worker Georges Maneuvrier, ref. 163H). The MAR was invented around 1900. P. Cooper-Hewitt patented a glass-envelope MAR in 1902 (ref. 163J), based on his mercury vapor lamp. He marketed a metal-envelope MAR in 1908. In 1914, Irving Langmuir patented the concept of using a controlgrid between the anode and the mercury-pool cathode (ref. 163K). This made it possible to arbitrarily choose the actual moment of arc initiation ( = switch-on via phase angle control, instead of it being determined by the primary power), and thereby vary the DC output. MARs can also be configured as an inverter instead of a rectifier, i.e., as a DC-to-AC converter.
MARs are a form of cold-cathode gas discharge tube. The rectifier consists of a glass or stainless steel vessel. The vessel is evacuated, or filled with inert gas. There is a pool of liquid mercury at the bottom of the vessel. This is the cathode. The vessel has one or more upward arms with a graphite anode. Clearly, a MAR is a static rectifier, as opposed to the mechanical rotary converters that preceded the MAR.
Full-wave rectification of a single-phase AC voltage requires two anodes. See Figure 135. For rectifying 3-phase AC power, the MAR must have a multiple of three anodes.
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Fig. 85: left: Simplified principle diagram with a 2-anode MAR (left) and an active 3-anode MAR
Like a fluorescent lamp, a MAR must be started. Conduction is initiated by dipping the starting (igniting) electrode into the mercury pool, passing a high current, and retracting the electrode. This locally heats up the mercury (the "cathode spot" or "emission spot") and vaporizes it. This starts abundant emission of electrons by the mercury cathode. The mercury vapor is ionized by the stream of electrons that flows to the anode, and causes plasma discharge (arc) between the anode and the cathode. The mercury ions emit both visible blue-violet light, and a large amount of ultra-violet radiation. The light may have another color when the vessel is filled with an inert gas, e.g., pink as in Fig. 85 (probably argon). Evaporated mercury condenses on the cool wall of the vessel (hence the large bulbous form), and returns to the mercury pool at the bottom of the device. The plasma discharge stops as soon as the anode voltage drops below a certain level, or anode current is interrupted. Hence, for rectification of an AC voltage, ignition must be synchronized with that voltage. Alternatively, excitation electrodes may be used to maintain the plasma. The anode material does not emit electrons, so electrons can only flow from the cathode to the anode. I.e., current can only flow from the anode to the cathode. The ripple on the DC output current is smoothed with a series-inductance ("choke coil").
The heat of the mercury vapor must dissipate through the glass envelope in order to condense. To help keep the glass envelope cool enough, an electric fan is typically installed below the MAR (as clearly visible in Fig. 138 and the the right-hand photo of Fig. 90 below).
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Fig. 86: Effect of cooling on voltage vs. current curve ( = losses & efficiency) of a glass MAR
(source: Fig. 66 in ref. 192)
For operating temperatures below 10 °C (50 °F), special measures must be taken to protect the MAR against damage from instable operation, and the attached transformers against current surges. This may be done with surge diverters and cathode heating. Ref. 161. Note that mercury freezes around -39 °C (-39 °F).
The MAR anodes are connected to AC power via a transformer. Each phase of the secondary side of this transformer has an inductance: inductance of the secondary winding itself, and transformed inductance of the primary transformer windings and the AC power line. Inductance prevents current (here: anode current) from varying instantly. Hence, when one anode becomes conductive and its current is building up, the current of the adjoining previously conductive anode anode is still dying down: for a short time, both anode arcs are active simultaneously! This cyclic "overlap" phenomenon effectively short-circuits the main transformer's secondary phases that are associated with these anodes. For rectifier circuits, the "overlap angle" (a.k.a. commutating angle) is the commutation time interval when when both devices conduct. This causes the rectifier's DC output wave to temporarily drop to the average of the overlapping sinusoidal transformer phase voltages, which significantly distorts the ripple (Fig. 25, 29, 33, 37 in ref. 163C).
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Fig. 87: The effect of phase-overlap on the DC voltage wave
(overlap due to source reactance or load; source: Fig. 25 & 29 in ref. 163C)
The voltage drop during the overlap period is proportional to the output current, and is also a function of the number of anodes and of the transformer inductance. As load current is increased, the operating time (duty cycle) of each rectifier phase is increased, and more phases will overlap. In case of a short-circuit load, the significant voltage drop across the rectifier arcs and resistive losses in the transformer (and other parts of the rectifier circuit) prevent full-time overlap of all phases.
MARs can be constructed for hundreds of kilovolts and tens of thousands of amps. They have been used, and sometimes still are (!), as rectifiers for locomotives, radio transmitters, control of industrial motors, welding equipment, aluminum smelters, high-voltage DC power transmission, etc. Glass-bulb MAR designs are typically limited to 250 kW (500 volt, 500 amps). For higher power levels, a steel-tank version was developed around 1908. Siemens-Schuckert developed a compact double-wall water-cooled tank rectifier around 1920. Through the 1960s, high power (up to gigawatts, ref. 163L) high-voltage DC (HVDC) transmission line systems were designed with MAR rectifiers and inverters. MAR technology was succeeded by ignitrons and thyratrons (ref. 163F, 163S), and then solid-state Gate Turn-Off devices (GTOs, e.g., thyristors), ref. 163Q.
In May of 2015, I obtained the black & white photo shown below. It shows the large 6-anode MAR of the Be-10 "Bernhard" at Hundborg/Denmark. The MAR does not appear to have control-grid electrodes. It was installed in a typical MAR-cubicle, 2 m tall (≈ 6.7 ft). The required primary transformers were located in the adjacent MAR-control cabinet. Likewise, the chokecoil, though the DC-motors may have had enough inductance so as not to require such a DCcurrent smoothing coil.
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Fig. 88: The rectifier system of Be-10 at Hundborg/Denmark
(sources: (left) ref. 223; (right) Fig. 31 in ref. 93; the thick "disk" below the MAR is actually the spinning cooling fan)
The "Bernhard" MAR was made by the Gleichrichter Gesellschaft m.b.H company in Berlin, manufacturer of rectifiers since 1919. They were acquired by the Swiss company Brown-Boveri & Cie. (BBC) in 1921 (ref. 191). BBC became ABB (ASEA Brown Boveri), after a merger between BBC and ASEA AB of Sweden in 1988. The "Bernhard" MAR was a model S 18 T "Glasgleichrichterkolben" (glass bulb rectifier; pdf page 20 in ref. 189). The complete rectifier cubicle with all the equipment and controls was model DRA 300A / 220 V, also of the Gleichrichter G.m.b.H. (pdf page 20 in ref. 189). The model designator suggests that the MAR had a rating of 330 amps DC at 220 volt AC. The cubicle included the standard cooling fan as well as a bulb heater. The circuitry around this MAR included 17 fuses! Standard equipment of each "Bernhard" station included one spare MAR (pdf page 21 in ref.189).
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Fig. 89: Label of a BBC MAR-cubicle with a MAR built by Gleichrichter G.m.b.H. in Berlin (source: H.-T. Schmidt homepage; MAR for 220 volt 3-phase AC @ 75 amps, 100/140 volt DC @ 150 amps, 140/165 volt DC @ 65 amps)
Other German MAR manufacturers of the era included Siemens-Schuckert Werke, several German subsidiaries of BBC, and the AEG company Apparate-Werke Berlin-Treptow (AT) that was founded in 1928. A 6-anode MAR with a height of 90 cm (3 ft, about the size of the "Bernhard"-MAR) can typically handle as much as 350 amps at 650 volts. The photo on the left in Fig. 140 below shows a small MAR, rated for only 220 volt / 100 amps (22 kW), together with its transformers. This MAR was manufactured in the 1950s by Elektro-Apparate-Werke J.W. Stalin in Berlin-Treptow. This was the post-war continuation of AEG-AT in the Soviet-occupied part of Germany. The 6-anode MAR in the photo on the right is about 60 cm (2 ft) tall. It is part an elevator (lift) system in a defunct very large WW2 air-raid shelter 140 ft (43 m) below Belsize Park in London (ref. 243). This MAR is still operational in modern days (at least through the year 2014). The cubicle is similar to the "Bernhard" cubicle in Fig. 138.
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Fig. 90: Example of MAR cubicles (left: 1950s, right: 1936)
(sources: Technische Universität Bergakademie Freiberg (left); ©2000 Nick Catford Subterranea Brittanica (ref. 243); both used with permission)
The "Bernhard" MAR shown in Figure 88 has six anodes. For obvious reasons, these multi-arm MARs are sometimes referred to as "Krakengleichrichter" ("octopus-rectifiers"). Compared to a 3-arm MAR, a 6-arm MAR reduces the ripple in the rectified voltage. It also requires a more complicated main-transformer connection to AC power, e.g., a delta/double-star or star/doublestar configuration. See pp. 18-24 in ref. 163B. Six-anodes is typically sufficient. Having more anodes does reduce the output ripple (which has a lowest harmonic frequency of six times the 50 or 60 Hz main power). However, the reduction when going from 6-phase to 12-phase rectification is less than half the reduction when going from 3-phase to 6-phase (Fig. 35 in ref. 163C for no-load conditions). Also, cost increases rapidly without increasing rectifier output, and the already low power factor (due to an undesirably large phase angle between AC supply voltage & current) is further reduced.
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Fig. 91: A 6-anode Mercury Arc Rectifier with delta/double-star main-transformer connection to 3-phase AC power (source: Figure 7.41 in ref. 163E)
When the motor voltage is reduced to slow down the motors (e.g., to brake to standstill), the large inertia of the "Bernhard" turntable will reverse the load torque of the motors. However, current can only flow through the MAR in one direction - it is a rectifier/diode. So, regenerative braking ( = using the motor as a generator and feeding the generated electricity back to a power grid) is not an option, and the motor cannot exert a braking torque. As a result, the armature voltage will increase to undesirably high levels. This is typically handled by switching-in a large dummy-load resistors across the armature of the motors, and dissipating the generated power as heat.
Here is a 36 sec video clip of a 6-anode MAR in action (WARNING - MAR systems are very loud!):
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A six-anode Mercury Arc Rectifier in action
Source: YouTube; one of four MARs of the 300 kW, 600 volt DC power supply system of the tramway network in Melbourne/Australia. MARs made by Hewittic Electric Co. Ltd. (frmr. Westinghouse Cooper-Hewitt Co Ltd., estd. 1906) in Walton-on-Thames/England, installed in 1936, operational until 2019! Ref. 163T.
THE "CONZ" DC-AC POWER INVERTER
As stated above, a synchronous AC motor was used for obtaining and maintaining the required accurate locomotive speed. The frequency of the 3-phase AC power from the public power grid and the local backup generator was not sufficiently accurate. Hence, the fluctuating primary AC power had to be converted to constant-frequency 3-phase AC. Before the days of solid-state power electronics (1960s), the required power conversion was done by electro-mechanical means: an electric motor, an AC generator, and a closed-loop control system. The motor, generally referred to as the "prime mover", could be AC or DC. In the "Bernhard" system, a special DC-to-3-phase-AC inverter (D: "Umformer für Gleichstrom-Drehstrom") was used: a model NGJV So, 5/2 T built by the Conz company (sheet 16 in ref. 189). It was located in power generator building near the "Bernhard" beacon. The associated control panel had the following fuses: two for 350 V, 160 A, and three for 500 V, 35 A (per sheet 20 of ref. 189).
The Conz Elektricitäts-Gesellschaft mbH company was founded in 1887 by Gustav Conz. It
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was originally located in the Spaldingstraße in the southern German city of Ulm. The company moved to Hamburg in 1890, and acquired a plot of land in Hamburg/Altona-Bahrenfeld (Gasstraße 6-10) in 1911. An office building and two factory building were constructed here in 1912. Ref. 207. In 1962, Conz became a wholly-owned subsidiary of Deutsche MaschinenbauAktiengesellschaft (DEMAG) in Duisburg. The Hamburg plant was closed in 1995.
Fig. 92: advertizing of the Conz company from 1924
(source: Elektrotechnische Zeitung (ETZ), Nr. 16, 17 April 1924)
Fig. 93: advertizing of the Conz company from 1937 & 1940
A standard AC-generator has a field winding that is fed with DC power, and an armature
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winding that outputs the generated AC power. That is: a "singly-fed" generator. However, at the heart of a Conz converter is a high-power "doubly-fed AC generator" (D: "Doppeltgespeister Drehstromgenerator"). This is also called a "doubly-fed induction generator (DFIG) and "slipring generator" (D: "Schleifringläufergenerator"). Note: "doubly-fed" is somewhat of a misnomer, as it does not mean that the machine has two separate power inputs. Just that it has two sets of 3-phase connections. A DFIG is similar to the singly-fed generator, in that the stator outputs the generated AC power. However, now the rotor is excited with 3-phase AC power at variable frequency. The frequency of this AC excitation power is continuously adjusted to compensate for changes in the speed of the prime mover. The result is regulated 3-phase AC power (D: "geregelter Drehstrom") with a constant frequency. In modern times, DFIGs are widely used for large wind turbines, as solid-state inverters required for megawatt-scale wind turbines are larger and more expensive.
Fig. 94: singly-fed DC and doubly-fed 3-phase AC power generator
(the stator windings and DFIG rotor windings are shown in the standard WYE (a.k.a. "Y", "star") configuration rather than Delta ("Δ"))
The next figure shows the principle diagram of the AC-AC Conz generator, as patented in 1937:
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Fig. 95: Conz convertor - variable-to-fixed frequency AC-AC converter
(source: adapted from the 1937 Hans Gross / Conz patent nr. 692583; see ref. 188 for a list of related patents)
The green overlay in the figure above, shows the control loop for keeping the output frequency of the DFIG constant - independent of variations of the frequency and voltage of the primary AC power and of the output load: the 3-phase AC output power drives a synchronous AC motor. This motor is small (low power) compared to the DFIG and the primary mover. It drives a centrifugal governor. Based on the rpm, the governor adjusts a variable resistance. The resistance is placed in series with the field of a small DC motor, so as to change its speed. The DC motor drives a small 3-phase AC generator that excites the DFIG. Note that it is also possible to reverse the rotational direction of the three phases of the output. The efficiency of a Conz generator is higher than Ward-Leonard converter, especially under partial load or no load (idling).
The Conz generator configuration above uses an AC motor as prime mover, and a DC generator. This configuration can be simplified if a high-power DC source is available. This is the case in the "Bernhard" system, for powering the DC motors in the four locomotives. The DC-to-AC converter configuration is also mentioned in the Conz patent. So, a DC motor was used as prime mover, and the small DC generator was eliminated:
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Fig. 96: Conz generator - DC-AC configuration as probably used in the "Bernhard" motor drive
(source: adapted from the 1937 Hans Gross / Conz patent nr. 69258; see ref. 188 for a list of related patents)
The output frequency of the "Bernhard" Conz-generator is not known. However, the title of the related 1937 Reichs patent is "Frequenzwandlergruppe zur Erzeugung konstanter Mittelfrequenz", i.e., "Frequency converter for generation of constant mid-frequency". In modern times, 400 - 2000 Hz mid-frequency AC is used in high-power application such as resistance welding. Ref. 10 (1946) states that the the "Bernhard" Conz-generator generated 50 Hz power. Ref. 290A also suggests this.
The photo below was taken in the power supply bunker or barrack of Be-10 at Hundborg/ Denmark. The Conz converter is on the right and is about 2 m tall. It is installed upright, probably due to the centrifugal governor. The cabinets on the left house the MAR rectifier and associated circuitry and controls.
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Fig. 97: The Be-10 Hundborg installation - cabinets with the MAR and its controls, and the "Conz" generator
(photo courtesy Mike Dean, US National Archives & Records Adm. (NARA) image nr. 111 SC 269041; US gov't = no ©)
ROTATIONAL SPEED
The official German Bernhard/Bernhardine system description documents (ref. 181, 183, Blatt 8 & 10 in 198A) clearly state that the Bernhard-beacon rotated once every 30 sec, i.e., at 2 rpm. The specified diameter of the center of the circular rail track ( = midway between the two rails) was 2x10.55 = 21.1 m (ref. 193). Hence, the track length was 66.28 m. This means that at 2 rpm = 120 rph, the small locomotives that turned this enormous antenna installation, moved at a respectable speed of 8 km per hour (5 mph). As described in the "Locomotive system" section below, the rotational speed of the system was determined by a single synchronous 3-phase AC motor in one of the four locomotives. The 3-phase AC power was provided by a DC-AC inverter that had a fixed reference frequency. Hence this motor could only turn at the reference speed, and the speed was monitored very accurately. The rotational speed of the beacon was kept constant to within -0.2 to +0.3% ! See p. 80 in ref. 181 and p. 8 & 18 in ref. 183.
The official German system descriptions and the manual of the FuG 120 "Bernhardine" printer
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system also state the same 2 rpm speed (top of Section B on p. 5 in ref. 15, sheet 8 & 10 of ref. 198A; also §3 in ref. 10). Moreover, the "Bernhardine" printers were simply not compatible with antenna rotational speeds that deviated more than a couple of percent from that nominal speed! As with other types of synchronized teleprinter systems, the motor of the Bernhardine-printer had to turn 1-2% faster than nominal system speed (p. 18 in ref. 183).
There are some persistent statements (e.g., ref. 5A) that the "Bernhard" beacons rotated with a period other than 30 sec. Some of the sources for these statements are the following:
The French resistance explicitly reported in 1943 that this station Be-3 at Le-Bois-Julien rotated once per minute (p. 62 in ref. 91).
A British 10 cm [ = 300 MHz] radar station at Fairlight (on the East Sussex coast, east of Hastings, 87 km [54 miles] northwest of Be-3 at Le-Bois-Julien) concluded that the station appeared to rotate once a minute (ref. 173B, December 1943). Likewise, extensive radar measurements early-June 1944 also concluded that the rotation period was 52-60 sec (ref. 173A).
Luftwaffe POWs in the UK reported 1 rpm for the Bernhard system in general (§10, 18, 19 in ref. 6C)
A 1946 US air force survey of German electronics development, stating that "... information is printed once per minute" (ref. 93). This may have been based on wartime "intelligence" from other sources.
A "reliable informant" who saw the Be-6 station at Marlemont/France station in operation, reported to the British that he was told [correctly] that it rotated with a speed of 8 km/h. Based on the wrong British photometric assumption that the Bernhard-ring had a diameter of 82 ft [25 m], a rotational period of 36 sec. was estimated. Ref. 173E. For the actual diameter of 71 ft [21.5 m], the correct period of 30 sec. would have been obtained.
Ref. 225 and 226A state that the system rotated at 1 rpm with and the beam was received by aircraft during 10 sec per rev.
Ref. 175B (RAF 192 Squadron) mentions that the beam was received by aircraft during 5-10 sec per minute. This may suggest 1 rpm. Note that typical reception time was actually 3-5 sec per rev, so 5-10 sec / minute could be 1 or 2 rpm...
The 1936 Lohmann/Telefunken patent 767528 states that the limiting factors for the upper limit of the antenna's rotational speed, are the printing speed of the Hellschreiber and the required pixel resolution of the printed information. Given the large size and weight of the antenna system, there are obviously also mechanical considerations for the upper speed limit. The patent proposes to resolve this, by quadrupling the number of antenna beams, spaced at 90o intervals. Each optical encoder disk would simply have four "light source plus photocell" pairs (two pairs shown in the diagram above), that could be adjusted to account for angular offsets between the beam centerlines.
THE ANTENNA SYSTEM
The "Bernhard" ground station is the rotating radio-navigation beacon of the "Bernhard/ Bernhardine" system. This means that its transmission is not simultaneously in all directions (i.e., omni-directional), but it sweeps the horizon with its directional radio beams. Secondly, it is
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a 2-channel transmission: one channel is used to transmit the momentary azimuth value of the beam-direction. This is done in Hellschreiber-format. The second channel is used to transmit a signal that disappears ("null") when the associated beam is pointing straight at the antenna of the navigation-radio receiver in the aircraft.
So, how is this all done? Clearly, we need two antenna sub-systems: one for each transmission channel. The antenna sub-system for the azimuth-data must create a single-beam radiation pattern. The antenna sub-system for the pointer-channel must create a twin-beam radiation pattern, with a sharp null between the two beams. That is, two narrow beams, that point slightly to the left and right of the centerline of the single-beam. To get the sweeping effect, the two antenna systems must be aligned (pointing in the same direction), and be continuously rotated together.
Let's look at the actual antenna systems to see how all of this was implemented. Figure 55 below shows three development stages of the antenna system of the initial UHF Bernhard system. It operated at a frequency of 300 MHz, which is equivalent to a wavelength of 1 meter. The fourth development form (1940) used a parabolic antenna with an aperture of 6 λ (ref. 3, 181), which translates to a beamwidth of almost 10°. This antenna is shown in the far right photo in the next figure.
Fig. 98: Three antenna systems developed for the UHF-version of "Bernhard" (left: 1935 (twin-beam only), center: 1936 , right: 1940; source: ref. 2B)
The antenna system in the center photo of Figure 98 most clearly shows the arrangement of a large number of vertically-oriented antenna elements:
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Fig. 99: The three antenna sub-systems and the resulting radiation pattern (top view) (dipole arrays diagram: adapted from Reichspatent 767532)
The antenna sub-system at the top (see the green box in the above Figure) shows a group of five identical, parallel antennas that are interconnected. These antennas are simple dipoles. As the name suggest, a dipole has two identical "poles", typically straight metal rods or wires. The two halves of the dipole are connected to the transmitter via a 2-wire feedline. The next figure illustrates the radiation pattern of a single, vertically oriented dipole antenna in free-space (i.e., sufficiently far from other objects and ground). It is a doughnut-shaped omni-directional pattern which is not what we want!
Fig. 100: The omni-directional radiation pattern of a single vertical dipole antenna in freespace
(left: 3D pattern of the strength of the radiated signal; center: vertical cross-section of the pattern, right: top view)
So how do we get a directional beam-pattern? This requires that we concentrate the transmitted energy in the desired direction(s), and radiate less energy in all other directions. This can be done in at least three ways (ref. 139K1): By placing a reflector surface behind the (dipole) radiator.
By using a "longwire" antenna (such as a Rhombus antenna), with a radiator length of several wavelengths. This is not very practical for a rotary antenna system that
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operates at a wavelength of ca. 10 m. By combining the radiation pattern of multiple dipoles. or a combination thereof.
Let's keep it simple and use several identical dipoles that are placed in parallel, and arranged on a straight line ( = linear). As all dipoles lie in the same plane, this is called a planar array. Let's use the same spacing ( = equi-distant) between adjacent dipoles: a linear array. We feed all dipoles of this array with the same radio-frequency current (= same amplitude and same phase). I.e., a uniform distribution of the dipole excitation. The in-phase (co-phase) feeding makes the array concentrate the radiated energy in the direction that is perpendicular ( = broadside) to the plane that is made up by the parallel dipoles. We now have a uniform linear broadside array. The actual pattern depends on the distance between the dipoles. The next two Figures show this dependency for an array of 3 and of 4 parallel dipoles, respectively. This is beginning to look like what we want!
Fig. 101: Top view of radiation patterns of a uniform 3-dipole broad-side array (dipoles spaced by 0.2 - 1 λ)
Fig. 102: Top-view radiation patterns of a uniform 4-dipole broad-side array (dipoles spaced by 0.1 - 2 λ)
We can make several general observations:
The direction with maximum radiation intensity, is independent of the number of parallel dipoles and their spacing.
For spacing larger than 0.1 λ, all patterns have one or more clear side-lobes, in addition to the "forward" radiation of the main beam. This is not desirable.
For spacing equal to, or larger than, half a wavelength (0.5 λ), some side-lobes become very significant and approach the strength of the main beam. These are called grating lobes. This is also not desirable. Note: maximum gain for a given number of dipoles is typically obtained for a spacing of 0.5 to 0.7 λ.
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The main beam becomes narrower ( = directivity) and stronger ( = gain), as the number of dipoles and/or spacing increases. This is desirable. However, each additional dipole adds less gain than the previous addition ("law of diminishing returns"). The latter is illustrated in Figure 37 below. Also, if the number of dipoles is increased without increasing the overall size (span) of the array, then the directivity is reduced (main beam becomes wider), ref. 139J.
As, for a given number n of dipoles, the spacing increases, the number of (equal strength) maxima increases. Between these maxima, there are n - 2 smaller lobes. Ref. 139J.
Fig. 103 The radiation pattern of uniform broadside arrays of 1-8 parallel dipoles (spacing < 0.5 λ)
(source: Figure 3.46 in ref. 139A)
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Fig. 104: Radiation pattern of uniform broadside array of 3 parallel dipoles (spacing = 0.5 λ) - without reflector screen or dipoles
(left: array configuration; center: 3D radiation pattern; right: top view of the radiation pattern; my 4NEC2 model is here)
How get can we reduce, or even eliminate, the side-lobes and rear-lobe? There are several techniques, with varying degrees of complexity. The basic parameters that we can adjust, are amplitude and phase of the dipole current, spacing between the dipoles, and the use of reflectors (ref. 139A-139H):
One way is to not feed all dipoles with the same current amplitude. That is, non-uniform excitation instead of uniform. Instead, use amplitudes that taper off towards the dipoles at both ends of the array. This is called current "grading" or "tapering". The amplitude coefficients should be equal to those of a binomial series. For dipole spacing less than 0.5 λ, this eliminates all side-lobes. Current-grading coefficients may also take the form of a Fourier-series distribution, Power-of-Cosine distribution, Taylor-series distribution, etc. However, such distributions significantly complicate the system for feeding the dipoles. Also, the uniform distribution of the "Bernhard" arrays produces the highest directivity (see p. 518 in ref. 139H), though the first side-lobe is at best only about 13 dB down from the main-lobe.
Side-lobes can be reduced by putting a reflector surface behind (and parallel to) the plane of the dipoles. This is what we see in the three "Bernhard" antenna systems shown in Figure 55 above. For UHF frequencies (λ ≤ 1 m), constructing such a conductive "mirror" surface is easy. Likewise for small VHF arrays (2-3 dipoles). Reflectors are typically placed at a distance of 0.25 λ from the dipoles (1939 Telefunken/Lohmann patent 767531, lines 114-116; also p. 18 in ref. 138).
For true broadband operation (i.e., a bandwidth of at least ±10% about the center frequency, whereas "Bernhard" is "only" ±5%), a reflector distance of 0.34 λ actually provides much less variation in feedpoint impedance, with that impedance closer to being purely resistive (ref. 139K2).
Instead of a solid metal surface, as wire mesh may be used, as long as the size of the openings is less than 0.1 λ. Instead of a mesh, it is also possible to use a sufficiently dense "curtain" of un-tuned wires that are parallel to the dipoles.
To get the desired effect, the planar surface or mesh may have to be extended around the array. I.e., the array is placed inside a shallow metal box or wire basket, that is open in the broadside / forward direction of the array.
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Instead of a reflector surface or mesh, a reflector-dipole can be placed behind (and parallel with) each primary dipole, e.g., at a distance of 0.25 λ (e.g., p. 18 in ref. 138). The reflector-dipoles may be passive ("parasitic"). In this case, they are not connected to the transmitter, but receive radiation from the other dipoles and re-radiate it. The reflector-dipoles may also be actively powered by the transmitter. The result of the latter method is illustrated in Figure 105 below.
Fig. 105: The radiation pattern of uniform broadside arrays with identical array placed 0.25 λ behind it, fed with 90° phase difference (source: Figure 3.42 in ref. 139A)
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Fig. 106: Close-up of the upper antenna system of the "Bernhard" beacon (sources: Museums Center Hanstholm, Hanstholmregistreringen; used with permission)
Figure 107 shows my simulation of the effect of placing a reflector mesh-screen (0.1 λ mesh openings) at 0.25 λ behind the 3-dipole array: a significant reduction (14.4 dB) of the side-lobes, compared to Fig. 61 above. In the actual "Bernhard" antenna system, there is a mesh-screen above, below, and to the sides of the array. I.e., a mesh cage that is open in the forward direction. This further reduces the side-lobes. I did not expand my simulation model this way.
Fig. 107: Radiation pattern of 3-dipole broadside array with a reflector screen (not a cage)
(left: array configuration; center: 3D radiation pattern; right: top view of the radiation pattern; my 4NEC2 model is here)
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Radiation pattern of the 3-dipole array with reflector screen - rotating at 2 rpm
(©2016 F. Dörenberg; simulation in free-space, i.e., effect of the earth's surface not taken into account)
So, now we basically know how to create a single-beam pattern with acceptably small sidelobes. But how do we get the required twin-beam pattern? How about using two separate broadside uniform arrays that are placed side-by-side? Then we "just" have to create a small angular difference between the two main beams. This can be done both mechanically and electrically. The mechanical approach was used in the small version of the "Knickebein" beacon system. The left-hand and right-hand dipole arrays are clearly placed at an angle, and the two beams cross-over in front of the antenna system. See Figure 108. Looking down onto the antenna system, the two sides form a shallow "V". Hence the name "Knickebein" ("crooked leg" = "dog leg").
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Fig. 108: Knickebein ground-station - large (left) and small (right) (source: Fig. 36 & 37 in ref. 181; red circle shows size of a man)
Fig. 109: The alternating "E" (dot) and "T" (dash) beams of the Knickebein beacon
The main beam of a single broadside-array can be pointed electrically in a direction other than perpendicular. This called "beam slewing" or "electronic beam steering" (see Section 3.14 / pp. 271 in ref. 139A). Here, the dipole-current in the dipoles of one half of the array, is given a phase-angle difference with respect to the dipoles in the other half of the array. The size of the phase-offset determines the amount of beam-slew or beam-tilt.
To get a twin-beam pattern, one could use two such split-arrays and put them side-by-side. However, this is unnecessarily complicated for what we are trying to achieve! All we need to do, is put two uniform linear broadside arrays side-by-side, and simply feed them in an anti-phase manner (180° phase difference between the two arrays). See p. 72 in ref. 137A. The resulting radiation pattern has two main-beams, symmetrically with respect to the normal (perpendicular) direction, with a small angle and a sharp, deep null between the two beam directions. See Figure 110 for a 4+4 dipole array configuration. Other than the rear-lobes, this is what we want!
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Fig. 110: Radiation pattern of two side-by-side arrays that are fed anti-phase (without reflector screen or dipoles)
(left: array configuration; center: 3D radiation pattern; right: top view of the radiation pattern; my 4NEC2 model is here)
Before discussing the VHF-version of "Bernhard", there are two aspects of the UHF antenna system that need to be mentioned: The configuration of the twin-beam dipole array. Feeding a rotating antenna system from stationary transmitters.
The twin-beam UHF antenna system comprised two side-by-side arrays (see the magenta boxes in Figure 99 above). The photo shows that each array actually comprised two sub-arrays of five dipoles each. These sub-arrays are placed one above the other (a so-called "stacked" array). This was either done as part of side-lobe suppression (see Section 3.16 (p. 276) in ref. 139A), or to increase gain. However, the latter is less likely. The system used two identical transmitters (i.e., same output power). The power of one of these transmitters was split in two, for the two halves of the twin-beam array. Increasing the gain of the twin-beams would increase the operational range of the beacon, but it would not make sense for the twin-beam system to have a significantly larger (or smaller) range than the single-beam system above it.
The VHF "Bernhard" system operated at frequencies in the 30-33.1 MHz band, instead of 300 MHz. That is, a nominal wavelength of about 10 m instead of 1 m. This basically means that the antenna system of the VHF "Bernhard" is about ten times as big as its UHF predecessor. Figure 68 below shows the large antenna system. It measured about 35x25 m (WxH, ≈82x115 ft). We recognize the upper dipole array (green box) for the single-beam transmission, above the two side-by-side arrays (magenta boxes) for the twin-beam transmission.
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Fig. 111: The two antenna sub-systems of the Be-10 "Bernhard" station at Thisted/ Denmark
Note that the dipole-array configurations are not the same as those of the UHF "Bernhard" system:
The single-beam array now comprises three parallel dipoles instead of five. Clearly, having used arrays of five dipoles would have made the entire antenna system much larger. Also, the single-beam has to be at least as wide as the combined twin-beams.
There is no solid reflector surface behind the top-array. Instead, there is a wire-mesh reflector behind and around the array. Obviously the mesh is much lighter, and also has less wind resistance. It is placed at a distance of 0.25 λ from the dipoles (p. 19 in ref. 183; note that in ref. 183 and some similar German literature of the era, "dipole" refers to a dipole-leg, i.e., only half ( = one pole) of a complete dipole antenna).
The twin-beam array now comprises two side-by-side sub-arrays of four dipoles each, instead of five. These two sub-arrays are fed in an anti-phase manner: 180° phase difference between the two arrays.
There is no reflector surface behind the bottom-array. Instead, there are reflectordipoles. They are placed at 0.25 λ behind the primary dipoles. The reflector-dipoles are "active", i.e., powered by the transmitter. Active reflectors are more effective for sidelobe reduction than passive/parasitic reflector dipoles or rods (see p. 71 in ref. 137A). The phase angle of the excitation of the reflector-dipoles leads that of the main dipoles in front of them by 90°. There are two different configurations of reflector-dipoles, see Figure 112A/B:
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A reflector-dipole behind each primary dipole (Figure 112A).
A reflector-dipole only behind the two primary dipoles of each sub-array that are closest to the centerline of the antenna system (Figure 111 and 112B).
Fig. 112A: Top view of the 2x(4+4) array configuration of the twin-beam antenna
Fig. 112B: Top view of the 2x(4+2) array configuration of the twin-beam antenna
Fig. 113: Radiation pattern of two side-by-side arrays that are fed anti-phase (with active reflector dipoles at 0.25 λ)
(left: array configuration; center: 3D radiation pattern; right: top view of the radiation pattern; my 4NEC2 model is here)
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Animation of the radiation pattern of the 2x(4+4) arrays - rotating at ≈2 rpm (©2016 F. Dörenberg; simulation in free-space, i.e., effect of the earth's surface not taken into account)
Below are illustrations of the radiation pattern of the "Bernhard" antenna system. They are from pre-WW2 Telefunken/Lohmann patents, and therefore relate to the early UHF-version of "Bernhard". However, they are similar to the final VHF-version of "Bernhard" (see p. 95 in ref. 3). Note that the pattern has significant side-lobes and rear-lobes. This is confirmed by the official manual of the FuG 120 "Bernhardine" bearing-printer system that was used in the aircraft (p. 6 in ref. 15).
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Fig. 114: Single-beam and twin-beam radiation patterns of the UHF "Bernhard" antenna system
(left: Figure 2 in the 1936 patent 767354; right: Figure 1 in 1938 patent 767523)
In the Cartesian plot (right-hand plot in the Figure above), the side lobes of the twin-beam curve are roughly a factor 24.4 : 5.2 = 4.7 below the main lobes. This voltage attenuation factor is
equivalent to a power attenuation of about 10•log(4.7)2 = 13.4 dB. This is consistent with the maximum theoretical value of about 13 dB for arrays with a uniform current distribution (see p. 518 in ref. 139H).
My simulation model generates very similar patterns:
Fig. 115: Cartesian plot of the radiation pattern, based on my antenna simulation model for the 2x(4+4) arrays
(both curves converted from power gains; the grayed area corresponds to the -50° to +50° range of the plot in Fig. 47 and 48 above)
Based on available photos, the 2x(4+4) dipole array configuration was used at the "Bernhard" installation of Trebbin (Be-0), Mt.-St.-Michel-de-Brasparts (Be-2), and La Pernelle (Be-4). The
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2x(4+2) configuration was used at the installation of Bredsted (Be-9), Thisted (Be-10), and Arcachon (Be-7). This suggests that the latter configuration may have been introduced with station Be-8 in Schoorl (constructed 1942/43). Why remove the outermost reflector-dipoles (other than to save material)? Figure 73 shows the difference in the 3D radiation pattern of the two configurations. Without the outermost reflector dipoles, there are more (and slightly stronger) rear-lobes. However, this may not necessarily have had a negative impacted on the performance of the "Bernhard/Bernhardine" system, based on the time-constants of the automatic receiver-gain control of the SV 120 printer-amplifier in the aircraft.
Fig. 116: Radiation pattern of the 2x(4+4) array configuration (left) and of the 2x(4+2) configuration
(My 4NEC2 models for these two configurations are here and here)
Note that the colorful radiation patterns illustrated above, were generated with modern computer-based simulation tools. Clearly, such tools were not available before and during World War 2 - the days of mechanical analog calculators, notably the slide rule (D: "Rechenschieber"). However, radiation patterns and other characteristics of directional antenna systems (incl. arrays with reflectors) were well understood in those days, and were indeed calculated (ref. 253A (dated 1938), 253B), albeit in a very time consuming manner.
The next figure shows a 1944 Cartesian plot of pointer-beam of the VHF "Bernhard" - it corresponds to my simulation of the 2x(4+2) dipole configuration:
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Fig. 117: Cartesian plot of the radiation pattern of the VHF "Bernhard" antenna system (source: adapted from Fig. 1 in ref. 201, 1944)
Transmitter power output (TPO) is the radio-frequency (RF) power that the transmitter produces at its output. This is not the same as the effective radiated power (ERP) of the antenna system. ERP is basically the product of transmitter output power, losses in the feedline system (and radiation by that system) including splitters & connectors, and gain of the antenna system in the direction of maximum gain. ERP is referenced to the same TPO being input to a single dipole. The "Bernhard" dipole arrays concentrate the RF energy into the main lobes of the radiation pattern. This provides a gain with respect to a omni-directional reference antenna. Based on my simulations of the antenna arrays, the main lobes of the twin-beam pattern had a gain of 13.81 dBi. That is: 13.81 dB compared to an isotropic radiator. ERP is referenced to a dipole, which has a gain of 2.15 dBi. Hence, the twin-beams had an estimated gain of 13.81 - 2.15 = 11.66 dBd. With a 500 watt "Bernhard" FuSAn 724 transmitter, this would have resulted in an ERP of 7.3 kW - far from the unrealistic 5 MW ( = 500 W + 40 dB) that is sometimes suggested in literature. FuSAn 725 was intended to increase the beacon's range by increasing the transmitter power by a factor of ten = 10 dB: from 500 W to 5 kW. Note: a 10-fold increase of transmitter power implies a range increase of SQRT(10) ≈ 3x. Of course, the power increase can also be used to increase immunity to enemy interference (e.g., jamming).
The next Figure illustrates the length, thickness, and spacing of the dipoles, as well as the spacing between the dipoles and the reflector-cage. The size of the two men in the photo can be used as a reference. Note that the VHF "Bernhard" operated at a frequency of 31.55 MHz ±1.55 MHz. Hence, the nominal wavelength λ is 9.5 m.
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Fig. 118: Dimensions and spacing of the dipoles and the wire-mesh reflector cage
The table below shows the dipole and spacing dimensions for three different assumptions regarding the height of the men in the photo above:
Table-2: Estimated dipole and spacing dimensions, based on photogrammetric analysis
I have run antenna simulations for about 30 different combinations of parameters in Table-2, for the array of 2x(4+4) dipoles. A table with the results is here. General conclusions are consistent with antenna theory:
When the dipole length is decreased, gain of the main beams decreases (not desirable), the front-to-back ratio decreases (not desirable), and the strength of the first left & right side-lobe increases relative to the two main beams (not desirable)
The gain of the two main beams is relatively insensitive to spacing between the front dipoles and the reflector dipoles.
The strength of the first side-lobes left & right of the two main beams, is relatively insensitive to the variation of the parameters. The relative strength is -9 to -12 dB.
The strength of the other side- and rear-lobes is sensitive to spacing between the front dipoles and the reflector dipoles, and to the dipole diameter.
The width of the two main beams is relatively insensitive to array dimensions, other than the number of dipoles. The beam-width is 10°-12°.
The angle between the two main beams is relatively insensitive to array dimensions, other than the number of dipoles. The angle is 20°-24°.
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The total number of pattern-lobes increases when lateral spacing between the dipoles is increased. For the evaluated configurations, this even number varied from 10 to 16.
As in the UHF "Bernhard" system, the dipoles are "full-wavelength": they have a nominal tip-totip length of 1 λ. Note that this appears to be contrary to the original 1936 Telefunken/Lohmann Reichspatent nr. 767531 and 767532, in which all dipoles are referred to as "half-wave" (1⁄2 λ). However, literature of the era generally referred to the length of a dipole leg, instead of the overall dipole length.
Anyway: why use dipoles 1 λ dipoles instead of 1⁄2 λ dipoles?
The radiation pattern of a 1 λ dipole has slightly more gain and slightly narrower beams than that of a 1⁄2 λ dipole.
1 λ dipoles are more suitable for broad(er)band operation than antenna systems with 1⁄2 λ dipoles: for the same relative thickness, a full-wavelength dipole has bandwidth that is about 1.3x larger than that of a half-wave dipole (see Section 4.3 in ref. 135).
A 1 λ dipole may be harder to properly match to a modern solid-state transmitter via a 50 Ω coax cable than 1⁄2 λ dipole (at resonance), but that is not a real argument against 1 λ dipoles when using a transmitter with tube amplifiers in combination with an openwire feedline.
The mid-point of each "leg" of a 1 λ dipole in principle has (near) zero voltage. This makes it a convenient point for attaching the dipole to a support structure. This is discussed further below. The neutral point of a 1⁄2 λ dipole is the feedpoint, which is not convenient for attachment.
Implementing a uniform array is easier with 1 λ dipoles. They have high feedpoint impedance. Therefore, they are voltage-fed instead of current-fed. A feed-system for supplying all dipoles of an array with same amplitude and phase, is easier with voltage than with current. The dipoles need to be fed in-phase ( = co-phase). That is, the phase difference between all dipoles within a sub-array must be 0° = n x 360°, where n is an integer number. Half of the 360° is simply obtained by having a feedline length of 1⁄2 λ = 180° between adjacent dipoles. Obviously, this is easy to do: just space the dipoles 1⁄2 λ (with a small adjustment for the velocity factor of the feedline wire). A tuned 1⁄2 λ section of feedline also has the advantage that it does not act as an impedance transformer: the impedance at one end, is transferred 1:1 to the other end. In the Bernhard antenna arrays, the feedline between adjacent dipoles is a balanced open-wire transmission line (TL). I.e., two parallel wires that are suspended in the air. The second half of the required 360° is simply obtained by connecting the top element of the even dipoles and the bottom element of the odd dipoles to the same feedline wire, and vice versa. I.e., switching polarity at each dipole. The latter switch-over can be implemented two ways, see Figure 53:
By crossing the feedline wires between dipoles. This (standard) approach is illustrated in the 1936 Telefunken/Lohmann Reichspatent nr. 767531 and 767532, However, with this method, the feedline wires are not perfectly parallel. This may disturb the characteristic impedance Z0 of the feedline.
Without crossing the feedline wires between the dipoles. Instead, the cross-over is made at the feedpoint of every other dipole (see p. 72 in ref. 137A).
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Fig. 119: In-phase voltage feeding of a uniform array of 1λ dipoles
The top and bottom antenna array are each connected to a separate transmitter in the large equipment cabin that rotates with the antenna system. The conduit was attached to the steel trusses of the antenna support structure. This feedline connection ("Energieleitung") consisted of a shielded two-wire transmission line (a.k.a. shielded two-wire TL, shielded balanced TL, shielded pair; D: "(ab)geschirmte symmetrische Bandleitung", "(ab)geschirmte Zweidrahtleitung", "(ab)geschirmte symmetrische Doppelleitung", "zweiadriges abgeschirmtes Kabel"), see p. 110 and 111 in ref. 181. This is basically two parallel wires in metal tubing or braiding, with a dielectric material between the wires and a round or oblong metal conduit. See Fig. 120. In modern times, we refer to this cable type as "twinax".
Fig. 120: "symmetrische Hochfrequenzleiting mit Abschirmung" - shielded balanced transmission line
(source: Fig. 28 in ref. 197)
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Fig. 121: Antenna system interconnections (source: derived from ref. 181)
The null-direction of the bottom array must calibrated ("Nullverstellung", see cable item 56 in ref. 189) so as to be exactly aligned with the centerline of the beam of the top array. This nulldirection is not only affected by the accuracy of the construction of the antenna system, but also by the phase and amplitude equality between left-hand and right-hand sub-arrays of the bottom antenna. As shown in Figure 121 above, there was a means to adjust the amplitude of the signal fed to the left-hand and to the right-hand sub-array. It was located at the transition from the shielded 2-wire cables from the transmitters, and the open-wire line from there to the dipoles. Note that there was no means to adjust the phase. Per ref. 181 (p. 111), the amplitude adjustment was mechanical, and the amplitude adjustment did not cause a phase shift. This suggests that the amplitude adjustment was implemented by selecting a tap of a transformer. The 1935 Telefunken/Runge/Krügel/Grammelsdorff patent nr. 737102 proposes using a fixedlocation remote receiver to check the direction of the beam-null, as measuring and balancing antenna feed-currents does not guarantee its correctness. Antenna feed-currents could then be adjusted with variable capacitors across the feed-lines or adjustment of the coupling at the transmitter or the feed-point.
The following photo shows two antenna cables emanating from the bottom of a hexagonal box that is mounted to the right of the top of the door, and a ventilation screen below it, at floor level. This suggests that the transmitters were located in the cabin section behind it. This is consistent with the layout drawing of Bernhard station Be-0 at Trebbin.
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Fig. 122: The entrance end of the cabin at Bredsted - antenna feedline and ventilation panel highlighted
(source: Australian War Memorial photo SUK14634; also part of photo on page 5 in ref. 5)
The following two photos show more details of the routing of the twin-lead cable and the associated open-wire feeedline of the lower dipole array (the barely visible feedline wires have been highlighted)
Fig. 123: Routing of the twin-lead cable (pink) and open-wire feedline (yellow) for the bottom dipoles at Bredsted
(unedited photo taken May 1945 by Flt. Lt. Herbert Bennet, RAF Mobile Signals Units of No. 72 Signals Wing; © David Bennet; used with permission)
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Fig. 124: Antenna feedline routing from transmitter at Bredsted
(unedited photo taken May 1945 by Flt. Lt. Herbert Bennet, RAF Mobile Signals Units of No. 72 Signals Wing; © David Bennet; used with permission)
It is unclear why there is a downward split in the open-wire feedline before it reaches the level of the dipole feed-points. At the center of the photo, the open-wire feedline splits into separate open-wire feedline for the left and right front dipole sub-arrays. Note that at that split point, the open-wire feedlines are also connected to two porcelain insulators that are located slighly above and behind the split point (but on the front side of the trusses between the front and rear dipoles). Additional insulator pairs are also visible (marked with blue circles), but no wiring is visible. Their purpose is unclear...
Available documentation does not mention the spacing between the dipole feedline wires and the diameter of those wires. It can also not be determined accurately enough from the photos. Hence, the characteristic impedance of the feedline cannot be calculated. However, it was probably at least several 100 Ω. As shown in Figure 121 above, the feedline from the transmitter was attached to the end of each 4-dipole sub-array. The 3-dipole top array was fed at the center dipole. This provided a sufficiently broadband feed system for the "Bernhard" operating frequency range of 31.5 MHz +/- 5%. See Fig. 2 in ref. 139K3. Note that open-wire feed-lines do have some disadvantages (ref. 139K4):
The wires always radiate to some extent, which may affect the radiation pattern of the antenna system Snow and rime accretions affects the impedance characteristics
Depending on their placement, capacitance between insulators (see Fig. 125 below) affects the impedance characteristics.
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Obviously, thick dipole radiators have a large cross-section. At the dipole feedpoint, these large cross-sections would be facing each other and form a significant capacitance. This is undesirable. Furthermore, the large cross-section of the radiators must be connected to the feedline wires. The feedline wires have a much smaller diameter, typically no more than several mm. A large step-transition in conductor diameter is also undesirable in antenna systems. A standard solution is to give the feedpoint-end of the dipole radiators a pointed shape, like the tip of a sharp pencil. See p. 70/71 in ref. 135. This is illustrated in Figure 125. As explained just above Fig. 119, the "Bernhard" antenna configuration required that adjacent dipoles be connected to opposite feedline wires. With flat or conical feedpoint tips, this requires crossing the two feedline wires between adjacent dipoles. This is impossible while at the same time maintaining a constant spacing between the two feedlines. Varying wire spacing causes undesirable disturbance of the feedline impedance, and may even cause arcing where the wires come close to each other. The pointed tip concept can also be adapted to implement the alternating connection to the feedline wires - without having to cross those wires between adjacent dipoles:
Fig. 125: Flat, conical, and off-center feedpoint-ends of dipole radiators
Fig. 126: Pointed radiator-feedpoint tips of several "Bernhard" installations and of a small "Knickebein" (far right)
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The feedpoint tips appear to have been made of metal strips, attached to the end of the dipole tube, and are pinched (or folded over) to form a point. The point is actually angled away from the tube, as the distance between the feedline wires is larger than the tube diameter.
Fig. 127: Close-up of pointed dipole feedpoint-tips of Be-9 at Bredsted/Germany. (unedited photo taken May 1945 by Flt. Lt. Herbert Bennet, RAF Mobile Signals Units of No. 72 Signals Wing; © David Bennet; used with permission)
Note that the above pointed tips are a direct copy of those used on the tubular dipoles of the ca. 1939 Small Knickebein beacon system! See here on the Knickebein page..
The dipole radiators were made of large diameter tubing (note: "tube" is specified by outside diameter, "pipe" by inside diameter). Ref. 13. A solid rod would have been much heavier, cost a lot more material, and not perform any better as an antenna: only the "skin" radiates. It is unknown what material they were made of: steel, copper, brass,... Copper would have had less losses, but steel pipe would have been much more easily available, stronger, and could easily be welded to an attachment arm.
The next Figure shows the standard textbook diagram of the sinusoidal distribution of current and voltage along the radiators of a full-wave dipole. The curves are only valid for "vanishingly thin" radiators!
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Fig. 128 current & voltage distribution of a very thin full-wave dipole in free-space
(note: only valid in free space (far enough away from ground and objects), and for very thin radiators)
When the radiators have a diameter that is not infinitesimally small, the current distribution is no longer purely sinusoidal - see Figure 129 below. The current at the feedpoint and tips becomes significant, due to charge concentration and displacement current. This explains the reduction in feedpoint resistance when the diameter of the radiators is increased (i.e., the λ/d ratio is decreased).
Fig. 129: Current distribution of a thick full-wave dipole in free-space (dashed line: Fig. 4.9 in ref. 135; solid line: my 4NEC2 model)
The feedpoint resistance and resonance length of a dipole depend on the ratio of the wavelength λ, and the diameter of the dipole radiators. The diagram below shows this dependence for a single dipole. For the same relative thickness, a full-wavelength dipole has bandwidth that is about 1.3x larger than that of a half-wave dipole (see Section 4.3 in ref. 135). Note that each "Bernhard" system operated at a fixed frequency in the range of 31.5 MHz ±5%. It was obviously highly desirable to be able to use the same dipoles at all "Bernhard" installations. This required a relatively broadband antenna system, which was facilitated by using full-wave dipoles.
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