The first German radars were developed and manufactured by the GEMA (Gesellschaft für Elektroakustische und Mechanische Apparate) company in Berlin. The history of radar onboard U-Boats began a little later than in the case of surface warships. At the beginning of 1937 experimental radar sets were installed on the torpedo boats G-7, G-10, G-11, and the beginning of 1938 decimeter FuMO 22 radar was installed on the heavy cruiser Admiral Graf Spee.

In the summer of 1939 there were plans to test radar equipment onboard U-Boats (most likely U-39 and U-41), but due to the lack of space for the equipment, they were never conducted. In 1940, BdU returned to the concept of radar on U-Boats - an order was placed with GEMA for radar equipment small enough to fit inside the crowded U-Boat pressure hull. Two FuMO 29 prototype radar sets were delivered in April of that year.

From mid-1941, for tactical reasons, the U-Boats were no longer fitted with active sonar equipment (S-Gerät). It was decided that the free space would be used for radar sets (it has to be remembered, that one of the main modifications which led to the type VIIC was lengthening the pressure hull by one frame - 60 cm - to make additional space for active sonar.) The sonar equipment was installed in the control room, on the port side, between the chart table and the forward bulkhead.

Installation of FuMO 29 radar began in 1942. Its characteristic feature was its antenna, which consisted of twelve dipoles located in two horizontal rows on the forward part of the conning tower casing. The upper row consisted of receiving dipoles, the lower - transmitting. The first U-Boats which were fitted with this radar were the type IXC boats: U-156, U-157 and U-158. FuMO 29 radar operated at a frequency of 368 MHz, had an output power of 10 kW and a detection range for surface vessels of 7500 meters, and for aircraft - 15000 meters. The main disadvantage of FuMO 29 radar was caused by its antenna. Its detection angle was limited to 25-30° which resulted in the boat having to make a complete turn to search the whole horizon.

Photo 1. U-195 (type IXD1) equipped with FuMO 29 during trials near Bornholm [1]

Photo 2. U-156 (type IXC) equipped with FuMO 29 [2]

Photo 3. U-231 (type VIIC) equipped with FuMO 29 [3]

Photo 4. U-230 (type VIIC) equipped with FuMO 29 [4]

Early in 1943 FuMO 30 radars entered service. This version differed from the FuMO 29 in that its antenna now consisted of eight vertical, center-fed, half-wave dipoles which were arranged in two horizontal rows. Wire-mesh 1,4 m wide and 1 m high that acted as a reflector was located behind the dipoles.

This antenna was mounted on the top of a rotatable shaft located on the port side of the bridge. When unused, it was lowered into a dedicated recess in the conning tower casing. The antenna was raised by means of a pneumatic piston (similar to the radio direction finder antenna). The first boat equipped with FuMO 30 was U-194 (type IXC/40).

Photo 5. Herbert Werner on the bridge of U-953 (type VIIC), standing in front of a FuMO 30 antenna [4]

Photo 6. U-441 showing her FuMO 30 antenna while operating as U-Flak 1 during the period 22 May - 13 July 1943

Photo 7. U-643 (type VIIC) showing her FuMO 30 antenna. On the port side of the conning tower you can see the
UAK symbol given to the boats built in the Blohm & Voss yard)

Photo 8. U-643 (type VIIC) showing her FuMO 30 antenna. On the port side of the conning tower you can see the
UAK symbol given to the boats built in the Blohm & Voss yard) [5]

Photo 9. FuMO 30 antenna on the bridge of U-995

Drawing 1. The antenna shaft and FuMO 30 antenna of a type VIIC U-Boat [3]

The electronic equipment of the FuMO 29 and FuMO 30 radars was the same. Due to its size it could not be placed in the radio room - so it was installed in the control room, on the port side, near the antenna shaft. The antenna shaft had a hand-wheel connected to it by which the radar operator manually rotated the antenna.

Photo 10. Type VIIC U-Boat control room - showing the FuMO 30 antenna shaft
together with the hand-wheel for rotating the antenna [3]

Photo 11. FuMO 30 antenna shaft in the control room of U-995

The electronic equipment of the FuMO 29 and FuMO 30 sets was modular and consisted of five modules: R, T, N, Z i X.

Drawing 2. FuMO 30 radar set [6]

Photo 12. FuMO 30 radar set in the control room of U-505 [7]

Photo 13. FuMO 30 radar set in the control room of U-505, near the left edge
showing the hand-wheel for rotating antenna [8]

The modules operated as follows:
Module R - power supply
Module T - transmitter with high frequency generator
Module N - receiver with intermediate frequency converter and imaging module
Module Z - timing generator
Module X - calibration module for adjusting receiver to transmitter frequency

There are parts of a FuMO 30 radar set - modules T, Z and X in the control room of U-995.
Externally there are some visible differences in its construction, which suggests that it is a late-war version. It is not known, what happened with other two modules.

Photo 14. FuMO 30 radar set in the control room of U-995

Drawing 3. Module N of FuMO 30 radar set - receiver with imaging module [6]

Drawing 4. Intermediate frequency converter of FuMO 30 [6]

Drawing 5. Module T of FuMO 30 radar set - high frequency generator [9]

FuMO 30 radar was installed on German U-Boats till the end of war.

There is little that can be said about the efficiency of FuMO 29 and FuMO 30 radars. In the KTB of U-172 (equipped with FuMO 30) describing her fifth patrol there is statement that radar was useless. But to evaluate the usefulness of these radars, you would need to review the KTBs of all the boats that had them.

A FuMB 5 Samoa antenna was often integrated with the FuMO 30 antenna. It had two flat, drop-shaped dipoles which were tilted at a 45° angle to allow receiving both horizontal and vertical polarized waves. The wire mesh of the FuMO 30 antenna also acted as a reflector for the FuMB 5 Samoa antenna. This antenna was used together with the FuMB 4 Samos radar detector.

The next radar commonly installed in U-Boats was FuMO 61 "Hohentwiel" U. It was a modification of the aircraft radar FuG 200 developed and manufactured by the Lorenz Company.

FuMO 61 operated at a frequency of 556 MHz, its output power was 30 kW, and the detection range for surface vessels was 7000 meters and 20000 meters for aircraft. From March to September 1944 sixty-four U-Boats were fitted with this radar set. The antenna of FuMO 61 consisted of 24 vertical, end-fed, half-wave dipoles arranged in four horizontal rows. Behind the dipoles was wire-mesh 1,4 m wide and 1 m high, which acted as a reflector. This antenna was mounted on the top of a rotatable shaft located on the port side of the bridge. When unused, it was lowered into a dedicated recess in the conning tower casing. The antenna was raised by means of a pneumatic piston (similar to the radio direction finder antenna and FuMO 30 antenna).

Photo 15. FuMO 61 antenna

Photo 16. FuMO 61 antennas on the U-Boats shored in Lisahally, at the end of June 1945. From right to left,
in the first row: U-1109, U-1058, U-278, U-901, behind U-293, U-826, U-1105, U-1022 and U-2326. [3]

Because the electronic equipment was quite small (compared to FuMO 30), it could be located in the radio room. To make it possible for the radar operator sitting in radio room to rotate the antenna, there was a flexible shaft with a hand-wheel which connected the radio room to the antenna shaft.

Drawing 6. Arrangement of FuMO 61 radar set in the radio room of a type IXC U-Boat [2]

Drawing 7. Arrangement of FuMO 61 radar set onboard U-515 – showing the driving shaft of the antenna [10]

Like FuMO 30, FuMO 61 was modular. It consisted of six modules:

• transmitter (Sendegerät)
• receiver (Empfangsgerät)
• imaging module (Sichtgerät)
• power supply (Netzgerät)
• control module (Feldregler)
• antenna multiplexer (Simultan)

Photo 17. Transmitter (at right, top) and receiver (below) of FuMO 61 radar set
in the radio room of U-889 [2]

Photo 18. Imaging module (at left, top) of FuMO 61 radar set
and hand wheel for rotating antenna (below) in the radio room of U-889 [2]

Photo 19. Receiver, imaging module and transmitter of FuMO 61 radar set

Photo 20. Imaging module of FuMO 61 radar set (most likely from U-234) [11]

Photo 21. Imaging module of FuMO 61 radar set [12]

The imaging module of FuMO 29, FuMO 30, and FuMO 61 radars was a type A display - the sweep was vertical - the distance to target could be read on the vertical axis. The bearing was determined by rotating the antenna to get maximum echo in the center of the screen (contrary to the airborne version FuG 200 which determined bearing by lobe-switching).

Drawing 8. Type A display of the FuMO 61 radar set

A version of the FuMO 61 radar, designated FuMO 65 "Hohentwiel-Drauf" was developed for the new type XXI U-Boats. The main difference was in the display type - this latter one had a type PPI display like modern radar today. There were also small differences in the construction of the antenna.

Photo 22. FuMO 65 antenna on the type XXI U-Boat [5]

Photo 23. FuMO 65 antenna on U-3008 (type XXI) [13]

Drawing 9. FuMO 61 and FuMO 65 antennas [14]

The efficiency of FuMO 61 radar also cannot be evaluated without a thorough query of the KTBs. There is one report that was sent on 17 September 1944 by U-862 after a 99 day cruise from Narvik to Penang, which said:
"FuMO Hohentwiel has worked well throughout the voyage. The range against loaded freighters in the tropics amounts to 7,2 km. No impairment of performance has been occasioned by the tropical climate" [5].

FuMO 84 "Berlin II" U1 (by Telefunken) and FuMO 391 "Lessing" (manufactured by GEMA) radars were developed near the end of the war. They were destined for the type XXI U-Boats and their advantage was the ability to operate when submerged at periscope depth.

FuMO 84 was a naval version of the air force's microwave FuG 224 Berlin A radar. Its construction was based on the Cavity Magnetron - in previous models of German radars vacuum tubes were used to generate high frequency waves. Due to high parasitic capacitance the upper frequency was limited. The Germans tried to use a very early version of the magnetron (split-anode type) before World War Two, but could not generate stable frequencies. Based on these experiments they came to the conclusion that it was impossible to build microwave radar. After investigating the remains of the British microwave H2S radar extracted from wreckage of the English Short Stirling bomber which crashed near Rotterdam (on 2/3 February 1943), the Telefunken Company was able to build a stable high frequency generator based on the Cavity Magnetron invented by the British engineers Harry Boot and John Randall in 1940. This generator was then used in the construction of the microwave radar FuG 224. FuG 224 Berlin A radar was installed as a navigational aid in German aircraft from mid-1944, but soon it was adapted for use onboard warships (e.g. under the designation FuMO 81 it was installed on the battleship Tirpitz and the heavy cruiser Prinz Eugen). A FuMO 84 version was developed for type XXI U-Boats. It had a type PPI display and a dielectric type antenna which consisted of four polystyrene rods of about 40 cm in length, enclosed in a spherical dome made of Opanol (polyisobutylene). In the first version of this device (FuMO 83 Berlin-U1), the whole dome was installed on a retractable, rotating shaft. In the FuMO 84 Berlin-U2 version, an antenna which rotated (at a speed of several hundred RPMs) was installed inside the dome, which in turn was installed on a stationary shaft.

Drawing 10. Imaging module of the FuG 224 radar set [15]

Drawing 11. Type PPI display of the FuG 224 radar set [15]

Drawing 12. Dielectric antenna of the FuG 224 radar set - showing four
polystyrene rods placed on the rotating dish [15]

FuMO 391 was a special kind of radar - the radio waves were emitted by an omnidirectional antenna located on the Schnorchel head. This radar detected aircraft at distances up to 30 km, but did not allow for determining range or bearing. It was only a warning device - early detection of the aircraft allowed the U-Boat to dive below periscope depth safely.

The war ended before FuMO 84 and FuMO 391 radars could be used in combat.

Comparison of technical data of U-boat radars

The attitude of the U-Boat commanders toward the operational use of radar affected the frequency of its use. From the time radar was first available aboard U-boats (end of 1941) to mid-1943 they were not used in any significant way. This was caused mainly by concern that allied aircraft and surface warships were equipped with radar detectors which could easily locate U-Boats. Moreover, the aircraft with its radar turned on could be detected from a greater distance by means of their own radar detector (FuMB) than by means of their own radar. Additionally, a high failure rate and imperfections of the first radars (FuMO 29 and FuMO 30) did not tend to encourage its usage. Despite all of this, from mid-1943 BdU started to encourage the operational use of radar:
"Our own radar sets are well suited to establish the presence of airplanes independently of their use of radar, for example during surface cruise altitudes of 500 meters and more at a distance of 10 kilometers at the least." [16].

In March 1944 the reluctance of the commanders was suppressed to some degree. There were more and more reports of successful usage of radar. By means of radar the aircraft were detected so early that they could be evaded before they could attack. The radar was used to shadow vessels, to control anti-aircraft fire, and to conduct torpedo attacks (accurate distance measurement). Hohentwiel radar was also used as a navigation aid during bad weather.

It seems, that when U-Boats were forced to stay permanently submerged, radar was on the decline - there were less and less occasions for successful usage. Also the height of the antenna had a significant impact on the radars range, just like with visual observation.

Developed especially for type XXI U-Boats Berlin and Lessing radars were allocated to detecting aircraft to prevent sudden attack on a snorkeling U-Boat. Probably - if not for the end of the war - the further development of electronic sensors would have been in the direction of the acoustic devices - passive and active (as it happened after the war).

Sources:
Courtesy of Nicolas Bracco [1]
Vom Original zum Modell, Uboottyp IX C, Fritz Köhl, Axel Niestle [2]
Vom Original zum Modell, Uboottyp VIIC, Köhl, Fritz, Axel Niestle [3]
Iron Coffins, Herbert A. Werner [4]
The U-Boat: The Evolution and Technical History of German Submarines, Eberhard Rössler [5]
Courtesy of Günter Hütter (www.cdvandt.org/gema-x-geraet.htm) [6]
www.uboatarchive.net/U-505Photographs.htm [7]
Courtesy of Ken Dunn [8]
Courtesy of Arthur O. Bauer (www.cdvandt.org/seetakt.htm) [9]
Report on the interrogation of survivors from U-515 (uboatarchive.net/U-515INT.htm) [10]
Courtesy of Geoff Fors (www.wb6nvh.com/SG200.htm) [11]
airandspace.si.edu/collections/artifact.cfm?id=A19602645000 [12]
www.uboatarchive.net/DesignStudies-U-3008.htm [13]
Drawings by Simon Morris [14]
Bordfunkgerät Fu G 224, Geräte-Handbuch [15]
Battle of the Atlantic, Volume IV Technical Intelligence From Allied Communications Intelligence (www.history.navy.mil/library/online/techintellallcomm_battatl.htm) [16]