German Naval Radar

Small Units

The German S-Boats which mainly operated at night and had to navigate the difficult waters of the Channel, also requested radar sets. As the Luftwaffe replaced their nightfighter FuG 202 Liechtenstein-B/C with more modern units, the Navy took over the old sets, as they worked at high frequency, required only small antennas. It was navalised, largely by discarding the high-bearing tupe, and redesigned FuMO 71 Liechtenstein-B/C. An experimental set was installed aboard an S-Boat and, as the fixed array necessitated training aboard an the boat in order to detect a target, a rotating antenna (1.6m x 1.3m) was developed under the designation FuMO 72. However, this increased the silhouette of this particular experimental boat in such a way that it was always sighted first, consequently receiving the nickname 'grenade collector'.

Later a Vorpostenboot, the Vp 1107, was fitted with an experimental FuMO 72. As with all Luftwaffe sets, but unlike most of the Kriegsmarine sets, the Liechtenstein had vertical polarsation. As far as known, no German S-Boat received an operational radar set.
 

Submarines

The increasing intensity of the 'microwave war' caused a steady re-equipment of all German submarines with detection sets which became more and more sophisticated. To detect the British ASV (air to surface vessel) radar all German submarines were equipped, from the summer of 1942, with 'Metox' detectors manufactured by the French firms Grandin and Sadir. The antenna was a simple wooden cross, nicknamed 'Biscay cross', which had to be turned by hand and withdrawn into the boat when diving. As the British switched to microwave radar in the spring of 1943, German submarines were attacked so often, usually without a chance to take countermeasures and sometimes on totally dark nights, that losses became catastrophic. German industry was, therefore, ordered to develop new radar emission detection receivers which resulted in the production of several sets (Naxos, Cypern I, Cypern II, etc.) which employed two interchangeable antenna systems serving two different purposes:

 
• The pressure-proof round dipole Bali for improved omni-directional reception
• The Palau antenna for direction finding, whose distinctive butterfly shaped antennas were situated at the rear side of the Hohentwiel frame 

Although Germany was leading in this field in 1939 certain factors meant that German submarines could not use radar extensively for observation and detection purposes.

Firstly, German scientiests believed strongly, that it was impossible to bring down radar wavelength to the centimetre (VHF) band so all German sets, in the first half of the war, suffered form the bearing inacuracies inherent with metric radar and served merely as navigation aids. Only after the Germans had examined a British H₂O set, containing the revolutionary cavity megatron, were they able, by copying it, to cross the VHF barrier. By then it was too late to regain all the lost technological and scientific development of the previous years.

Secondly, the German Navy having overestimated the value of passive versus active detection, created a philosophy of 'radar silence' to avoid passive detection in the same way as 'radio silence' was intended to prevent signal interception and direction finding. This might also explain why the Navy had radar with vertically polarized transmission throughout the war; an arrangement which resulted in water surface disturbance of the radar signal ('grass'). In contrast, even the first Luftwaffe ASV detection sets had horizontal polarization to avoid this problem. Hence, nearly all German wartime naval radars had aerials with vertical dipoles.

Lastly, the radar antennas were fitted in such a way that they could not used while snorkelling so, although the U-boats had radar sets, they were outdated, largely silenced and, for most of the time, useless.

The first radar employed on U-boats was a submarine version of the standard German Seetakt (82cm wavelength, 386 megacycles). The FuMO 29 set had a 2x6 fixed dipole array fitted on the forward face of the conning tower while FuMO 30 had a small turnable frame aerial with only 2x4 dipoles. Results with these sets were unsatisfactory but the Luftwaffe which held more advanced views on radar than the Navy and was now the leading service in this field, had a better type, the FuMG 200 Hohentwiel (56cm wavelength, 556 megacycles). This set became the FuMO 61 Hohentwiel-U which employed a rectangular frame aerial (1m x 1,4m) carrying 4x6 dipoles.
 
 

Also see:  "U-boat Radar Detectors" and "U-boat Radars" at the U-Boat Net.