Originally Posted by Metox receiver; Battle_of_the_Atlantic_(1939–1945) @ wikipedia

By August 1942, U-boats were being fitted with radar detectors to enable them to avoid the sudden ambushes which a radar-equipped aircraft or corvette might spring. The first such receiver, named Metox after its French manufacturer, was capable of picking up the metric radar bands used by the early radars. This not only enabled U-boats to avoid detection by Canadian and U.S. escorts, which were equipped with obsolete radar sets, but allowed them to track convoys where these sets were in use.

Originally Posted by Short_Sunderland @ wikipedia

While the bright Leigh searchlight was rarely fitted to Sunderlands, ASV Mark II radar enabled the flying boats to attack U-boats on the surface. In response, the German submarines began to carry a radar warning system known as "Metox", also known as the "Cross of Biscay" due to the appearance of its receiving antenna, that was tuned to the ASV frequency and gave the submarines early warning that an aircraft was in the area. Kills fell off drastically until ASV Mark III radar was introduced in early 1943, which operated in the centimetric band and used antennae mounted in blisters under the wings outboard of the floats, instead of the cluttered stickleback aerials. Sunderland Mark IIIs fitted with ASV Mark III were called Sunderland Mark IIIAs. Centimetric radar was invisible to Metox and baffled the Germans at first. Admiral Karl Dönitz, commander of the German U-boat force, suspected that the British were being informed of submarine movements by spies. In August 1943, a captured RAF airman misled the Germans by telling them that the aircraft were homing in on the signals radiated by the Metox,[9][10] and consequently U-boat commanders were instructed to turn them off.