Metox

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Metox was a radio receiver installed on German U-boats at the later part of World War II (1943) that could detect ASV (Air to Surface Vessel radar) transmissions from patrolling Allied aircraft. ^[1]

From July 1940 onwards, the British fitted the RAF Mk II AI (Airborne Interception) radar into Coastal Command aircraft for use as the MkII "1½-metre ASV".^[2] The radar's known AI weaknesses -problems due to land clutter and inability to determine height effectively which caused its failure in night fighters were no handicap in this new role. With 2 range scales, 0-9 and 0-36 miles, it could detect surfaced U-boats at up to 12 miles and land at up to 70 miles, though a typical U-boat detection range was 5 miles. The radar had a fairly crude display by today's standards, but was able to give the range and an approximate direction within an arc either side of the aircraft heading. Returns were lost in sea clutter once the aircraft was within about 1 mile of the U-boat, but usually by then, the aircraft was within visual range - and the U-boat was well into a crash dive!

To counter this, a Wing Commander Humphry de Verde Leigh (later OBE DFC AFC) developed the Leigh light, effectively a powerful flood light steered by the ASV radar. This allowed ASV radar equipped aircraft to search for U-boats at night. The U-boat was initially tracked by the radar with the light following the radar track but switched off. Once the returns were lost, the light would be switched on and the U-boat would be bathed in light and very vulnerable, the first successful attack was on U502 on 5th July 1942. The sudden light was often the first indication that the U-boat had been found and the Leigh light was initially very successful, particularly in the Bay of Biscay.

Metox was the German answer to the Leigh light rendering it completely ineffective. The Metox sets received the transmitted pulses from the ASV and rendered them as audible beeps. It enjoyed the usual advantage of radar detectors over radar in that the signal is direct and only had to travel one way whereas the radar has to detect the very weak reflection from the submarine.^[3]. Most radars increase the number of pulses and decrease the width of the pulses when switched to a shorter range, the shorter pulse widths allow the radar to look at closer objects. The Metox exploited the fact that once the radar operator changed the range indication from 36 miles to 9 miles (15 km), the pulse repetition frequency of the radar's transmitter doubled. Radar cannot detect any reflections returned earlier than half a pulse width so when the U-boat was closer than 9 miles the operator would change to the shorter scale. If the Metox set started beeping at twice the rate, the U-boat knew that they had been detected. By the time the aircraft approached the U-boat's position enough to energise the Leigh light, the U-boat was well under the water. As a bonus, the Metox set would also provide warning in excess of visual range in daylight.

Metox was eventually countered by a version of the 10 centimetre H2S which Metox couldn't detect and once again the Leigh light forced U-boat crews to refuse to run surfaced at night. Even during the day, the U-boats were easy prey as the new radar was easily able detect the U-boat's periscope.

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