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High-frequency direction-finding

One of the more important developments was ship-borne direction-finding radio equipment, known as HF/DF (high-frequency direction-finding, or Huff-Duff), which started to be fitted to escorts from February 1942. These sets were common items of equipment by the spring of 1943. HF/DF let an operator determine the direction of a radio signal, regardless of whether the content could be read. Since the wolf pack relied on U-boats reporting convoy positions by radio, there was a steady stream of messages to intercept. An escort could then run in the direction of the signal and attack the U-boat, or at least force it to submerge (causing it to lose contact), which might prevent an attack on the convoy. When two ships fitted with HF/DF accompanied a convoy, a fix on the transmitter's position, not just direction, could be determined. However, the standard approach of anti-submarine warships was immediately to "run-down" the bearing of a detected signal, hoping to spot the U-boat on the surface and make an immediate attack. Range could be estimated by an experienced operator from the signal strength. Usually the target was found visually. If the submarine was slow to dive, the guns were used; otherwise an ASDIC (Sonar) search was started where the swirl of water of a crash-diving submarine was observed. In good visibility a U-boat might try and outrun an escort on the surface whilst out of gun range.

The British also made extensive use of shore HF/DF stations, to keep convoys updated with positions of U-boats. HF/DF was also installed on American ships.

The radio technology behind direction finding was simple and well understood by both sides, but the technology commonly used before the war used a manually-rotated aerial to fix the direction of the transmitter. This was delicate work, took quite a time to accomplish to any degree of accuracy, and since it only revealed the line along which the transmission originated a single set could not determine if the transmission was from the true direction or its reciprocal 180 degrees in the opposite direction. Two sets were required to fix the position. Believing this to still be the case, German U-boat radio operators considered themselves fairly safe if they kept messages short. The British, however, developed an oscilloscope-based indicator which instantly fixed the direction and its reciprocal the moment a radio operator touched his Morse key. It worked simply with a crossed pair of conventional and fixed directional aerials, the oscilloscope display showing the relative received strength from each aerial as an elongated ellipse showing the line relative to the ship. The innovation was a 'sense' aerial, which, when switched in, suppressed the ellipse in the 'wrong' direction leaving only the correct bearing. With this there was hardly any need to triangulate—the escort could just run down the precise bearing provided, estimating range from the signal strength, and use either efficient look-outs or radar for final positioning. Many U-boat attacks were suppressed and submarines sunk in this way—a good example of the great difference apparently minor aspects of technology could make to the battle.