Here are a few words about the history and development of this radar system.
AI (Airbourne Interception) and ASV radar sets were developed on parallel paths, with advances on one side often being used to advantage on the other. Unfortunately the ASV Mk I which was installed on both Hudsons and Sunderlands and was not very reliable. Early experiments with different aerials improved things no end until ASV Mk II was introduced in 1940.
The first ASV success was recorded on 30th November 1940, when a Whitley Mk.VI equipped with ASV Mk II damaged U-71 in the Bay of Biscay. By mid-1941 the ASV radar had increased daytime attacks on U-boats by 20%, and made nightly attacks possible.
Late 1942 saw ASV Mk III which gave the operator a new PPI (plan position indicator) radar tube. By May 1943 Bay of Biscay U-boat sightings improved dramatically and shipping losses decreased accordingly.
When ASV Mk.VI was introduced, it was much more powerful than the older Mk.III. It also had an attenuator fitted to reduce the radar power level (which the enemy could detect) thus misleading them into thinking that the aircraft was flying away from them as it approached.
ASV Mk.VII system was a development of the H2S radar system to try and detect a new type of U boat that was equipped with a "Schnorkel". A device that made it unnecessary for the submarine to surface to charge it’s batteries. This was an issue that was never really solved even with the later ASV Mk.XI