output it could block all VHF transmissions over 32-48 MHz. However, enclosed in its own pressurised container, (to prevent arcing of the high voltages inside), it was large and at 600 lb took up the entirety of the bomb bay of the Boeing Fortresses used by No. 100 Group RAF. Due to the high transmitter power, test flights had to be carried out in the vicinity of Iceland, otherwise the jamming would have blanked out all frequencies in the specified range, over a large area, as well as giving the Germans warning of the impending arrival of a jamming system. Radar The development of radar for defensive and offensive operations was of paramount concern during the war. Early work was on Airborne Interception radar (AI) able to be carried in night fighters and used for locating enemy aircraft in the dark, as Britain was soon facing The Blitz. The first tests had been carried out as early as 1936-7 using a Handley Page Heyford and later an Avro Anson at the initial suggestion of Henry Tizard then Chairman of the Aeronautical Research Committee. Initial aircraft used operationally were Bristol Blenheims converted to fighters with belly gun packs, followed by a brief usage of the AI-equipped Turbinlite Douglas Havoc paired with Hawker Hurricanes, but later the Bristol Beaufighter was chosen, followed by the de Havilland Mosquito which later became the standard RAF night fighter for the remainder of the war. Initial versions of AI were metric-wavelength, the antennas being arrow-shaped or dipoles, later centimetric versions used a rotating paraboloid aerial carried under a streamlined nose radome. Airborne Interception radar progressed from the initial AI Mk I version to the AI Mk 24 Foxhunter used in the Panavia Tornado. Parallel work was carried out on Air-to-Surface-Vessel radar (ASV) for use by Coastal Command aircraft for hunting U-boats at sea, initially using the Lockheed Hudson equipped with an early version of ASV. Success with the new equipment led to mounting the equipment onto Vickers Wellingtons and Sunderland flying boats, the early metric-wavelength ASV-equipped types carrying an array of transmitting and receiving "Stickleback" aerials on the rear fuselage top and sides and under the wings. Later a version of the centimetric-wavelength H2S was used. ASV-equipped aircraft such as the Wellington, Sunderland, Catalina and Liberator, made a substantial contribution to winning the Battle of the Atlantic for the Allies. ASV-equipped Fairey Swordfish and Fairey Barracudas