Radar, Airborne Interception, Mark VIII, or AI Mk. VIII for short, was the first operational microwave-frequency air-to-air radar. It was used by Royal Air Force night fighters from late 1941 until the end of World War II. The basic concept, using a moving parabolic antenna to search for targets and track them accurately, remained in use by most airborne radars well into the 1980s.
Low-level development began in 1939 but was greatly sped after the introduction of the cavity magnetron in early 1940. This operated at 9.1 cm wavelength (3 GHz), much shorter than the 1.5 m wavelength of the earlier AI Mk. IV. Shorter wavelengths allowed it to use smaller and much more directional antennas. Mk. IV was blinded by the reflections off the ground from its wide broadcast pattern, which made it impossible to see targets flying at low altitudes. Mk. VIII could avoid this by keeping the antenna pointed upward, allowing it to see any aircraft at or above the horizon.
The design was just beginning to mature in late 1941 when the Luftwaffe began low-level attacks. A prototype version, the Mk. VII, entered service on the Bristol Beaufighter in November 1941. A small number of these were sent to units across the UK to provide coverage at low altitudes while Mk. IV equipped aircraft operated at higher altitudes. After a small run of the improved Mk. VIIIA, the definitive Mk. VIII arrived in early 1942, offering higher power as well as a host of electronic and packaging upgrades. It arrived just as production rates of the De Havilland Mosquito began to improve, quickly displacing the Beaufighter units in RAF squadrons. Mk. VIII equipped Mosquitoes would be the premier night fighter from 1943 through the rest of the war.
The Mk. VIII spawned a number of variants, notably the AI Mk. IX which included a lock-on feature to ease interceptions. A series of events, including a deadly friendly fire incident, so greatly delayed the Mk. IX that it never entered service. During the late-war period, many UK aircraft adopted the US SCR-720 under the name AI Mk. X. This worked on the same general principles as the Mk. VIII, but used a different display system that offered several advantages. Development of the basic system continued, and the Mk. IX would eventually briefly re-appear in greatly advanced form as the AI.17 during the 1950s.
Development
Prior work
The seminal Daventry Experiment of 1935 proved the basic concept of radar was feasible and led to the rapid formation of the Air Ministry Experimental Station (AMES) at Bawdsey Manor to develop them. The AMES team's primary concern was the development and deployment of the Chain Home (CH) system, providing early warning for raids approaching the UK. As the team grew, the work diversified, and by 1938 there were a number of teams working on other projects as well.
One of the first of these side efforts came about due to Henry Tizard's concerns about the potential effectiveness of Chain Home. He believed that the Luftwaffe would suffer so stiffly at the hands of the RAF's ground-controlled intercept system that they would switch to the night bombing role. At night, a fighter pilot could see a target at perhaps , an accuracy Chain Home and its associated Dowding system could not provide. Tizard's concerns were later brought up by Robert Watson-Watt at a round-table meeting at the Crown and Castle pub. "Taffy" Bowen offered to take up development of a new system able to be installed in aircraft to close the distance between CH direction and visual range at night.
Due to the physics of radio transmission, antennas have to be about as long as the wavelength of the radio signal to achieve reasonable gain. The half-wave dipole, with two poles each about one-quarter the length of the signal, is a particularly common solution. CH operated at anywhere from 10 m to 50 m depending on the version, meaning antennas would have to be at least long, which made it totally impractical for use on an aircraft. Bowen started development of a new system operating at shorter wavelengths, first at 6.7 m following work by the British Army, and then finally settling on 1.5 m, the practical limit of available technology. This became known as Airborne Interception radar (AI), and was the primary focus of Bowen's work from 1936 to 1940.