AI Mk. IX radar

AI Mk. VIII

AI Mk. VIIIA in the nose of a Bristol Beaufighter
Country of origin	UK
Introduced	1941 (1941)
Type	Airborne interception
Frequency	3.3 GHz (S band)
PRF	2500 pps (930 for beacons)
Beamwidth	~12°
Pulsewidth	1 µs (3 µs for beacons)
RPM	1020
Range	400 to 30,000 ft (120–9,140 m)
Altitude	500 ft (150 m) and up
Diameter	28 in (71 cm)
Azimuth	45° to either side
Elevation	45° up and down
Precision	1 to 3° ahead, less to the sides
Power	25 kW
Other Names	ARI 5093, ARI 5049 (Mk. VII)

Airborne Interception radar, 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. This provided the important advantage of allowing the radar to avoid ground reflections simply by pointing the antenna upward. The Mk. IV's semi-directional antennas were blinded by noise from the signal reflecting off the ground while flying at low altitude, while the Mk. VIII could see anything above the fighter's altitude.

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 supplies of the De Havilland Mosquito began to improve, quickly displacing the older 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 along 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.