AI Mk. IV radar
Group Captain "Paddy" Green achieved most of his 11 confirmed kills in this Mk. IV-equipped Beaufighter.
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| Country of origin | UK |
|---|---|
| Introduced | 1940 |
| Type | Airborne interception |
| Frequency | 193 MHz (VHF) |
| PRF | 750 pps |
| Beamwidth | ~175 degrees |
| Pulsewidth | 2.8 µs |
| Range | 400 to 18,000 ft (120–5,490 m) |
| Precision | 5 degrees |
| Power | 10 kW |
| Other Names | AIR 5003, SCR-540 |
Airborne Interception radar, Mark IV, or AI Mk. IV for short, was the world's first operational air-to-air radar system. Early Mk. III units appeared in July 1940 on converted Bristol Blenheim light bombers, while the definitive Mk. IV reached widespread availability on the Bristol Beaufighter heavy fighter by early 1941. On the Beaufighter, the Mk. IV arguably played a role in ending the Blitz, the Luftwaffe's night bombing campaign of late 1940 and early 1941.
Early development was prompted by a 1936 memo of Henry Tizard on the topic of night fighting. The memo was sent to Robert Watt, director of the radar research efforts, who agreed to allow physicist Edward George "Taffy" Bowen to form a team to study the problem of air interception. The team had a test bed system in flights later that year, but progress was delayed for four years by emergency relocations, three abandoned production designs, and Bowen's increasingly adversarial relationship with Watt's replacement, Albert Percival Rowe. Ultimately, Bowen was forced from the team just as the system was finally maturing.
The Mk. IV series operated at a frequency of about 193 megahertz (MHz) with a wavelength of 1.5 metres, and offered detection ranges against large aircraft up to 20,000 feet (6.1 km). It had numerous operational limitations, including a maximum range that increased with the aircraft's altitude and a minimum range that was barely close enough to allow the pilot to see the target. Considerable skill was required of the radar operator to interpret the displays of its two cathode ray tubes (CRTs) for the pilot. It was only with the increasing proficiency of the crews, along with the installation of new ground-based radar systems dedicated to the interception task, that interception rates began to increase. These roughly doubled every month through the spring of 1941, during the height of The Blitz.
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