Radar, Aircraft 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 just above the horizon, 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 aircraft interception (AI) radar, and was the primary focus of Bowen's work from 1936 to 1940.
While testing an early 1.5 m set the team failed to detect any aircraft, but easily picked out large objects like cranes and ships at nearby wharves. Further experiments demonstrated the ability to pick up ships at sea, leading to a live demonstration where the team was able to track down Royal Navy capital ships in terrible weather. This led to immediate interest on behalf of RAF Coastal Command which saw this as a way to find enemy ships and U-boats, and by the British Army, which was interested in using the radars to direct fire against shipping in the English Channel. Work on the system for AI use largely ended.
AI development
It was not until 1939, with war clearly looming, that the team once again returned to AI work. Compared to the successful and rapid development of the anti-shipping radars, the team found themselves facing a continual stream of problems in air-to-air settings. There were two primary problems, a lack of maximum range that made finding the targets difficult, and a lack of minimum range that made it difficult for the pilot to see the target before it became invisible to the radar.
Like Chain Home, the AI radar sent out a powerful pulse semi-directionally, lighting up the entire sky in front of it. Echoes from aircraft would be received on multiple directional antennas, and by comparing the signal strength from each one, the direction of the target could be determined. However, this also meant that the signal reached the ground and reflected off it, producing a return so powerful that it overwhelmed the receiver no matter where the antenna was positioned. Since this signal had to travel to the ground and back, it produced a line on the display at an indicated range equal to the aircraft's altitude. Flying at , a typical altitude for German bombers, meant anything beyond about was invisible in the noise. This left little range to detect the target.
A more difficult problem was the inability to detect targets at short range. The transmitter signal was difficult to cut off sharply and was still broadcasting a small signal when the returns from nearby targets started to be received. Moreover, the powerful signal tended to bleed through to the receiver, causing it to oscillate for a time, blanking out nearby targets. These effects limited the minimum range to in the best case, just at the very limit of the pilot's eyesight at night. Attempts had been made to address this problem, and Bowen and Hanbury Brown were convinced they had a workable solution.
However, the Air Ministry was so desperate to get AI into service that they had used the team as a production facility, having them hand-fit aircraft with the prototype Mk. III units that were nowhere ready for operational use. While these sets were rushed to the squadrons, further work on developing solutions to the "great minimum range controversy" ended. Arthur Tedder would later admit this was a "fatal mistake".
Early microwave work The Airborne Group had been experimenting with microwave systems as early as 1938 after discovering that a suitable arrangement of the RCA acorn tubes could be operated at wavelengths as low as 30 cm. However, these had very low output, and on top of this, the receiver's electronics were not very sensitive at these frequencies. This resulted in very short detection ranges, essentially useless. The group gave up on further development for the time being, and Bowen described the topic being pooh-poohed by the engineers for some time.
Nevertheless, pressure from the Admiralty kept microwaves in everyone's mind. While the 1.5 m sets were fine for detecting larger ships, they could not effectively see smaller objects, like U-boat conning towers. This was for the same reason that antennas need to be roughly the size of the wavelength; to provide a reasonable reflection, the objects need to be at several times larger than the wavelength. The Admiralty had the advantage of administering the UK's vacuum tube development efforts, under the Communication Valve Development Committee (CVD), and were able to continue development of suitable tubes.
Bowen and his counterpart at the Admiralty Signals Establishment (ASE), Canadian polymath Charles Wright, met at Bawdsey in the spring or summer of 1939 and considered the issue of a microwave aircraft interception radar. Bowen agreed that the main problem with the range limits of the AI sets was the floodlight-like transmissions and that the easy way to fix this would be to narrow the beam, focussing the power into a smaller area. He concluded that a 10-degree beamwidth would do the trick. Considering that the nose of an aircraft could hold a radar antenna about across, an antenna with poles shorter than 15 cm was desirable, and if that antenna had to move within the nose for tracking, 10 cm (~3 GHz) would be ideal. This strongly agreed with Wright's requirements for a shipboard system able to detect U-boats while having an antenna small enough to be mounted on small escort vessels.
With both forces desiring a 10 cm system, Tizard visited the General Electric Company's (GEC) Hirst Research Centre in Wembley in November 1939 to discuss the issue. Watt followed up with a personal visit sometime later, leading to a 29 December 1939 contract for a microwave AI radar set. This was followed by the CVD placing a contract for suitable valves with the Birmingham University. Bowen arranged a January meeting between GEC and EMI to coordinate AI work, which led to further collaboration.
The Birmingham group was led by Mark Oliphant, formerly of the Cavendish Laboratory at Cambridge University but recently moved to Birmingham to set up the Nuffield Laboratory. The team decided to base their development efforts on the klystron concept. The klystron had been introduced by the Varian brothers at Stanford University in 1936, but produced a relatively low power output. Oliphant's team began applying new tube making techniques and by the end of 1939 they had a tube capable of delivering 400 watts.
AIS begins Watt moved to Air Ministry headquarters in London and Albert Percival Rowe took over management of the radar teams at Bawdsey. He had a troubled relationship with Bowen and many others at AMES. At the opening of the war, the entire AMES establishment was moved from Bawdsey to a pre-arranged location at Dundee. The choice of Dundee was largely due to the university being Watt's alma mater. He had made little effort to prepare the university for use by AMES and the rector was surprised when they arrived one day out of the blue. Almost no room was available as the students and professors had returned from summer holidays. The AI team was sent to a small airfield at Perth that was miles away and quite small. Both locations were totally unsuitable for the work and the teams constantly complained.
In February 1940 Rowe began to organise a new AI team led by Herbert Skinner. Skinner had Bernard Lovell and Alan Lloyd Hodgkin begin considering the issue of antenna designs for microwave radars. On 5 March they were invited to the GEC labs to view their progress on a radar based on VT90 tubes, which had by this time been pushed to useful power levels at 50 cm wavelengths.