AI Mk. IV Mk.VIF mounts the AI Mk. IV. The transmitter antenna is (just) visible on the nose, the left-side receiver just outboard of the landing lights. A crude system using a hand-held antenna working at 1.25 m (220 MHz) was ready by August 1937 and fitted to
Avro Anson K6260 at
RAF Martlesham Heath. This unit demonstrated the ability to detect aircraft at the range of about in the air-to-air mode, but also demonstrated the ability to detect ships on the ocean at ranges up to . This ability led to the split between AI and
air-to-surface-vessel (ASV) radar systems, both of which would be widely used during the war. Practical ASV radars were operational in 1940, but the AI developments proved much more difficult. Because ASV proved simpler, the team spent most of their time on that development through 1938. It was not until 1939, with the war obviously looming, that the team was once again moved back to AI development full-time. A lingering problem was that the minimum range remained around 1,000 feet, too long to allow easy interception. This was due to the transmitter signal not turning off sharply, leaking through to the receiver causing it to oscillate or ring for a period. While this powerful signal was dying down, reflections from nearby aircraft were lost in the noise. Numerous solutions had been attempted, but proved to be of limited use. Starting in late 1939 the development team was asked to fit the existing Mk. III's, a barely functional design, to aircraft. This ended further attempts to address the minimum range issue while they worked on installations. While their development effort ended, the headquarters staff at the
University of Dundee attempted to develop their own solutions to the problem. This led to considerable strife and in-fighting between the two groups. The AI group was eventually broken up at the end of March 1940, leaving Bowen out of the AI effort. A solution was eventually provided by
EMI who had developed a new type of transmitter that was not based on the common
self exciting principle. Instead, a separate
squegging oscillator was used to produce pulses of the carrier signal using a timer. This timer also muted down the receiver, making it less sensitive to the transmission signal and solving the ringing issue. Minimum range was reduced to about 400 feet. The resulting AI Mk. IV went into production in July 1940 and all units were sent to newly arriving
Bristol Beaufighters. The Beaufighter/AI Mk. IV achieved its first victory on the night of 15/16 November 1940, when an aircraft from No. 604 destroyed a
Junkers Ju 88A-5 near
Chichester. Several advanced versions of the Mk. IV were also produced, which offered direct readings for the pilot and options to allow use in single seat aircraft. However, these developments were overtaken by the rapid improvements in microwave systems, and both the Mark V and Mark VI saw only limited production and service.
Mk. VIII NF Mark XIII of No. 604 Squadron shows the distinctive upturned "Bull nose" containing the Mk. VIII radar In February 1940,
John Randall and
Harry Boot at
Birmingham University successfully ran the first
cavity magnetron, eventually generating 1 kW at 9.8 cm (3,060 MHz). Supported by GEC, the device quickly developed into a practical 10 kW system, and several test units were available by May 1940. Microwave wavelengths are so much shorter than the Mk. IV's 1.5 m, fifteen times, that the
dipole antennas required for reasonable gain were only a few inches long. This dramatically reduced the size of the system, allowing it to fit entirely in the nose of the aircraft. While a team under
Herbert Skinner developed the electronics,
Bernard Lovell was put in charge of examining the use of a parabolic dish to improve the directionality of the signal. The resulting beam was so sharply focussed, spanning about 10 degrees, that it easily avoided ground reflections at even low altitudes. The narrow beam also meant that the radar could only see targets directly in front of the antenna, unlike the Mk. IV which could see anything in the entire volume in front of the aircraft. To solve this problem, the dish was mounted on a bearing system from
Nash & Thompson that allowed it to be rotated in a spiral pattern. The cockpit display was modified to spin the timebase at the same speed as the antenna, 17 times a second. The display still produced blips similar to those on the Mk. IV, but as the timebase now spun, they drew short arcs on the display during the period the antenna was pointed in that direction. Like the Mk. IV, the distance from the center of the CRT indicated the range. As the target moved closer to the centreline of the aircraft, the beam spent more time painting the target, and the arc spread out, becoming a ring when dead ahead. First introduced in March 1941, it was found that the ground reflection created a sort of
artificial horizon on the bottom of the display, a surprising side-effect which proved very useful. However, the limited power of the magnetron, about 5 kW, provided range of about , not a great improvement over the Mk. IV. Performance of the system at low altitude was so improved over the Mk. IV that it was decided to make an initial run of 100 units out of what were essentially prototype systems as the Mk. VII, requiring very large amount of aircraft space for the install. Conversions on the Beaufighter began in December 1941. This run was followed by the production Mark VIII that included the new "strapped magnetron" of 25 kW, improving range to about . This version also had several major clean-ups in the electronics, support for
IFF Mark III which caused a
sunrise pattern to appear when aimed at friendly aircraft, and beacon tracking allowing it to home in on ground-based transmitters emplaced by friendly units. In September 1942 a Mosquito NF.II was upgraded to the Mk. VIII, serving as the pattern for the Mosquito NF.XII. Starting in December, Beaufighter units were upgraded to the similar Mk. VIIIA, an interim type awaiting production quantities of the VIII.
Mk. IX Although the precise origins of the concept are unknown, on 8 March 1941 Lovell mentions the concept of "lock-follow" for the first time in his notes. This was a modification to the spiral-scan system that allowed it to track targets automatically without further manual operation. This became known as AIF.
"Freddie" Williams joined the effort, and by the autumn of 1941 the system was basically functional and plans began to introduce it as the Mark IX. Several unrelated events conspired to greatly delay further progress. On 1 January 1942 Lovell was sent to work on the
H2S radar project and was replaced by Arthur Ernest Downing. This delayed the project just long enough that it got caught up in a great debate that broke out in the summer of 1942 about the use of
window, today known as
chaff. Window caused false returns on radar displays that made it difficult to tell where the bombers were amid a sea of blips.
Bomber Command had been pressing to use window over Germany to reduce their losses, which were beginning to mount as the German defensive network improved. Fighter Command was concerned that if Bomber Command used it over Germany, the Germans would return the favour and use it over the UK. A series of tests carried out in September 1942 by Wing Commander Derek Jackson suggested that some changes to the display systems might solve the problems with window on the Mk. VIII. At this point it was suggested that the Mk. IX might ignore the window completely, as the light metal strips rapidly dispersed from the target being tracked, faster than the radar could follow. Further testing by Jackson demonstrated the opposite was true, and that the Mk. IX almost always locked-on to the window instead. Arthur Downing quickly implemented several changes to fix this problem. He was personally operating the system when he was shot down in a
friendly fire incident, killing him and destroying the only prototype. This so greatly delayed the program that the Air Ministry asked Jackson to test the US
SCR-720 unit as a stop-gap measure. This proved to be able to pick the bomber from the window, and work on the Mk. IX was given low priority while the UK version of the SCR-720, known as the Mk. X, was purchased. With the night fighter force certain of its ability to continue operating successfully if needed, Bomber Command received clearance to begin using window on 16 July 1943. Work on the Mk. IX continued, but it never saw operational service. In testing in 1944 it was found to be marginally better than the US SCR-720, but with the SCR-720 expected to arrive at any moment, the demand for another radar was not pressing. Instead, the Mk. IX was given more time to mature. Further development led to more testing in 1948, but it was again passed up for production and cancelled the next year.
Mk. X NF.11 The Mark X was the UK version of the SCR-720. This was originally promised for delivery in the summer of 1942, but ran into delays and only started arriving in December 1943. These were fit to the Mosquito to produce the NF.XVII and later versions. Conversions at operational units began in January 1944, and the Mk. X remained in service through the rest of the war. Compared to the Mk. VIII, the SCR-720 used a helical scan instead of spiral. The radar antenna was spun around a vertical axis through an entire 360 degrees 10 times a second, with the transmitter switching off when the antenna was pointed back towards the aircraft. This provided a 150 degree scan in front of the aircraft. As it spun, the antenna slowly nodded up and down to provide altitude coverage between +50 and -20 degrees. The resulting scanning pattern naturally produced a
C-scope display on the CRT. In the post-war period the Mk. X became one of the UK's most widely used fighter radars, largely because a lack of foreign exchange to purchase newer designs, and the poor economy in general which required the RAF to have a "make do" attitude. The Mk. X would go on to equip the first jet-powered night fighters, including the
Vampire NF.10 and
Meteor NF.11. Small numbers remained in service as late as 1957.
Mk. XI, XII, XIII For the
Fleet Air Arm, the TRE developed a series of AI radars operating at the even shorter 3 cm wavelength, the
X band, which further reduced the size of the antennas. The original model was the Mark XI, followed by the improved Mark XII and lightened Mark XIII. It is not clear if any of these models saw service, and few references mention them even in passing.
Mk. XIV, XV These designations were given to the US
AN/APS-4 and AN/APS-6 radars, small under-wing X band radars used primarily by naval aircraft. The APS-4 was originally developed as the ASH, a forward-aimed surface-search system. It was packaged into an underwing pod so it could be used on single-engine aircraft like the
TBM Avenger. It proved to have a useful interception function, and was modified to be able to scan up and down as well as just side to side. The Fleet Air Arm mounted it on the
Fairey Firefly, which had the size to carry a radar operator and the performance to operate as a fighter. Some were also used on the Mosquito. Considerably later, a single Meteor,
EE348, was fit with an APS-4 in a nose mounting as a test vehicle. The APS-6 was a modification of the APS-4 specifically for the interception role. It replaced the side-to-side scan with a spiral-scan system largely identical to the one in the Mk. VIII. It also included a switch that reduced the scanning pattern to a 15 degree cone in front of the aircraft, producing a C-scope view used during the final approach. This was paired with a new and much smaller display, allowing it to be fit to smaller single-seat aircraft. It was widely used on the
F6F Hellcat and
F4U Corsair. ==Post-war systems==