page_title: AI Mk. VIII radar
section_title: Continued development
hierarchical_section_title: AI Mk. VIII radar / Development / Continued development
caption_attribution_description: English: 5836 Reflex klystron made by Raytheon Português: Clistrão reflexo 5836 feito por Raytheon
context_page_description: 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, 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 its own 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.
context_section_description: Awaiting the arrival of a scanner, in the autumn of 1940 the AMRE had ordered the delivery of an aircraft with some sort of radio-transparent nose. The Indestructo Glass company proposed using 8 millimetres (0.31 in) thick Perspex, while the AMRE team preferred a composite material of polystyrene fabric and Egyptian cotton bound with phenol formaldehyde resin (the glue used in Bakelite), or a similar paper-based resin composite. The Perspex solution was chosen, and in December 1940 Bristol Blenheim N3522, a night fighter adaptation of the Blenheim V, arrived at RAF Christchurch, the nearest suitable airfield. A number of attempts had to be made to successfully mount the nose to their test aircraft. It was not until the spring of 1941 that Indestructo delivered suitable radomes and the mounting issues were wholly solved.
  While this work progressed, the teams continued development of the basic system. Burcham and Atkinson continued their development of the transmitter section, attempting to generate very short pulses of power to feed the magnetron. They finally settled on a solution using two tubes, a thyratron and a pentode, which produced 1 µs pulses at 15 kW. GEC preferred a design using a single thyratron, but this was eventually abandoned in favour of the AMRE design. Further work pushed this system to 50 kW, producing 10 kW of microwaves at a pulse repetition frequency of 2500 cycles per second.
  Skinner took up the task of developing a suitable crystal detector, which essentially consisted of endless trials of different crystals; Lovell noted that "an abiding memory of the days at Worth and Leeson is of Skinner, cigarette drooping from his mouth, totally absorbed in the endless tapping of a crystal with his finger until the whisker found the sensitive spot giving the best characteristics." This led to the use of a tungsten whisker on silicon glass, sealed into a wax-filled glass tube. Oliphant's team in Birmingham continued these experiments and developed a capsule-sealed version.
  The radio receiver turned out to be a more difficult problem. Early on they decided to use the same basic receiver system as the earlier Mk. IV radar. This had originally been a television receiver designed by Pye Ltd. to pick up BBC transmissions on 45 MHz. It was adapted to the MK. IV's ~200 MHz by using it as the intermediate frequency stage of a superheterodyne system. To do this, they had added another tube that stepped down the frequency from the radar's 193 MHz to 45 MHz. In theory this should be just as easily adapted to the AIS's 3 GHz, using a similar solution. The problem was that the magnetron's frequency tended to drift, in small amounts pulse-to-pulse, and much greater amounts as it heated and cooled. Any sort of fixed-frequency step-down like the one used in the Mk. IV wouldn't work. After trying a variety of designs based on klystrons and older-style magnetrons, they eventually gave up.
  The solution was provided by well-known tube expert Robert W. Sutton at the Admiralty Signals Establishment. He designed a new tube for this purpose, today known as the Sutton tube but at that time more widely known as a reflex klystron. This was essentially a conventional two-cavity klystron with one cavity removed. The remaining cavity was fed a tiny amount of the output from the magnetron, causing the electrons passing by it to take up the pattern of the radio signal (this is the basis of all klystrons). Normally this would then pass the second resonator where the output would be tapped, but in the Sutton tube, the electrons instead approached a high-voltage plate that reflected them back towards their source. By carefully controlling the voltage of the reflector, the electrons would arrive having gained or lost a controlled amount of velocity, thus inducing a different frequency signal in the cavity as they passed it the second time. The combination of the original and new frequency produced a new signal that was sent to the conventional receiver. Sutton delivered an example producing 300 mW in October 1940.
  One problem now remained, the 
caption: The Sutton tube ultimately solved two thorny problems for the AIS team, acting as both a local oscillator as well as a high-speed switch.
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