page_title: AI Mk. VIII radar
section_title: Window
hierarchical_section_title: AI Mk. VIII radar / Development / Window
caption_attribution_description: English: Avro Lancaster SR-B drops bundles of "window" during a raid on Duisburg on 15 October 1944. Still from film shot by the RAF Film Production Unit during Operation HURRICANE. Avro Lancaster B Mark I, NG128 'SR-B', of No. 101 Squadron RAF, piloted by Warrant Officer R B Tibbs, releases bundles of 'Window' over the target during a special daylight raid on Duisburg. Over 2,000 sorties were dispatched to the city during 14-15 October 1944, in order to to demonstrate Bomber Command's overwhelming superiority in German skies. Note the large aerials on top of the Lancaster's fuselage, indicating that the aircraft is carrying 'Airborne Cigar' (ABC), a jamming device which disrupted enemy radio telephone channels
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: At the same time, a fight between Fighter Command and Bomber Command was brewing. Bomber Command was ramping up its campaign, but was suffering mounting losses at the hands of Josef Kammhuber's increasingly effective defences. They began pressing for permission to use chaff, known in the UK under the code-name window, which in testing had demonstrated its ability to blind radar systems. Air Chief Charles Frederick Algenon Porter ordered Bomber Command to begin using window on 4 April 1942, but he rescinded that command on 5 May under pressure from Sholto Douglas. Douglas pointed out that the Germans would be able to copy window the first time they saw it, and it was unwise to use it until its effect on the UK's own radars was better understood.
  Under the direction of Frederick Lindemann, an extensive series of studies were carried out by Derek Jackson at RAF Coltishall. Starting in September, aircraft with Mk. IV and Mk. VII were tested against window in a series of 30 flights. Much to everyone's consternation, Jackson concluded that the Mk. VII's spiral-scan display proved to be affected by window more than the simpler display of the Mk. IV. When he learned the results, Douglas wrote a memo to the Air Ministry asking that window be held back until new radars could be developed that were not as susceptible to its effects.
  One of the interesting coincidences of the war was that the Germans had already developed their own version of chaff under the code-name Düppel, and had tested it near Berlin and over the Baltic. However, Hermann Göring was worried that if they used Düppel over the UK, the RAF would quickly copy the concept and use it against them. As Bomber Command's fleet was rapidly growing, the results would likely be greatly in the RAF's favour. Learning from past mistakes when older material had leaked, Göring had most of the paperwork on Düppel destroyed.
caption: Bundles of window are dropped from an Avro Lancaster during a raid on Duisburg.
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