The SCR-720 was a World War II aircraft interception radar designed by the Radiation Laboratory (RadLab) at MIT in the United States. It was used by US Army Air Force night fighters as well as the Royal Air Force (RAF) in a slightly modified version known as Radar, Aircraft Interception, Mark X, or AI Mk. X for short. SCR-720 was the first radar to successfully use the "helical-scan" technique, which became common in night fighter radars. The concept was first raised in early 1940 as part of UK research using the cavity magnetron as the basis of a microwave-frequency radar system. They abandoned this approach as they were unable to solve the problem of feeding microwave power to a spinning antenna. The concept was revealed to US researchers as part of the Tizard Mission during the summer of 1940, and the RadLab decided to press on with the concept. This led to the SCR-520 of 1942, designed for installation on large aircraft like the P-70 Havoc and P-61 Black Widow. Only 108 were produced, and most were later converted to the sea-search role as the SCR-517. Western Electric started a redesign and introduced a somewhat lighter and much simpler version as the SCR-720 in late 1942. It arrived in the midst of RAF Bomber Command's efforts to introduce the "window" which proved to be equally effective on German radars as well as the RAF's own. A search for a solution led to the SCR-720 being accepted by the RAF, and window was released for use in 1943. Production versions of the Mk. X did not arrive until much later than expected, in December 1943, and did not start replacing the older AI Mk. VIII radar in front-line units until early 1944. This was just in time; the Luftwaffe began using window over the UK in January 1944 as part of their Operation Steinbock. The SCR-720 was used by the US for only a short time as newer and longer-ranged radar systems were developed in the post-war era. The same was supposed to be true in RAF service as well, but a lengthy series of delays in various programs kept the Mk. X in service well into the 1950s. The last aircraft with Mk. X, the de Havilland Sea Vixen, remained in second-line roles until 1970.
Development
Cavity magnetron
The UK had led development of airborne radars with the introduction of the AI Mk. IV radar system, which reached operational service in 1940. This system was built using conventional vacuum tube (valve) electronics from an experimental television receiver. The tubes could operate at a maximum frequency of about 200 MHz before their efficiency fell off dramatically. Generally, an antenna has to be at least 1⁄2 the wavelength being used to get good gain; the Mk. IV's 200 MHz frequency corresponds to a 1.5 m wavelength, requiring antennas to be on the order of a metre. This proved difficult to arrange on an aircraft, and both resolution and detection range suffered as a result. The need for shorter wavelengths was also important to the Royal Navy, who needed improve resolution to detect the conning towers of semi-submerged U-boats. They led research into shorter-wavelength systems. As part of this research, they began funding the University of Birmingham's efforts with klystrons. These were not successful, but two Birmingham physicists with little else to do ended up producing a solution, the cavity magnetron. Their first example produced 500 W of radio power, better than the best klystrons in the world. They pushed this to over 1,000 W within weeks. The main Birmingham team gave up on the klystron and began work solely on the magnetron. GEC was introduced to the work and applied their tube-making knowledge to the system, almost immediately introducing models producing 5 kW and by the summer had examples producing 15 kW.
Tizard Mission
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