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 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
In the summer of 1940 the situation in the UK was dire; the British Expeditionary Force barely escaped a total loss during the Dunkirk evacuation and the RAF was outmatched perhaps three to one by the Luftwaffe. Although great effort had been made to install the Chain Home radars and the Dowding system to manage it, the possibility of losing an air war and subsequent invasion was very real. Henry Tizard, who played a founding role in the development of radar as leader of the Tizard Committee, was aware that the pressing needs of the immediate war effort meant that the UK would be unable to fully take advantage of the many technological advances they had made. He began to press for permission to show these technologies to his counterparts in the US, where American production capacity could be used to get these devices into the war effort in much greater quantity.
A great debate broke out over the relative merits of revealing so many advances to a country that would be able to make immediate use of them to great financial reward. In August 1940, Winston Churchill had enough of the arguments and personally directed Tizard to begin contacting US researchers. Tizard formed up a group of seven, including himself as the lead and John Cockcroft as his deputy. For the scientific portion of the group, Tizard brought "Taffy" Bowen who had led development of the AI Mk. IV before being sidelined due to managerial fighting with A.P. Rowe, who ran the Air Ministry's radar research establishment. Tizard and a military liaison flew to Washington, DC on 22 August 1940, while the rest of the team left for Canada on the Canadian ocean liner RMS Duchess of Richmond on 30 August.
In Washington, Tizard met with Vannevar Bush, who had arranged the formation of the National Defense Research Committee (NDRC) only weeks earlier in late June. The rest of the team arrived in Halifax on 6 September, and Washington on the 12th. The initial meetings did not go well as both groups seemed reluctant to tip their hand on their research. The logjam broke on 19 September when the topic of radar came up. The British team was surprised to learn that the US had already begun deployment of a longwave radar system similar to their own Chain Home.
The key moment came when one of the US Navy representatives demonstrated an experimental microwave tube that worked at 10 cm, but noted that it made only tens of watts and that they had reached a dead end in development. As soon as the statement was made, Bowen reached into his "black box" and pulled out E1189 No. 12, a magnetron operating at 9.1 cm and capable of producing at least 10 kW. The reveal had the effect of a bombshell, cutting away remaining reticence on both sides, who quickly began sharing details of all of their research.
Rad Lab
The magnetron came under the scrutiny of Alfred Loomis, a successful businessman turned researcher who had already set up a "Microwave Committee" within the NDRC. With this clear demonstration that microwaves were practical, Loomis began to form a dedicated research lab which opened in November as the Radiation Laboratory at MIT. In spite of great secrecy, news of the Rad Lab became well known in research circles, and scientists from around the country and Canada were travelling to Boston on nothing more than rumour. When they heard what was being done, they would instantly ask to join the effort. By mid-November this was up to one prominent scientist a day.
Bowen stayed in the US while most members of the team returned to the UK. He travelled to the General Electric labs in New Jersey and demonstrated the magnetron to the researchers from GE and Bell Labs. The device managed 15,000 W on 6 October. Bell took up production, delivering the first 30 by the end of the month. These were split among a number of groups, especially Loomis' group, and a number of microwave projects started. Western Electric, Bell's production arm, also took up production of the existing VHF Mk. IV set to speed production and help supply RAF needs. This emerged as the SCR-540, but saw limited use.
The Rad Lab started their magnetron experiments with a simple system using separate transmit and receive antennas, each consisting of a short dipole antenna positioned in front of a parabolic reflector. They set it up on the roof of the Rad Lab, and on 4 January 1941 they were able to pick up reflections from buildings on the other side of the Charles River. They then began modifying this "roof system" for a single-antenna using a duplexer similar to the ones used in the UK's experimental sets. This used a network of tuned conductors cut to specific lengths. As was the case in the UK, the system significantly reduced the output signal strength. Bowen notes that a hint of pessimism took over as they attempted to solve these problems. The system was finally tuned and working by early February, and on the 7th they detected echoes from an aircraft flying over Boston Airport some away.
Helical vs. spiral During this period, the UK researchers at TRE had been continuing to work on the problems caused by the tuned switching network. The solution came from a suggestion by Arthur H. Cooke of the Clarendon Laboratory, who suggested that a vacuum tube filled with a dilute gas would "flash over" due to the high energy from the transmitter, conducting the energy to the antenna, but then quickly de-ionize and stop conducting as soon as the signal ended. A perfect tube for this purpose turned out to have been just invented to solve another problem with microwave radars, and the resulting soft Sutton tube was able to rapidly move into production in March 1941. As the Sutton tube absorbed very little energy, the output of the radar systems immediately doubled, and the constant retuning of the network was eliminated.
With this problem finally solved, the team began plans to bring the system, known as "AIS" for "Aircraft Interception, Sentimetric", into service as soon as possible. The last remaining significant problem was how to use the radar's narrow beam to scan the sky in front of the fighter. They had originally considered a method known as "helical-scan", which spun the parabolic reflector dish around a vertical axis, causing the beam to make a series of horizontal stripes while the dish angled up and down to scan vertically. Feeding microwave energy from the magnetron to a spinning antenna presented a problem that would take some time to solve.
Looking for expedient solutions, they began to consider simpler ways to move the antenna that did not require a complex connection. Eventually, following experiments by Bernard Lovell, they decided on leaving the dipole antenna on a fixed mounting facing forward, and nutating the parabolic reflector in circles behind it. This "spiral-scan" system meant the antenna could be fed using a simple coaxial cable, with the downside that the beam became increasingly de-focused at greater angles from the centreline.
At the RadLab, the immediate need to solve the scanning problem was not as pressing, and they decided to continue to develop the helical-scan solution. They quickly solved the problems, and had a working system by March 1941. This was fit in the nose of an obsolete Douglas B-18 Bolo bomber. The aircraft took to the air for the first time on 10 March 1941, the very same day that the British AIS with spiral-scanning flew for the first time in the UK. During this flight, Bowen estimated the maximum range of the US unit to be about , and on their return flight they flew past the naval yards at New London, Connecticut and detected a surfaced submarine at about .
Having heard of this performance, Hugh Dowding, who was visiting the US at the time, pressed to see it for himself. On 29 April, after detecting a target aircraft at about Dowding asked Bowen about the minimum range which they demonstrated to be about . Dowding was impressed, and before leaving to return to the UK, met with his counterpart, General Cheney, telling him about the system's performance and pressing for its immediate development for purchase by the RAF.
Testing the system