[2] 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.

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.

[5] 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.

[6] 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.

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 RadLab became well known in research circles, and scientists from around the country and Canada were travelling to Boston on nothing more than rumour.

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 4 to 5 miles (6.4–8.0 km) away.

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.

[10] 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.

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.

[11] 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.

[14] On 29 April, after detecting a target aircraft at about 2 to 3 miles (3.2–4.8 km) Dowding asked Bowen about the minimum range[b] which they demonstrated to be about 500 feet (150 m).

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.

One of these would be kept by Western Electric, another by Bell Telephone, one would replace the original lash-up in the B-18, another sent to the National Research Council (NRC) in Canada, and the final one sent to the UK.

Instead, the Canadian NRC supplied a Boeing 247 airliner, and after a test fit to ensure the radar could be mounted properly, the aircraft was disassembled and shipped to the UK.

[15][c] AI-10 was similar in performance to the AIS systems of the same vintage, but Bowen found no strong desire on the part of the RAF to buy the device.

This has been attributed to a number of factors including overwork by the AMRE team fitting their own equipment and lacking time to test AI-10, as well as hinting of "not invented here" syndrome.

[19] There were two variations, the original SCR-520-A, and the -B which added an IFF receiver which could also be used for beacon tracking, the latter of which required the maximum range to be extended to 100 miles (160 km).

[21] The SCR-520-B, used in the P-70, weighed 600 pounds (270 kg) in twelve pieces, with six large units that had to be mounted near the radar dish, the largest of which was about a 1 yard (0.91 m) on a side.

These limitations meant it could be fitted in only the largest aircraft and so was unsuitable for the smaller, high-performance fighters used in the UK, like the de Havilland Mosquito.

With the introduction of the SCR-720, the older SCR-520 units were adapted for shipping detection, as this task was normally carried out from large patrol aircraft with ample room for the system.

The Luftwaffe responded by dramatically increasing the number and performance of their night fighter fleet, while also greatly improving command and control systems to make them effective.

One solution that had been developed was "window", today more widely known as chaff, which would confuse the German radars and make tracking individual aircraft difficult.

Window consisted of nothing more than aluminized paper strips, so it would be trivial for the Germans to make their own version as soon as they learned of it, and potentially re-open the air war against England.

On 23 December 1942 during the first tests of the improved version, the aircraft was attacked by a pair of friendly Supermarine Spitfires and shot down, destroying the only prototype and killing the lead designer.

Only minor concerns were found; the radio frequency feeder cable was poor, they wanted a new visor around the B-scope display, the range settings should be changed to 3, 5, 10 and 100 miles.

The poor results were attributed not to the German Dupple, but that the contacts were often Focke-Wulf Fw 190s, which the Mosquito was barely able to catch, as well as the "vigorous evasive tactics" used by the Luftwaffe pilots.

[27] P-61's, the first US aircraft with the system, normally the SCR-520, arrived in the UK in late June 1944 as part of 422 NFS and started flying operational missions in July.

[34] As Javelin ran into delays, the decision was made to extend the life of the Meteor and Vampire fleet by mounting new radars, either Mk.