The Magnetron - AI and the night-time raiders

In 1936, Sir Henry Tizard had been concerned that the proposed CH systems would be insufficient to direct fighters towards enemy planes if visibility was bad or if the enemy chose to raid at night. E.G.Bowen was tasked to set up a team to work on 'RDF2' based at Bawdsey. Trials using 6.7 m and then 1 m wavelengths took place in the summer of 1937. On September 3rd 1937 a very successful trial took place (using 1.5 m) when HMS Rodney and several aircraft were detected and the flight was in appalling weather conditions. Thus until the advent of the Magnetron, AI (and ASV) equipment would only be built to a 1.5 m wavelength specification.

Tizard's fears came true in 1941 when the Germans did indeed switch their efforts towards night-time raids but by now the Magnetron was in existence.

Whilst the 1.5 m systems were acceptable for clear daytime use because the fighter pilot could be vectored to within about 2 miles of an enemy plane and then see them, this patently was not going to work in cloud or at night. The last 2 miles to interception would have to be controlled by radar. The new 10 cm system was accurate enough to allow this because the narrower beam could be more precisely directed and there was no 'ground clutter' to confuse the receiver in the plan….