The Incredible Cavity Magnetron

By David Rhys-Jones

You may be more familiar with the Cavity Magnetron than you think. It probably generated the microwaves that jiggled the water molecules that heated the sauce in the bowl of pasta that you ate last night. However, there is rather more to the Cavity Magnetron than TV Dinners. This is how it began.

The Second World War was won by the resolution and bravery of a large number of people of many nations, but there were two British  inventions which may well have tipped the scales. Both were developed in a total secrecy which survived for the duration of the war, despite the huge numbers who were involved.

The first,which everyone now knows about, was Enigma. Because of the need to send all messages via radio, the Germans developed a coding machine which had a four disks with multiple electric pathways through them.  A letter or number key pressed would send a current through all four disks to light a bulb against a letter. Without a knowledge of how the disks were aligned, nobody reading a message could tell which key had been pressed. There was an almost limitless number of possible settings. The Germans believed this implicitly and it was never questioned. The code was broken by a diverse group of Cambridge Dons, complete eccentrics and Post Office Engineers led by someone who was gay.  They eventually reached the point where they could read the messages faster than the recipients themselves.

The Cavity Magnetron was almost as important to the outcome of the war and has a similar history of development. It was the key element that made centimetre wavelength radar feasible. Something that the Germans believed was impossible because they acknowledged themselves to be the radar experts. In most areas they were.

It had been known for some time that the radio signals could bounce off objects and that receivers could pick the signal and range and bearings could be calculated. The detail of the picture improved as the wavelength decrease. The wavelength depended on the size and layout of the antenna. Chain Home, the Radar used at the time of the Battle of Britain, was 12 meter wave length and aerials were lengths of wire suspended between high towers. Fighter Command’s success in the Battle of Britain was due, not to the quality of their radar which used fairly crude off the shelf transmitters, but the sophisticated system which channeled the information back to Fighter Command and guided the interceptions.

Freya, the much more sophisticated German system, operated  at 2.5 meters with correspondingly smaller antenna  which could be rotated. The German Night fighters ultimately used a form of VHF that required a huge stag’s horn array of antenna on the nose of a converted bomber. It told little more than whether the target was left or right or up or down. However, to get an accurate image of the aircraft or ground or to pick up a submarine conning tower, radar frequencies in the 10 centimetre band were needed.   This was technically impossible with an antenna.

The Magnetron started as an attempt to bypass the patents on the thermionic valve. Few members under the age 60 will know what a valve is. Basically it did what the transistor does today. Suck the air out of a glass tube with a heated filament and a negatively charged plate and the electrons will move to the plate and a current will flow.   You have a diode.    If you put a mesh between the filament and the plate and then charge it, the current will stop. You have an electronic switch, a thermionic valve and, in the 1930’s, a patent infringement problem.