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Birmingham and the RADAR revolution

Updated: Aug 13, 2020

Patrick McCarthy uncovers the link between your microwave, your uni and World War Two

How does a microwave oven work? Finely tuned electromagnetic (EM) waves from standing nodes inside the oven’s chambers, exciting the bonds in water, causing them to heat up as the contents spin on the plate. The source of these microwaves, the cavity magnetron, has a history directly linked to the University of Birmingham.

LEFT: replica of the original cavity magnetron invented at UoB

RIGHT: replica of the electromagnet for the original cavity magnetron

Photos: The University of Birmingham Research and Cultural Collections, House of Historic Physics Instruments

In 1939, the term RADAR (Radio Detection and Ranging) was coined. Systems had been developed over many years whereby objects could be detected with several-meter resolution from large distances. Originally intended to help prevent the collision of ships in fog, RADAR works by sending out short, regular radio wave pulses and measuring the reflections observed on an oscilloscope. By timing the pulses’ returns, the range of the object they reflected off can be determined.

However, for RADAR to work it needs a ‘coherent’ wave source – one which consistently produces radio waves of similar wavelength. This proved to become more difficult as the radio waves climbed to higher energies. However, higher energy waves correspond to shorter wavelength radiation, which means better ‘resolution’ – the smallest object the antenna can detect, this meant that, early in the Second World War, it was possible to detect objects such as approaching plans; but being able to resolve how many planes incoming proved to be difficult.

A maor breakthrough came from Birmingham in 1940, in the form of the cavity magnetron. This device uses several specially shaped cavities inside a vacuum tube, and passing a magnetic field through the length of the tube allows electrons emitted from a cathode in the centre to spiral around the cavities. This resonates and, as described by Maxwell’s laws, the moving charge creates electromagnetic waves corresponding to the frequency – in this case, radio.

John Randall and Harry Boot are credited with its creation, though it was not the first. A multi-cavity magnetron had already been patented in Berlin five years prior, however, it suffered from ‘frequency drift,’ where the radiated wavelength would change as the magnetron warmed up. This made it unreliable for radar, but Randall and Boot solved this problem by liquid cooling the chamber and increasing the magnetic field. This design could produce 1000 times the power of other devices at the time, and resolve objects with metre widths.

It has been argued that this breakthrough in coherent EM technology was a ‘simultaneous discovery,’ invented across several countries (such as Germany, the US, Japan and the Netherlands) in the space of over a year. However, Randall and Boot’s chambered design proved to be readily manufacturable, and as such it was easily deploted across most of Britain in radio stations, helping win the Battle of Britain and arguable, the war.

Today, there are more sophisticated devices such as ray tubes being used as sources, and the cavity magnetron has swindled in military use. However, you can still find them today in most conventional microwave ovens and older devices.

From Issue 12

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