Radar and the development of the cavity magnetron


Radar (Radio Detection And Ranging) was developed in many nations during the 1930s. The United Kingdom was the first to use this technology as the basis of a comprehensive air defence system, its earliest research being carried out at Orfordness in Suffolk. In 1939, a chain of 20 radar stations, known as ‘Chain Home’, tracked aircraft approaching the UK’s east and south coasts from the Isle of Wight to the Tay. During 1940, a new ground-based radar system, Chain Home Low, was introduced to detect both ships and low-flying aircraft.

Other radar systems developed throughout the war included IFF (Identification Friend or Foe, in which friendly aircraft identified themselves using radar signals from on-board ‘transponders’), H2S (a centimetric airborne radar which displayed a ‘map’ of the ground below the aircraft on its navigator’s cathode ray tube screen) and Oboe (a system for guiding bombers to their targets).

The cavity magnetron

The early Chain Home radar system was based on wavelengths of 15 to 30 metres, which involved very large aerials. It was obvious that if shorter wavelengths (i.e. below 1 metre) could be used then radar could be made far more compact, robust and effective, easy to fit into aircraft and ships.

In 1939, a team was set up under Professor Mark Oliphant at Birmingham University to attempt to create a radio valve capable of transmitting powerful emissions at a wavelength of approximately 10cm.  This team included H A H (Harry) Boot and J T Randall, who were allocated a device called the ‘magnetron’ for further development.

Many different types of magnetron, a vacuum tube which uses powerful magnets and resonant cavities to produce oscillating radio waves, had been developed in Germany, the US and USSR in the 1930s. In 1940, Randall and Boot made significant improvements in the design and produced very high power at wavelengths of 9.8 cm. Their magnetron design was then further improved by Dr E C S Megaw and his team at the GEC Laboratories, Wembley, who transformed the magnetron from a laboratory device needing vacuum pumps and a large electromagnet to a robust, workable, lightweight prototype.

GEC magnetron type E.1189. No.1, which operated for the first time on the 29th June 1940. (Image courtesy of the IET Archive)

The cavity magnetron became the preferred source of very high frequency radio waves in radar devices. Similar devices were developed independently by Yoji Ito in Japan.

Ink tracing of resonator magnetron drawing, [1940], from the papers of Harry Boot (IET Archive, NAEST 61)

For their work on the magnetron, Randall and Boot were awarded the Thomas Gray Memorial Prize of the Royal Society of Arts in 1943 for ‘improving the safety of life at sea’.  Further recognition followed with an Award by the Royal Commission on Awards to Inventors (1949), the John Price Wetherill Medal of the Franklin Institute, Pennsylvania (1958) and the John Scott Award of the City of Philadelphia (1959).