magnetron

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cavity magnetron, an electronic vacuum valve which originated in the UK in 1940 as a generator of microwaves suitable for high-power pulse (10 kW) airborne radar. With the aid of American resources and development skills, it brought about centimetric radar which was used with decisive effect during the battle of the Atlantic and the strategic air offensives from 1943 onwards.

The operational deficiencies of the existing 1.5 m. (200 MHz) airborne radar systems were the driving force behind the British development of the cavity magnetron. With AI (Aircraft Interception) radars, unwanted ground return from a broad antenna pattern prevented any target being detected which was beyond the intercepting aircraft's height. Small wavelengths (10 cm) obviated this problem and allowed the use of compact antennas of high gain and narrow beam-width. What was then needed was a valve that would deliver workable power at 10 cm.

Devised by J. T. Randall and H. A. Boot at the Nuffield Laboratory of the University of Birmingham, the cavity magnetron had a six-segment copper anode. It first operated on 21 February 1940; was redesigned by E. C. S. Megaw at the General Electric Company, Wembley; and eventually emerged in its final form suitable for airborne use in June 1940. One was taken by the Tizard mission to the USA in August 1940 (see Figures 1 and 2) and one American scientific writer, with understandable hyperbole, later stated that it was ‘the most valuable cargo ever brought to our shores’ ( J. Baxter, Scientists Against Time, Boston, 1946, p.142). It was the heart of the new radar's transmitter while another microwave valve, the klystron, was the equivalent vital organ in the receiver.

The basic magnetron diode valve can trace its origins to A. W. Hull of the USA, who in 1921 produced a cylindrical thermionic diode in which the behaviour of the electrons flowing from cathode to anode was controlled by an axial magnetic field. Pre-war studies on magnetrons were carried out in many countries. During 1936–7, in the Soviet Union, N. F. Alekseev and D. E. Malyarov produced demountable water-cooled cavity magnetrons, while, in Japan S. Nakajima and Y. Ito experimented successfully with cavity magnetrons from 1933 onwards. These achievements, however, were not translated into successful microwave programmes and were unknown to Randall and Boot, whose discovery in 1940 became the starting-point for the world-wide development of microwave radars.

On 2 February 1943, a Stirling bomber with an H2S ground mapping radar crashed near Rotterdam and its self-destruct mechanism failed to work. The Germans set up a special ‘Rotterdam’ Commission and by 22 June 1943 had constructed a prototype of the captured equipment which delivered some 40 kW peak power at 9.1 cm. Thereafter German cavity magnetron development was undertaken and well-engineered centimetric radars were produced, but, broadly speaking, Germany lagged behind the Allies in their microwave programme by almost two years.

Sean Swords

Bibliography

Bowen, E. , Radar Days (Bristol, 1987).
Collins, G. , Microwave Magnetrons (New York, 1948).

magnetron , vacuum tube oscillator (see electron tube ) that generates high-power electromagnetic signals in the microwave frequency range. Its operation is based on the combined action of a magnetic field applied externally and the electric field between its electrodes. The tube is a diode having a cathode and an anode and is surrounded by an external magnet. Without this external magnetic field, the tube would work much like a simple diode, with the electrons flowing directly from the cathode to the anode. The magnetic field forces the cathode-emitted electrons to assume a curved path and thus creates a rotating electron cloud about the tube axis. The magnetron is noted for its high efficiency (ability to convert electrical power input to microwave power output). Magnetrons are available for generating microwave energies ranging from a few kilowatts to a few megawatts and are used extensively in radar systems and microwave ovens .