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I am grateful to my colleague Jim Colville for bringing the following article from the Autumn 1962 edition of "The Locking Review" to my attention.


by Sqn. Ldr. G. D. BOLAM, M.A., B.Sc., Dip.EI., A.M.I.E.E.

One of the most important of the secret weapons developed during the war was the multi-cavity magnetron. It is not generally realised that the cathode of a magnetron is very quickly stripped of its coating and after passing current for a total time of only a few hours a magnetron is quite useless. Fortunately, a magnetron passes current for only a very small fraction of the time a radar is working.

Multi-cavity magnetrons are designed to work on centimetric wavelengths and must therefore be small (in physical size). In order to provide a high power pulse output the cathode must be capable of a very large peak emission - of the order of 1 A/cm2.

In order to secure such a high peak emission the cathode has to be oxide coated. With such a coating the electric field near the surface of the cathode should really be less than a certain value to avoid stripping the cathode a value corresponding to anode potentials of a few kilovolts. However, to provide a high power pulse the valve must not only pass a high peak current (about 20 A) it must also have a high anode potential of the order of 20 kV. This sets up an electric field which will strip the cathode in a few hours.

Magnetron cathodes are made to give as long a life as possible by special surface treatments but the life is still short only a few hours. Fortunately, pulse-working means that for each hour a radar is in operation the magnetron is in fact passing current for only a fraction of a second. Thus, although the working life of a magnetron totals only a few hours it can be used in an equipment for a few thousand hours.

Since the life of a magnetron is limited it is important to have a convenient way of testing magnetrons in use. This is done by using a spectrum analyser and rejecting a magnetron when the side lobes in the spectrum exceed 40 per cent. of the main lobe.

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Updated 06/03/2002

Constructed by Dick Barrett

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