An X-band magnetron Q-measuring apparatus

The Q-factors of a resonant-cavity magnetron are usually estimated, in terms of the frequency bandwidth of a resonance curve, from measurement of the standing wave which exists in a feeder through which power is supplied to the magnetron at, and near to, its resonant frequency. The accuracy of this method is high, but when changes in Q-factors are to be measured which, for example, may result from changes in the output coupling or tuning of the magnetron, its application is tedious. The use of a directional coupler replacing the standing-wave indicator to monitor the reflected power is well known. Such a coupler may be used in conjunction with a frequency-swept oscillator and cathode-ray tube to give a visual display of the magnetron resonance curve. The paper describes an apparatus of this type which was used for the qualitative examination of magnetron resonances in the 3 cm wave-band, and details of an improved equipment which may be used to measure magnetron Q-factors are given, with a discussion of the accuracy of measurement which may be achieved.