Simplified measurement of Q of microwave transmission cavities

A simple method for measuring the Q of high Q microwave transmission cavities is presented. A klystron source is frequency modulated first at a low frequency (1 kc/s) and then at a high modulating frequency (about equal to the estimated cavity BW/2). The ratio of the indicated frequency deviation at the output of the cavity slope detector is used to enter the universal curve derived to determine the cavity bandwidth and hence its Q. This method requires only the use of easily available test instruments and is quickly performed. The accuracy is dependent only on relative values and not on absolute values of the frequency deviation detected.