The Q of a Microwave Cavity by Comparison with a Calibrated High-Frequency Circuit

A comparison method has been developed for the direct measurement of cavity Q at microwave frequencies. It is particularly useful for high values of Q, such as 5,000 to 15,000. The over-all error in the measurement is estimated to be less than ±3 per cent. A two-channel superheterodyne technique is used, in which both channels are driven by the same frequency-swept oscillator, and both channels use the same local oscillator. The cavity is inserted in the radio-frequency stage of the first channel, and a comparison circuit is inserted in the intermediate-frequency stage of the parallel channel. The two resonance curves are displayed on the same oscilloscope for alternate sweeps of the oscillator. When the resonance curves are made to coincide, the Q of the cavity is n times the Q of the comparison circuit, where n is the ratio of radio frequency to intermediate frequency. Cavity shunt resistances can be measured on the same apparatus.