A Method for Measuring the Complex Dielectric Constant of Gases at Microwave Frequencies by Using a Resonant Cavity

Abstract

A sensitive method has been developed for the measurement of the complex dielectric constant of low pressure gases at X-band microwave frequencies. The gas, whose complex dielectric properties are to be determined, is contained in a resonant cavity which is a part of the microwave circuit. With the introduction of the gas, the real part of the dielectric constant changes the resonant frequency of the cavity, while the imaginary part changes the amplitude and breadth of the cavity response curve. By a rapid variation of the frequency across the cavity resonance, the real and imaginary parts can be conveniently and accurately determined from measurements on an oscilloscope. Tests on such a system have been performed with satisfactory results. Sample data are presented for methylchloride and deuterated ammonia. This method can be used for the measurement of the resonant dispersion and absorption of microwaves by gas molecules. Also, the same method can be used for the measurement of loaded and unloaded Q-values of a cavity.