Microwave Measurements of the Dielectric Properties of Gases

Abstract

An accurate and sensitive method for measuring the complex dielectric constant (ε′‐iε″) of gases in the microwave region is described and critically investigated for sources of error. A resonant‐cavity method is used in which the cavity response curve is displayed on a cathode‐ray tube. The variation of resonance frequency and Q of the cavity when filled with gas are determined from measurements with two frequency markers. The equipment is operated at 9000 Mc, and cavities with Q's of 1×104 are used. In the determination of ε′‐1 (dimensionless) of very low loss gases, an accuracy of 0.4 percent and a sensitivity of 4×10−7 can be obtained. In the determination of the loss factor ε″, an accuracy of 2 percent and a sensitivity of 5×10−7 can be obtained. This method is useful for measuring dispersion and absorption in solids and liquids as well as gases. Experimental results are given and briefly discussed for ε′‐1 of O2, N2, CO2, He, and air, and for ε″ of NH3 as a function of pressure up to 20 cm Hg.