Dielectric Measurement Methods for Solids at Microwave Frequencies

An experimental method of measuring the dielectric constants of solids is described for the case of medium and high loss materials. The dielectric sample is enclosed within a section of wave guide terminated by a reflecting plane, and measurements are made of the standing wave ratio and position of the voltage minimum for both a short-circuit and an open-circuit termination. The equations for the dielectric properties of the substance may then be expressed in a relatively simple form in terms of elementary functions for media having tanδ ≥ 0.03. In order to obtain reasonable accuracy for the determination of the loss factor of low loss materials using only standard wave-guide components, it was found to be necessary to adjust the operating conditions to obtain maximum power absorption by the dielectric sample. A frequency variation method of obtaining this maximum loss sensitivity is described, the apparatus being the same as that used for the medium and high loss materials. A set of experimental values is given for the dielectric constants of a group of common plastics at 24,000 mega-cycles. A brief analysis of the effect of certain of the equipment losses is included.