Something old-something new about microwave dielectric measurements

Microwave dielectric measurements differ from low frequency measurements because wavelengths are small enough to be the same order of magnitude as the specimen dimensions. The theory of low frequency lumped circuit measurements must therefore be expanded to include wave propagation and distributed constant transmission line impedance calculations. The equipment necessary to cover several ranges of frequency is usually several times more costly than low frequency bridges and specimen holders. Transmission line, resonant cavity, and free space methods are in use and the basic theory of measurement has been adequately covered by many authors, including the References listed at the end of this paper. The shorted transmission line and resonant cavity perturbation method have been recently published in ASTM Test Method D-2520 with emphasis on step-by-step procedures and complete equations necessary for calculation of results. The TE₀₁₁ cavity for rod or disk specimens is presently being written up as an additional ASTM test method. This method is quite accurate and capable of evaluation of dissipation factor on low loss materials. An ASTM Task Force has recently completed round-robin testing and analysis of a microwave stripline test method. A report of results and conclusions is now in progress. Microwave dielectric measurements are no more accurate than the accuracy in measurement of the dimensions of the specimen.