Measurement of the microwave dielectric constant for low‐loss samples with finite thickness using open‐ended coaxial‐line probes

This work addresses the effect of finite sample thickness on microwave dielectric constant measurements for thin, planar, low-loss samples using the open-ended coaxial-line probe method. Detailed measurements of the dielectric constant were carried out on a wide range of thicknesses of air samples which were backed by infinitely thick teflon and alumina dielectric media. The measurements were made at room temperature for various (50 Ω) coaxial-line dimensions, microwave frequencies 4-8 GHz, and power levels near a fraction of a mW. The results provide strong support for previously published theoretical calculations based on a boundary value problem which uses a spectral domain formulation for the aperture fields. From thin, planar samples, values of 10.44±0.5 and 25.9±1.3 were obtained at 5 GHz and 300 K for the bulk dielectric constant of MgO and LaAl[sub 2]O[sub 3], respectively. The applicability of a simple empirical model based on an exponential fit is discussed.