A free-space, non-destructive method for measuring the complex permittivity of a double-layer bulk dielectrics and thin film oxide layers at microwave frequencies have been developed. The method utilizes a spot-focusing antenna system in conjunction with a vector network analyzer in the range of 18-26 GHz. The bulk dielectric was measured using the Transmission Method and Metal-Backed Method, while the Metal-Backed Method was used to investigate the thin films. Both types of samples were sandwiched between two quarter-wavelength Teflon plates to improve the mismatch at the frequencies of measurement. The thin film sample arrangement was backed by an additional metal plate. The double-layer bulk dielectric samples were Teflon-PVC and Plexiglas-PVC, while the thin film samples consisted of SiO 2 layers of different thicknesses grown on doped and undoped Si wafer substrates. The relative permittivity obtained for PVC ranged between 2.62 to 2.93, while those for Plexiglas exhibited values between 2.45 to 2.63. The relative permittivity of SiO 2 deposited on these wafers was between 3.5 to 4.5. All these values are in good agreement with published data. The advantage of the method is its ability to measure the dielectric properties of the films at the midfrequency band irrespective of the substrate type used. Simulations of the measurement setup were carried out using CST Microwave Studio and the simulation results agreed closely with the measurements.

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