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A novel differential method based on the open resonator is developed for permittivity measurement of thin dielectric films on optically dense substrates at millimeter wavelengths. The method is based on measurement of resonant frequency shifts due to appearance of a thin film on upper and lower sides of the substrate. The advantages of the method are that there is no need to know the geometry of the open resonator nor the thicknesses of the film and substrate, though one has to measure separately dielectric properties of the substrate.
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Dudorov et al. (2005) studied this question.
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