A TE01n Cavity Resonator Method to Determine the Complex Permittivity of Low Loss Liquids at Millimeter Wavelengths

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Abstract
Two oversized TE01n cavity resonator systems have been constructed that are suitable for measuring the complex permittivity ε=ε′ (1 − j tanδ) of low loss liquids at millimeter wavelengths. One of them operates at frequencies from 26 to 70 GHz, while the second, which is described in detail, was tested at frequencies around 136 GHz where a quality factor of about 60 000 was obtained. ε′ is determined from the wavelengths measured in the filled and unfilled resonator. The loss factor tanδ is obtained from the half‐widths of the resonance curves in both resonator states. For that purpose, the frequency of the oscillator is swept, and the resonance curves vs frequency are displayed on an oscilloscope. Frequency calibration marks are generated by the absorption resonances of a second long open resonator operating in the HE11m dipole mode which is guided by a thin dielectric thread. To restrict the influence of traces of water in the liquids to be measured, the resonator is permanently connected to an apparatus that permits drying the liquids and filling and emptying the resonator without contact with atmospheric moisture. By this method, the complex permittivity at 136 GHz of seven nonpolar liquids (tanδ=3×10−4−30×10−4) has been determined. Insofar as possible the data are compared with published values.