An Improved Free-space Method for Measuring Electromagnetic Parameters of Liquids

An improved free-space measurement method is proposed in this paper to address the requirements for measuring the millimeter-wave broadband electromagnetic parameters of liquids. In this method, the receiving and transmitting horns are installed at a far-field positions above a liquid reservoir. Interference signals are mitigated through time-domain windowing, ensuring a cleaner received signal. The permittivity and loss tangent of the liquid sample are determined from the phase and amplitude of the scattering parameter S21 at two distinct liquid depths.Compared with the conventional free-space method, this method eliminates the interference of systematic inherent errors on the EM parameter measurement results of liquids, avoids the influences from the reservoir's sidewalls, and enables straightforward increment of the liquid sample depth for repeated measurements; the measurement precision is thus conveniently enhanced via averaging. The factors affecting the measurement errors of the electromagnetic parameters of liquids are analyzed, and the validity of the method is verified through the testing of the electromagnetic parameters of silicone oil and a synthetic oil.