Swept frequency reflectometer design for in-situ permittivity measurements

A prototype, broadband vector reflectometer for in-situ field measurements of permittivity is presented. The system was designed primarily to measure permittivity of sea ice in the 2-18-GHz frequency range but works equally well on other materials. The reflectometer design is based on FM-CW radar concepts. A linear frequency-modulated waveform is generated, and a portion of it is coupled off and passed through a delay line to serve as the local oscillator for the incident and reflected channels which receive most of the generated waveform. The reflectometer measures the complex reflection coefficient of a short monopole antenna probe. Permittivity is obtained by constructing a normalized input impedance of the probe. The impedance measured in a standard medium is based to construct a fourth-order rational function for the normalized input impedance, which is used with the measured impedance in the medium under test to obtain the permittivity. Measured permittivity data for air, 1-Butanol, and Teflon are presented.<
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