Quantitative determination of small dielectric and loss tangent contrasts in liquids

Abstract

Accurate measurement of dielectric constant and loss tangent in liquid media in the frequency range 2-6 GHz is proposed in this paper. The measurement set-up is based on an open-ended coaxial probe instrumented with a home-made microwave interferometer. The interferometer is used to match the impedance of the probe to the 50 ohms input impedance of the vector network analyzer to enhance the measurement sensitivity for detection of small contrasts of the material complex permittivity. An accurate modeling based on a calibration procedure is proposed to relate the measured microwave signal to the complex permittivity of the liquid under test. First, a modified one-port calibration model is developed to relate the microwave signal to the aperture-plane reflection coefficient. Secondly, both capacitive and quasi-static approaches are proposed to relate the complex permittivity of the liquid under test to the aperture-plane reflection coefficient. As a demonstration, the complex permittivities of different sodium chloride solutions are determined. Accuracies better than 0.4% at 2 GHz and 0.2% at 6 GHz are exemplary shown for both real and imaginary parts of the complex permittivity.