Slot antennas for measurement of properties of dielectrics at microwave frequencies

Single and stacked microstrip antennas were used in the past for the measurement of dielectric constants and thicknesses of lossy and lossless dielectrics at microwave frequencies. The two large slots at the two ends of the microstrip antenna produce a significant fringing field which couples to the material under test. One of the limitations of the microstrip antenna is its spatial resolution, which is determined by the length (the separation of the two end slots) of the antenna. This problem can be overcome by the use of the microstrip-fed slot antenna. Compared to the microstrip-fed microstrip antenna, a microstrip-fed slot antenna offers a better isolation between the feed and the material under test. The paper introduces the use of the microstrip-fed slot antenna for measuring the dielectric constants of materials. Like many techniques for characterisation of materials, a precise determination of the complex permittivity of a dielectric using the planar slot antenna, depends on the accuracy of the mathematical modelling or experimental calibrations available for the interpretation of measurement data. An accurate mathematical technique for the analysis of the microstrip-fed slot antenna, loaded with a lossy dielectric, is explained. This technique can be used to generate inversion curves for the translation of measured data into dielectric constant and loss tangent. Results for an example planar slot antenna when applied to different dielectrics are given.