Input impedance of an antenna and its application in complex permittivity measurement at high frequencies

A simple probe, a monopole antenna, was used to measure the complex permittivity of media at high frequencies. A rational polynomial was used to relate the normalized input impedance of an antenna in a homogeneous medium to the complex permittivity of the medium surrounding the antenna. The input impedance of an antenna was measured at different frequencies when the antenna was first surrounded by a standard medium. The measured impedance was used to determine the coefficients in the polynomial by using the least-squares method. Then the antenna was put in a medium for which the complex permittivity was to be measured. The complex permittivity was calculated from the roots of the polynomial once the input impedance of the antenna was measured.;To test the new measurement technique, experiments were done and the complex permittivities of saline solutions were measured with concentration varying from 1 kppm to 10 kppm in the frequency range 725 MHz to 1300 MHz at room temperature. The measured complex permittivities are in good agreement with those determined by an empirical formula. For practical use, experiments on cylindrical samples of different diameters have also been done to investigate the boundary effects. It is found that if the minimum diameter of a sample is larger than 2 skindepths of the medium, the boundary effects can be neglected. This finding with regard to the minimum size of the samples has been verified numerically as well.;Theoretical and numerical analyses for an antenna inserted in a cylindrical sample are then presented. An integro-differential equation is formulated for the current distribution on the antenna. The moment method is used to calculate the current distribution on the antenna. The numerical results are compared with the ones from our experiments from an analytic formula. The agreement is shown to be good in all cases. The range ;A measurement system was set up. The system consisted of a monopole antenna, a conducting plate, a network analyzer, and a computer and its peripherals. The system was automatically controlled by the computer. Thc measurement procedure is given in this dissertation.