MEASUREMENTS AND OPTIMIZATION OF RADAR ABSORBING MATERIALS: PRELIMINARY CONSIDERATIONS

摘要

Currently new organic materials are being developed for use as radar absorbing materials (RAM). These materials are thin, and in general characterized by the complex permittivity and permeability. During their development it is essential to be able to measure their electrical properties in a range of frequencies of interest. There are numerous methods that may be used for measurement of properties of thin samples. However, positioning of thin samples always remains a problem that may significantly increase measurement errors. In the initial phase measurement are limited to a relatively narrow frequency range in X-band (8.2-12.4 GHz). This allows for the measurements to be performed in the waveguide using an automatic vector analyzer. Two methods are explored for measurements of thin samples that are deposited on a low-loss dielectric support e.g., acrylic. In one of the methods, an identical acrylic block is measured, and used as a reference. In the other method, the presence of the acrylic is taken into account in numerical computations. Actually, by selecting a proper length of this sample support an improved accuracy can be obtained for test samples of some permittivity and permeability values. Initial tests and numerical evaluation indicate sufficient accuracy of measurements, as long as the support block is well fitted into the vertical dimension of the waveguide. To retrieve the permittivity and permeability values from measured reflection and transmission coefficients of the test sample in the waveguide two methods are being developed, namely the nonlinear square fit and genetic algorithm. In addition to measurements, methods of multi-layer RAM structures optimization are considered. Very promising results are obtained in this application with use of genetic algorithms.