Thick Frequency Selective Structures

Abstract: This thesis presents a new method that can handle a general thick Frequency Selective Structure (FSS), that consists of an arbitrary number of aperture layers and dielectric layers. The method is based on the Mode Matching Technique (MMT) and the finite element method (FEM). The thesis consists of a General Introduction and five scientific papers. The General Introduction gives an overview of filters and FSS. Some applications of FSS are studied, e.g. low observable radomes and reflector antenna systems, and the MMT is described. An example of an FSS is studied in detail, where the reflection and transmission curves are depicted, and the internal mode coefficients and fields are plotted. Paper I treats a perfectly conducting thick screen perforated with a periodic array of apertures with arbitrary cross-section. The scattered fields are determined by the MMT. Excellent agreement between computed and measured transmission is found. The present method and the spectral Galerkin method are compared for very thin perforated screens. The transmission depends on the thickness, and this dependence is investigated for a wide range of thicknesses. Paper II introduces a method that can handle a thick FSS, that consists of an arbitrary number of aperture layers and dielectric layers. An aperture layer is a conducting plate with a periodic array of apertures. The periodicity in the different layers must be in accord. The layers can be of any thickness and in any order, and the cross-section of the apertures is arbitrary. Paper III analyzes dissipation in FSS of aperture/slot type. The dissipation in an FSS is due to losses in the dielectric material, and losses due to finite conductivity in the metallic plate. The dissipation on the metallic structure arises both on the plane metallic surface and on the metallic walls of the apertures. The attenuation and the power losses are calculated for a number of different FSS. Based on these results the performance of an FSS with losses is discussed. Paper IV deals with the performance of an FSS with defects. In this case the structure is a periodic pattern of apertures in a conducting plate. The defects can be deviations in the placing of the apertures, in the material parameters, or in the shape of the apertures. First the perturbation to the farfield pattern from a deviation in one aperture is analyzed. This is then utilized for a statistical analysis of an FSS with a stochastic variation of the apertures. Paper V focuses on the radio wave propagation through energy saving windows. These panes have a metallic shielding that keeps the heat inside the building during winter and outside the building during summer. Unfortunately, this covering also has an opaque behaviour at microwave frequencies. A design of energy saving window panes with high transmission at 900 MHz and 1800 MHz is presented.