Analysis of frequency selective surfaces with arbitrarily shaped apertures by finite element method and generalized scattering matrix

Frequency selective surfaces (FSS), consisting of two-dimensional arrays of metallic patches or apertures in a metal screen, are usually analyzed by method of moments or CG-FFT techniques. In this work, we present a hybrid technique to compute the generalized scattering matrix (GSM) of a metal sheet perforated periodically with arbitrarily shaped apertures. The GSM can be used as a building block to analyze multilayer FSS with different geometries, by cascade connection. The GSM of each periodic surface can be computed by the most efficient technique, and then, they are combined in the cascade process. The technique is based on FEM and cascade of GSM, but FEM is only used to compute the modes at the irregular shape aperture, considered as a short length of waveguide. This approach takes into account the real thickness of the metal sheet. The numerical modes are computed only once, and then they are used in a modified mode matching technique, in order to reduce the intensive computation of FEM. For canonical apertures (rectangular, circular or coaxial) in a metal screen the GSM is obtained directly by mode matching (MM).