Multi-modal Circuit Model Derivation for Finite Thickness Frequency Selective Surfaces

We propose a simplified approach to compute the multi-modal equivalent circuit model of a Frequency Selective Surface (FSS) with periodic unit cells, under normal plane wave incidence. Floquet's periodic boundary conditions are replaced by perfect electric and magnetic boundaries arranged according to the incident field polarization. The electromagnetic scattering from the FSS is modeled as a generalized rectangular waveguide (GRW) discontinuity problem. The Generalized Transverse Resonance technique is proposed to analyze the FSS unit cell aperture which is modeled as a waveguide of a finite thickness. The cutoff frequencies and the cross-sectional profile of the FSS waveguide modes are found. Projection matrices are then computed between the FSS's GRW modes and the GRW modes outside the FSS, leading to the full modal network. The proposed method is validated by full-wave numerical results for finite-thickness patch-based FSS.