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A hybridization of the finite element method (FEM) [Jin, 1993] and the spectral domain method of moments (MoM) [Zavosh and Aberle, 1994; Aberle and Zavosh 1994] is utilized in the analysis of cavity-backed patch antennas mounted on an infinite ground plane. The spectral domain MoM models the fields in the external region of the cavity through the use of the half-space Green's function whereas the FEM models the fields inside the cavity using linear edge-based tetrahedral elements. The two regions are coupled through the continuity of the fields in the aperture. The MoM formulation can be completely decoupled from the FEM formulation with the use of the physical equivalence principle after a surface magnetic current is introduced just above and below the aperture plane. The use of edge-based tetrahedral elements inside the cavity volume results in a triangular discretization of both the aperture and patch surfaces. The spectral domain implementation of the Rao, Wilton and Glisson  basis functions with triangular support is one of the most appropriate choices in our case, if indeed arbitrary shaped apertures and/or patches are to be considered in the analysis.