Sub-structure characteristic mode analysis of microstrip antennas using a global multi-trace formulation

A characteristic mode (CM) method that relies on a global multi-trace formulation (MTF) of surface integral equations is proposed to compute modes and associated resonant frequencies of microstrip patch antennas with finite dielectric substrates and ground planes. Unlike the traditional full-structure CM methods, the global MTF allows for implementation of a sub-structure CM method. This is achieved by representing the coupling of the electromagnetic fields on the substrate and ground plane in the form of a numerical Green function matrix, which yields a more compact generalized eigenvalue equation. The resulting sub-structure CM method avoids the cumbersome computation of multilayered medium Green function (unlike the CM methods that rely on mixed-potential integral equation) and the volume discretization of the substrate (unlike the CM methods that rely on volume-surface integral equation) and numerical results show that it is more accurate than full-structure CMs in predicting modal behavior of electromagnetic fields on examples of practical microstrip antennas.