  A characteristic mode (CM) method that relies on a global multi-trace
formulation (MTF) of surface integral equations is proposed to compute the
modes and the resonance frequencies of microstrip patch antennas with finite
dielectric substrates and ground planes. Compared to the coupled formulation of
electric field and Poggio-Miller-Chang-Harrington-Wu-Tsai integral equations,
global MTF allows for more direct 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 the
multilayered medium Green function (unlike the CM methods that rely on
mixed-potential integral equations) and the volumetric discretization of the
substrate (unlike the CM methods that rely on volume-surface integral
equations), and numerical results show that it is a reliable and accurate
approach to predicting the modal behavior of electromagnetic fields on
practical microstrip antennas.
