A space-domain mixed-potential integral equation approach is applied in conjunction with the method of moments to compute the radar cross-section (RCS) of coax-loaded microslrip patch antennas having arbitrary or irregular shapes. The effects of Lhe substrate—which may be electrically thick and may consist of any number of planar, possibly uniaxially anisotropic dielectric layers, backed by a ground plane—are rigorously incorporated in the analysis by means of the vector and scalar potential Green's functions. The latter are expressed in terms of the voltages and currents on transmission line analogs of the layered medium, associated with TM and TE partial fields. The current distribution on the microstrip patch is approximated using vector basis functions defined over triangular elements and the coax probe current is expanded in terms of piecewise-lincar subdomain basis functions. A simple probe-to-patch attachment mode, compatible with the triangular element model of the microstrip patch, is used to enforce current continuity at the junction, and the coax aperture is modeled by a magnetic enrrent frill. The far zone fields are found by the stationary phase method, and are expressed in terms of the Fourier-transformed basis functions and the transmission line voltages and currents evaluated at the stationary phase point value of the transverse wavenumber. Computed RCS results are presented for several loaded and unloaded microstrip patch antennas of various shapes and are shown to be in agreement with published measured data and with computed results obtained by specialized techniques, which—unlike the method presented here—are not easily extendable to arbitrary shapes.