Radial propagation and steepest descent path integral representations of the planar microstrip dyadic Green's function

A radial propagation integral representation of the microstrip electric dyadic surface Green's function is developed. This representation is very efficient for a numerical evaluation of the field when the source and observation points are laterally rather than vertically separated with respect to the plane of the substrate. Furthermore, when the integration contour is deformed to the steepest descent path, the Green's function exhibits an even faster convergence. In contrast, the conventional Sommerfeld integral representation of the microstrip Green's function converges very poorly for this case. Numerical examples are presented which indicate that the representations obtained here are surprisingly efficient even for relatively small lateral separation of the source and field points. This work is especially useful in the moment method analysis of microstrip antenna arrays where the mutual coupling effects are important.