Efficient physical optics approximation for the calculation of radiation patterns of planar antennas located on a finite ground plane

A physical optics (PO) model is applied to investigate the radiation characteristics of a microstrip patch antenna located on a finite ground plane. The standard PO approach consists of the replacement of the dielectric and the ground plane by electric volume and surface polarization currents. In this paper these polarization currents are obtained as a combination of polarization currents of the infinite structure and auxiliary polarization currents, only present beyond a certain distance from the source. Simple expressions are derived for these auxiliary currents, based on the expansion wave concept. The radiation patterns consist of two terms: the radiation pattern of the antenna on the infinite ground plane and a diffraction term, which takes into consideration the finiteness of the layer structure. The diffraction term is expressed in terms of integrals containing the auxiliary polarization currents. Some of these integrals can be evaluated analytically. Numerical and experimental results are presented.