Free-space antenna far-field extraction from near-field measurements above metallic ground

The unknown spherical mode coefficients of an antenna radiating above an infinite perfectly electrically conducting ground plane are determined from near-field measurements by considering the ground plane in the forward operator of the inverse problem which is efficiently solved for the antenna sources by employing the iterative generalized minimum residual solver (GMRES). However, two mechanisms may lead to deviations of the reconstructed sources from the free-space antenna sources. First, dependent on the height of the antenna above the ground, the forward operator might not be exactly invertible such that the sources can not be accurately determined. Second, the reconstructed sources are different from the free-space sources due to inevitable coupling effects between the antenna and the metallic ground. Based on numerical experiments, the capability of the applied spherical field transformation approach to extract the free-space antenna sources from measurements above a metallic ground plane is investigated in detail.