Spherical field transformation for hemispherical antenna measurements above perfectly conducting ground planes

The spherical multipole based near-field far-field transformation is extended to near-field antenna measurements above a perfectly electrically conducting (PEC) ground plane. As the effect of the ground plane is considered in the transformation by applying the image principle to the spherical modes radiated by the device under test (DUT), the near-field measurement points above the ground plane are sufficient to fully characterize the radiation behavior of the DUT above PEC ground. The nonequispaced fast Fourier transform (NFFT) is employed in the forward operator of the inverse problem in order to apply the transformation to e.g. spiral scans which are favorable to large and heavy scanner systems. If the elevation axis is located above or below the ground plane, an additional translation operator is integrated into the transformation to consider such an offset in the mechanical system. The proposed method is applied to synthetic and simulated automotive antenna near-field data in order to show its effectiveness.