Spherical Field Transformation above Perfectly Electrically Conducting Ground Planes

A new spherical field transformation approach for near-field measurements above perfectly electrically conducting (PEC) ground planes at arbitrary heights is presented. It provides higher order probe correction and treatment of irregular sample point locations on the measurement surface but has the same low computational complexity as the known fast Fourier transform (FFT) accelerated spherical field transformation with first-order probe correction. The infinite PEC boundary is considered by applying the image principle to the spherical modes of the device under test (DUT). Only the measurements above the ground are required to iteratively solve the inverse problem for the unknown spherical mode coefficients. Due to additional modal translation operators, the scan center of the spherical measurement system does not need to lie in the ground plane. Moreover, if the DUT is positioned at an appropriate height above the ground plane, the free-space sources of the DUT are extractable with good accuracy. By employing the nonequispaced FFT, the near field can be irregularly sampled on the spherical measurement surface. Numerical investigations show the capabilities of the proposed field transformation approach. Its practical relevance and efficiency are demonstrated for simulated automotive antenna measurements and real near-field measurements above a metallic ground.