A Reduced Sampling Scheme for Planar Near-field Measurements Using Pointwise Probe Correction in the Spherical Harmonics Basis

Planar Near-Field (PNF) measurements are a reliable way to determine the radiation characteristics of an Antenna Under Test (AUT) when only a plane is of interest and far-field measurements are not an option. This plane is classically sampled in Cartesian coordinates on a grid following the Nyquist criterion, i.e., using sampling points with a distance of at least λ/2 in both axes. This sampling is normally kept independently of the distance between the plane and the AUT. Spherical Near-Field (SNF) measurements, on the other hand, are classically performed by sampling the surface of the minimum sphere enclosing the AUT following the Nyquist criterion. In this work, a reduced sampling grid for PNF measurements is introduced by using the radial projection of the sampling points on the minimum sphere of a classical SNF measurement onto a plane. Depending on the sampling, different solvers in the spherical harmonics basis are considered and the spherical mode coefficients are calculated. To describe the sampling points on a plane, pointwise probe correction is used, where different probe response constants are calculated for each measurement point. In this way, a different probe distance and probe orientation at each point are considered. Reconstruction results are compared to those of standard PNF sampling, showing that a reduction of measurement points by processing the acquired data in another basis is possible.