Correction of non-ideal probe orientations for spherical near-field antenna measurements

Spherical near-field scanning is a standard method to measure the radiation characteristic of an antenna under test (AUT). Due to the required near-field to far-field transformation of the measured data, an accurate position and orientation of the probe during the measurement is crucial. This might be difficult to achieve if, for example, a robotic arm system is used to position the probe. Different methods for correction of non-ideal measurement positions have been presented in the past. In contrast, the related non-ideal probe orientations in a spherical near-field measurement system have not been comprehensively analyzed due to the assumption that the error is small since the probe receiving pattern is typically broad (e.g. an open-ended waveguide). In this paper, it is shown that non-ideal probe orientations can be included in the spherical near-field to far-field transformation. This is achieved by additional rotations of the probe receiving coefficients in the probe response calculation. The introduced pointwise higher-order probe correction scheme allows an exact spherical wave expansion of the radiated AUT field. The proposed method is used to investigate the error due to non-ideal probe orientations by simulation and measurement. The presented results can be used to estimate the error due to non-ideal probe orientations. It is verified that the error is typically small compared to other error sources in a practical measurement even if a directive higher-order probe is used. Nevertheless, including the probe orientation generally improves the accuracy of the measurement result.