Computational electromagnetic modelling of a probe employed in planar near field antenna measurements

Planar near field antenna measurement techniques have become very popular. This is partly due to the ease and efficiency with which asymptotic far field parameters can be obtained from near field data using spectral techniques, and the removal of the requirement to displace the antenna under test during the sampling process. In order that reliable far field patterns can be obtained from planar techniques, the plane wave receiving coefficients of the scanning probe must be taken into account during the transformation process. Characterisation obtained from an auxiliary measurement of the probe can prove expensive in terms of facility time and cost, and inevitably require a degree of post processing. It would be attractive to obtain the plane wave receiving coefficients of the scanning probe directly from theory or from numerical simulation. The paper discusses an approach taken to obtain reliable far field parameters from the Ansoft high frequency structure simulator (HFSS), a commercially available finite element modelling tool that is not always considered when analysing electrically small antennas. Results are presented that illustrate the degree of agreement attained between numerical simulation and measurement obtained by using a quasi far-field polar spherical facility.