Positioning of orthogonally-polarized waveguide probes for near-field antenna measurement

In planar near-field antenna measurement, the full polarization characterization time of an antenna-under-test can be shortened by simultaneously using orthogonally-polarized probes. This paper discusses the positioning of two orthogonally-polarized probes in a near-field scanning system. The studies are conducted through full-wave simulations of tapered open-ended WR-75 waveguides operating at Ku-band. The studied probe characteristics are the mutual coupling between the orthogonallypolarized probes and the radiation pattern. The simulation studies concluded that the positioning along the major axes of the probe affects the radiation pattern more whereas the positioning along the diagonal direction increases the mutual coupling between orthogonal polarizations. Thus, the appropriate positioning is a tradeoff between these two parameters. For example, in order to maintain the ripples in the radiation pattern below 0.3 dB within +/- 60 degrees and port-to-port isolation below -35 dB, the orthogonally-polarized probes are recommended to be separated by more than 2-lambda distance and aligned along the diagonal direction.