Uncertainties Introduced by the Probe in Millimeter-Wave Planar Near-Field Measurements

No measurement is complete without a thorough investigation of the involved uncertainties. This article investigates uncertainties introduced by the probe in planar near-field (NF) measurements at the mm-wave frequencies 90 and 140 GHz. This includes probe compensation and multiple reflections between the probe and the antenna-under-test (AUT). A combination of simulations and measurements is used to achieve an as accurate as possible estimation of each uncertainty. Manufacturing uncertainties in the probe as well as the placement of electromagnetic (EM) absorbers on the probe flange show a significant influence on the transformed planar NF radiation patterns. When using an open-ended rectangular waveguide (OEWG) as a probe, manufacturing errors can reduce the trueness of the probe pattern and the resulting far-field (FF) pattern of the AUT to below 90% for θ > 17° (90 GHz) or θ > 22° (140 GHz). The presence of EM absorbers on the flange can cause the precision to drop below ±0.25 dB at broadside. Therefore, both these effects should be modeled beforehand to obtain an accurate radiation pattern of the AUT.