Investigation of Spherical higher order modes sources in antenna measurement probe design

Classical probe corrected Spherical Near Field (SNF) measurement assumes probes with limited |μ|=1 spectrum [1]-[3]. This requirements is needed to fully compensate the effect of the probe during the NF/FF transformation [3]. If the probe spectrum contains higher order modes, a residual error will affect the measured pattern (probe modal truncation). This requirement leads to challenging probe designs, especially if the required bandwidth is wide and/or there is a limitation on the acceptable physical dimension. Consequently, in many practical cases, higher order spherical modes are unavoidable and the design task is to minimize their impact on the probe quality. In this paper, the source of higher order spherical modes of the MVI SP1100 L-band probe is investigated by post-processing the calibration data from NF measurement. Reconstructed equivalent currents associated to higher order modes have been computed on a conformal geometry encompassing the probe using MV-INSIGHT processing [6]. Using a combination of spherical mode filtering and equivalent currents, the origin of such higher order modes is evidenced. Knowledge of these sources are useful to improve future probe designs.