Design and Verification of Innovative Wideband Spherical Near Field Probes with High Modal Purity

Measurements of modern multi-service antenna systems require ever increasing bandwidths of the measurement equipment. The main limiting factor of traditional Spherical Near Field (SNF) systems is mainly the probe, as it should radiate only first-order azimuthal spherical modes to apply first-order Probe Correction (PC). Conventional first-order probes are based on Ortho-Mode Junctions (OMJ) with externally balanced feeding. Unfortunately, realistic 3dB/180° couplers are characterized by excitation errors and, consequently, higher order spherical modes are launched into the probe. Besides the possibility of using full PC, the propagation of the spurious spherical modes can be mitigated at probe level with dedicate designs. In this paper, two innovative high performance SNF probes will be presented. An advanced feeding geometry allows the first probe to provide a high modal purity in the 617-960 MHz band, rejecting errors induced by the coupler. In the second probe, a highly accurate coupler has been designed to minimize the higher order modes on a large bandwidth, 1427-4200MHz. The two probes have been designed as part of an upgrade of a gantry arm system for modern base station antennas. The same measurement system has been used to calibrate the two probes and verify the expected performance in terms of radiation pattern and spherical modal content.