Study of Dual-Polarization Scattering Probe for Near-Field Antenna Measurement

Abstract

In this dissertation, a dual-polarization scattering probe for near-field antenna measurement is investigated and experimentally validated. Generally, a scattering probe used in the near-field of the antenna under test (AUT) should cause minimal disturbances in the measured field, ensuring that the field in the presence of the probe remains as close as possible to the field in its absence. Simultaneously, the probe should have a large scattering effect to enhance the measurement system's performance. Balancing these conflicting requirements poses a significant challenge. This study proposes a scattering probe based on an electrically invisible dipole (EID) that improves modulation effects while reducing environmental influences. Before this proposal, a one-port near-field antenna measurement technique employing a geometrically simple wire scatterer was investigated. The wire scatterer was scanned on a planar surface at intervals smaller than a quarter wavelength, and measurements were taken exclusively at the AUT's port without using a probe antenna. A 2-dimensional phase unwrapping method was applied to eliminate the 180° phase uncertainty in near-field data, enabling its transformation into a far-field pattern. Additionally, a compensation technique using the scatterer's scattering pattern was proposed to enhance measurement accuracy. The proposed measurement technique and scatterer compensation method were validated using a 2 x 1 horn antenna as the AUT. To further improve measurement accuracy and reduce measurement time, an electronically controllable dual-polarization scattering probe with high modulation depth (MD), based on the EID concept, was developed. This probe features dual-polarization control capabilities, such as vertical and horizontal polarization. The probe was implemented on a substrate using four PIN diodes and four capacitors. Frequency shifts introduced during manufacturing due to nonidealities of diodes and substrate were compensated with an offset length. The performance of the proposed probe was verified through experiments with actual implemented probes, which showed good agreement with simulation predictions. The proposed dual-polarization scattering probe exhibited high MD values for both polarizations across a wide bandwidth, suggesting its potential to increase the sensitivity of the measurement system.