Far-Field Characterization of High-Gain Antennas at 300-GHz Band Based on a Photonics-Based Near-Field Measurement

This paper presents a photonics-based near-field measurement approach for characterizing far-field radiation patterns of high-gain antennas operating at 300 GHz. Utilizing an electro-optic (EO) probe for electric near-field sensing, the system enables stable and precise acquisition of amplitude and phase over long measurement durations, which is essential for evaluating antennas with nominal gains exceeding 40 dBi. Far-field patterns reconstructed from near-field data using cylindrical mode expansion showed excellent agreement with direct far-field measurements conducted at an outdoor site with an antenna separation of 53 m. This result confirms that tabletop photonics-based near-field systems can serve as practical alternatives to outdoor far-field ranges at sub-terahertz frequencies. Quantitative evaluation using the equivalent noise level (ENL) metric demonstrated high pattern fidelity, with average ENL values below −35 dB within the −10 dB beamwidth region. Fine structural features, including low-level sidelobes, were also accurately reproduced. Comparisons with the ITU-R F.699-8 reference radiation pattern revealed narrower predicted beamwidths and underestimated sidelobe levels, likely due to model limitations and radome effects, suggesting the need for updates above 86 GHz. The proposed method offers a scalable and practical solution for antenna metrology in frequency regimes where conventional direct far-field measurements and CATR systems face significant implementation challenges.