Accurate Determination of Antenna Gain in the 300-GHz Band Using Amplitude Center Estimated From Electrooptic Near-Field Measurements

Antenna gain determination using the Friis equation typically employs the geometrically determined physical distance between antenna apertures, which often underestimates the gain. Herein, we propose and demonstrate an accurate antenna-gain determination method using the distance between the amplitude centers (ACs) of the antennas determined from the spatial phase distribution of the near field measured based on an electrooptic technique. Employing a WR-3.4 horn antenna, we show that at 286 GHz, the proposed method determines antenna gain with the same level of error even when the antenna separation distance is 1/5 shorter than that obtained using the conventional method that employs the physical distance between the antenna apertures in the Friis equation. Furthermore, the gain calculated using the proposed method achieves an error margin of <0.08 dB compared with that determined using the extrapolation method, which also utilizes the geometric physical distance between the antenna apertures. Notably, as the AC can be directly measured from the near-field distribution, the proposed method enables accurate antenna gain measurements without requiring prior knowledge of the characteristics and structural details of the antenna.