Near-field gain calibration for large spherical antennas

In the near field of the Arecibo spherical antenna radar system, i.e., at ranges less than about 260 km, the gain is a function of range and a knowledge of gain is necessary for deducing electron-density information from the power backscattered from the ionosphere. A method of obtaining the phase taper across the aperture (and hence the near-field on-axis and backscatter gains versus range), given a knowledge of the far-field on-axis gain versus frequency, and the amplitude of the illumination over the aperture is suggested. Our studies show that measurements of on-axis far-field gain over a bandwidth of MHz are adequate to give an accurate indication of on-axis gain versus range in the near field, while a MHz frequency spread is needed to give accurate information on the backscatter gain versus range. The near-field correction for the antenna has also been estimated from measurements made on a model of the new line feed. Confidence in the validity of this approach has been obtained by comparing the measured far-field on-axis gain versus frequency with that calculated using the data from the model feed.