Study of element patterns and excitations of the line feeds of the spherical reflector antenna in Arecibo

The element patterns of the circularly polarized line feeds for the spherical reflector in Arecibo are calculated, showing strong endfire radiation. The radiation fields of the line feeds are then obtained by adding up the contributions from all elements. The optimum excitation of the feed is found by an approximate stationary-phase solution of the radiation field. This shows that the element pattern gives a large phase contribution to the excitation, because of the finite diameter of the feed. If not corrected for, this phase error can cause losses up to 1.5 dB. The excitation is optimized further by cutting and trying. The validity of the analytical models is checked by calculation of the radiation pattern of the existing 96.6 ft 430 MHz feed. The excitation is modeled from measured phases along the feed. The calculated radiation patterns show good agreement with the measurements, including the phase errors and the dip in the center. By using the proposed new excitation it should be possible to increase the efficiency of the 96.6 ft feed by 0.7 dB and to increase the efficiency of the 40 ft feeds at higher frequencies by even more.