Shaped beam antenna for Earth coverage from a stabilized satellite

The feasibility and practicality of achieving improved performance from a stabilized antenna on a synchronous satellite by shaping the beam for optimum earth coverage is shown. Due to the difference in range and atmospheric attenuation from a synchronous satellite to various points on the earth, a conventional beam with maximum gain toward the center of the earth is inefficient because it has the highest gain where the least gain is required. Since the paths tangential to the earth are longest and since they traverse through more atmosphere, the antenna gain should be highest in this region and decrease to a minimum for the path normal to the earth (see Fig. 1). The flat portions at the edge of the "ideal" pattern allows for stabilization errors of the satellite. The ideal pattern has been calculated to give equal effective signal (including noise due to atmospheric attenuation and assumed preamplifier noise figures of the ground terminal and satellite) over the entire portion of the earth covered by the antenna beam. The ideal pattern is rotationally symmetrical and has the capability of dual orthogonal polarization. A relatively simple low-loss antenna that gives a good approximation to the ideal pattern is described. The antenna is a nine-horn array consisting of a large central horn surrounded by a ring of eight smaller horns. The central horn is multimoded to provide rotational symmetry and polarization purity. An optimum power division and relative phasing between the central and outer horns is made to give a pattern that closely approximates the ideal. Both theoretical and measured data are presented. Measured results confirm the beam shaping, rotational symmetry, polarization purity improved gain, and broad bandwidth expected from the array. A gain improvement of more than 2 dB over a well-designed conventional horn antenna has been demonstrated.