Page 190 - Satellite Communications, Fourth Edition

170 Chapter Six

Figure 6.24 Offset Gregorian antenna. (Courtesy of Radio Electr. Eng.,
vol. 54, No. 3, Mar. 1984, p. 112.)

directions and reduce it in others. The design steps start with a map
of the ground coverage area desired. A grid is overlaid on the map, and
at each grid intersection a weighting factor is assigned which corre-
sponds to the antenna gain desired in that direction. The intersection
points on the coverage area also can be defined by the azimuth and ele-
vation angles required at the ground stations, which enables the beam
contour to be determined. The beam-shaping stage starts by selecting
a smooth parabolic reflector that forms an elliptical beam encom-
passing the coverage area. The reflector surface is computer modeled
as a series of mathematical functions that are changed or perturbed