Experimental verification of a low cross-polarization contoured beam antenna

An offset Gregorian dual reflector antenna whose configuration satisfies tan(/spl alpha//2)=(e+1)/(e-1)tan(/spl beta//2) radiates a pencil beam without geometrical optics induced cross-polarization, In the equation, e is the eccentricity of the subreflector, /spl beta/ is the angle between the major axis of the ellipsoidal subreflector and the axis of the paraboloidal main reflector, and /spl alpha/ is the angle between the feed axis and the major axis of the subreflector. A contoured beam is often required for satellite communication applications, instead of a pencil beam. Furthermore, it is highly desirable that the antenna contour be provided by a shaped reflector due to its simplicity. There are two fundamental uncertainties in this approach: 1. Since a shaped reflector has no focal point, the above equation can no longer be satisfied explicitly. 2. For a highly shaped contour antenna, the peak of the cross-polarization pattern will be in the coverage area. Thus, it may not be possible to achieve a 33 dB C/I, a typical requirement for polarization reuse antennas. Nevertheless, the shaped dual-reflector antenna appears to be a good candidate for achieving low cross-polarization contoured beam. In order to verify this proposition, the authors designed, fabricated and tested a dual shaped reflector antenna.