An antenna includes a main reflector with a focal circle about an axis. A waveguide horn extends from the main reflector along this axis. A sub-reflector is mounted beyond the distal end of the waveguide horn with an annular gap remaining between the sub-reflector and the distal end of the waveguide horn. The annular gap forms a radial transmission line between a first wall on the sub-reflector and a second wall having outer annular corrugated surfaces on the distal end of the waveguide horn. The first wall of the transmission line is a body of revolution of elliptical shape on the sub-reflector. This ellipse is tilted at an angle with respect to the axis, and has a first focal point on the axis. The other focal point is preferably on or near the focal circle. The phase center of the waveguide horn does not coincide with either foci.