Off focus characteristics of the offset fed parabola

The results of a theoretical and experimental investigation are presented for a dual mode horn fed offset parabola for feed displacements of wavelengths about the focus in a direction along the offset axis. Beamwidths, sidelobe levels, relative gain, and crosspolarization patterns exhibit approximately equal E and H plane beamwidths as the feed is moved, along with wide angle sidelobes outside the main ridged beam that are similar in level to those of the same antenna with the feed on focus. The cross-polarization pattern levels diminish as the feed is moved off focus. The ridged main patterns have a shape similar to that of a corrugated horn with the same quadratic phase error. Therefore for some applications, it should be possible to use a defocused offset parabolic reflector rather than a corrugated horn, thereby effecting a sizeable decrease in antenna volume.