A variation on the basic Axisymmetrical parabolic reflector fed by an on-axis horn antenna is the Offset horn-fed parabolic reflector. One main advantage of offsetting the feed is that it eliminates aperture blockage caused by the feed horn and the mounting struts when using an on-axis feed.
The version of this topology included in Antenna Magus is a Symmetric offset parabolic reflector fed by a pyramidal horn antenna - although a variety of other feed antennas may be used, including simple conical horns, corrugated conical horns, axial-choke horns and coaxial-choke horns, to name a few.Ray collimation of the offset parabolic reflector antenna
Positioning the feed is more complex when compared with the on-axis fed reflector antenna, as the feed horn has to be carefully placed so the phase center coincides with the reflector's phase center. ...
In addition to reducing aperture blockage losses sidelobes caused by diffraction and cross-polarization effects are reduced by offset feeding. The asymmetry of the offset reflector, however, introduces other anomalies and asymmetries in the radiation pattern. 'Vertical' symmetry prevents cross-polarization in that plane while cross-polarization is increased in the 'horizontal' plane.
Also included in Antenna Magus 2.3 is a complimentary pattern fed version of this antenna which is very useful for quick simulations where coupling between the reflector and feed can be ignored.