Diagram showing the most common types of parabolic antenna feeds.Axial or front feed.  This is the most common type, with the feed antenna suspended in front of the dish on the beam axis, at the focal point, pointed back toward the dish.  One drawback of this type is that the feed and its supports partially block the beam.
Off-axis feed or offset-feed.  The primary reflector is asymmetric, with the focus, and the feed, located to one side, outside the beam area.  This avoids the problem of the feed obstructing the beam.  It is used in home satellite dishes. Offset feed can also be used in multiple reflector designs such as the Cassegrain and Gregorian, below.
Feed with secondary reflector. In in these designs the feed is located on or behind the dish, and its beam illuminates a small secondary reflector. Cassegrain design use hyperboloidal secondary reflector, in Gregorian design the secondary reflector has a concave ellipsoidal shape, one of possible Ring focus designs is derived from Gregorian offset and uses elliptical conical secondary reflector
This file was derived from: Parabolic antenna types2.svg:
Vsatinet