A Feed Horn is the VSAT antenna feed component completed by a waveguide. Feed horns typically have a beam width of 30-90 degrees, and are used to illuminate the main reflector, which then funnels the energy into a narrow beam. Feed horn apertures come in various shapes such as rectangular, circular, or even elliptical. The horn usually has a feed window, which is a non-conducting film stretched over the opening to keep moisture out.
You can also view the placement of the Feed Horn below in the VSAT Installation Manual Components Chapter: