Offset satellite antenna is a section of a paraboloidal reflector with oval form. The antenna surface is not round anymore, but oval (ellipse) and symmetric. The focal point is not mounted in the middle of the dish, but to the side of the dish (offset). Thus, the advantage of this technology, is that the antenna surface is not being shadowed by the feed anymore (from the satellite's point of view).
Another advantage is the less likelihood that snow will build up on the dish. The offset satellite antenna does not "look" straight up to the satellite as a Prime Focus antenna does, but it's inclined about 25° downward (almost upright position). Nevertheless, an offset dish appears for the satellite to be circular having the horizontal diameter.
Offset Gregory double focus antenna
The offset Gregory antenna has an additional subreflector. This way, the antenna efficiency and the gain of this antenna type increase (Gregory double focus principle)!
The antenna efficiency of an offset Gregory with 90 cm size corresponds to a normal 110 cm offset antenna.
A Gregory antenna is more expensive but manufactured to much tighter specifications. Compared to a normal offset antenna with the same size of the main reflector, it brings about 12% more gain.
Technical data of a Masterfocus/Fibo antenna:
Antenna diameter: 90 x 99 cm
Antenna efficiency: 82%
Cross polarisation isolation: <= -35 dB
VSWR: < 1.12
Beam width (3dB): 1.57°