1. Have suitable radiation pattern: Primary radiation pattern of feed can’t be too narrow, or the paraboloid won’t be shined completely; but it also can’t be too wide for fear that the power leakage is overmuch. Furthermore, the primary radiation pattern should close to rotational symmetry, without sidelobe and end lobe will be the best.
2. Have perfect wavefront: Round parabolic antenna requires the wavefront of feed with sphere so that the phase of paraboloid caliber field is distributed uniformly when phase center and focal point conincide. Or, the antenna radiation pattern will happen distortion, gain reducing and sidelobe increasing.
3. Without cross-polarization: without cross component to interfere co-polarization, which requires the cross component of feed radiation field as small as possible.
4. Steady impedance change: within operating frequency range, input impedance of feed shouldn’t change excessively to assure feed line match with it.
5. Size is as small as possible: A whole feed system mainly consists of feedhorn, 90°phase shifter and circle moment converter. The usage mode of feed can be divided into forward feed and back feed. According to satellite frequency, it can be divided into C-band feed and Ku-band feed, also we have C&Ku dual band feed system at present. The forward feed is always used in common parabolic antenna, while back feed often is used in Cassagrain antennas.
6. Parabolic antennas often use the following feed types: pyramid horn, conical horn, opening waveguide and ripple horn etc. Among the forward feed, ripple slot feed is used mostly. There is also another coaxial waveguide feed with choke groove.
The medium loaded horn is often used as back feed horn, which is made by adding a Teflon bush in common conical horn. Offset antenna should choose offset feed. The ripple of offset feed plate is funnel-shaped, while the ripple plate of forward feed is horizontal type.