Feed, the heart of antenna, serves as junior radiator of high-gain assembled antenna and provides efficient illumination for parabolic antenna.

1. It has suitable directional diagram. Feed junior directional diagram cannot be too narrow, or the paraboloid cannot be totally irradiated. But it cannot be too wide so as not to leak too much power. Besides, junior directional diagram should be near to rotational symmetry without sidelobe and tail lobe.
2. It has ideal wave front. Circle parabolic antenna requires that feed wave front is sphere to ensure that parabolic diameter phase is evenly distributed when phase center coincides with focus. Otherwise, it will give rise to distorted antenna directional diagram, reduced gain and increased sidelobe.
3. No cross-polarization. That is, there is no cross component of main polarization. It requires that cross-polarization component of feed radiation field should be as little as possible.
4. Impedance changes stably. It requires that input impedance of feed shouldn’t change dramatically to match with feed line within operational band.
5. Size should be as small as possible. The whole feed system mainly consists of feed horn, 90°phase shifter and rounded rectangle converter. The feed can be divided into feed of feed forward and feed of feed back according to usage mode. And it can be divided into C-band feed and Ku-band feed according to satellite band. At present, C-band and Ku-band shared feed has been developed. Feed of feed forward is applied in ordinary parabolic antenna and feed of feed back is applied in Cassgrain antenna. Common feed form of parabolic antenna has pyramid horn, conical horn, open-end waveguide and corrugated horn, etc. feed of feed forward uses corrugated slot feed mostly, and also coaxial waveguide feed with choke groove. Feed of feed back horn uses dielectric load horn which is common conical horn with the addition of a length of Polytetrafluoethylene liner. Offset antenna chooses offset feed. Offset feed’s ripple disc is funnel-shaped while primary antenna’s ripple disc is horizontal.

1. Polarized wave

When radio wave propagates in the space, its electric field vector’s instant orientation is called polarization. When radio wave propagates, the electric field vector’s space locus is a straight line and always propagates in a plane, so it is called linear polarized wave. Linear polarized wave can be divided into two categories which are horizontal wave and vertical wave. If the electric field vector’s space locus is a circle, namely, the field vector constantly rotates around axis of propagation direction, so it is called circular polarized wave.

Radio wave polarization performance depends on polarization performance of transmitting antenna feed system. Receiving antenna should possess same polarization and rotation performance with transmitting antenna to realize polarization matching, thus it receives all energy. If matching partially, it only receives the portion of energy. A linear polarization wave can be decomposed into two circular polarization wave rotating in opposite direction while a circular polarization wave can be decomposed into linear polarization wave which is mutually orthogonal. Therefore, antenna receiving linear polarization wave can also receive circular polarization wave. And antenna receiving circular polarization wave can also receive linear polarization wave, but it will lose 3dB energy. Either linear polarization antenna or circular polarization antenna cannot receive their own orthogonal component.

2. Polarizer

Polarizer is the device to control polarization direction of antenna feed system, which is used to choose polarization pattern which is in line with satellite TV signal and to restrain other patterns of polarization wave in order to acquire polarization matching and realize best receiving. In the modern satellite TV transmission, different TV programs can be transmitted by making use of mutual isolation performance between vertical and horizontal polarization, left-hand circular polarization and right-hand circular polarization, so as to improve satellite’s transmission capacity. Circular polarization or linear polarization has pluses and minuses. When circular polarization traveling through rain-mist layer and the ionosphere, its entry loss is low and it doesn’t have the problem of polarization panel rotation of linear polarization wave. And the greatest advantage of linear polarization wave is simple realization.