The antenna is a kind of converter, the guided wave propagation of transmission line, transform into media (usually in unbounded free space) electromagnetic wave propagation, or reverse transform. Transmit or receive electromagnetic wave used in radio equipment parts. Radio communication, radio, TV, radar, navigation, electronic warfare, remote sensing, radio astronomy and so on system engineering, who use electromagnetic waves to transmit information, depend on the antenna to work. In addition, in the use of electromagnetic wave transmission of energy, non signal energy radiation is also required for the antenna. The general antenna is reversible, that is, the same antenna can not only be used as a transmitting antenna, but also can be used as a receiving antenna. The same as the basic characteristic parameters of antenna transmitting or receiving the same. This is the reciprocal theorem of antenna.