An antenna (All antennas) is a converter that transforms a guided traveling wave propagating over a transmission line into electromagnetic waves propagating in an unbounded medium (usually free space), or the searcher performs the reverse transformation. A component used in radio equipment for transmitting or receiving electromagnetic waves. Radio communication, radio, television, radar, navigation, electronic warfare, radio astronomy, remote sensing and other engineering systems usually use electromagnetic waves to transmit information, rely on to carry out the work of antenna. In addition, non signal energy radiation is also required for the transmission of energy with electromagnetic waves. The general antenna has reversibility, that is, the same antenna can be used as a transmitting antenna or a receiving antenna. The basic characteristic parameters of the same antenna as transmit or receive are the same. This is the reciprocity theorem of the antenna.