An antenna is a converter that converts a traveling guided wave traveling along a transmission line into an electromagnetic wave traveling in an unbounded medium (usually free space), or vice versa. A component in radio equipment used to transmit or receive electromagnetic waves. Radio communication, broadcasting, television, radar, navigation, electronic countermeasures, remote sensing, radio astronomy and other engineering systems, all use electromagnetic waves to transmit information, rely on antennas to work. In addition, in terms of transmitting energy by electromagnetic waves, non-signal energy radiation also requires antennas. General antennas have reversibility, that is, the same antenna can be used as both transmitting antenna and receiving antenna. The basic characteristic parameters of the same antenna as transmitting or receiving are the same. That's the reciprocity theorem for antennas.