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