An antenna is a transformer that transforms guided waves propagating on a transmission line into electromagnetic waves propagating in an unbounded medium (usually free space), or vice versa. The component used in radio equipment to transmit or receive electromagnetic waves. Engineering systems such as radio communication, broadcasting, television, radar, navigation, electronic countermeasures, remote sensing, radio astronomy, etc. rely on antennas to transmit information using electromagnetic waves. In addition, when using electromagnetic waves to transmit energy, non signal energy radiation also requires antennas. Generally, antennas have reversibility, meaning that the same pair of antennas can be used as both transmitting and receiving antennas. The basic characteristic parameters for transmitting or receiving the same antenna are the same. This is the reciprocity theorem of antennas.