An antenna is a converter that converts a traveling wave traveling on a transmission line into an electromagnetic wave traveling in an unbounded medium (usually free space), or vice versa. A part of a radio device used to transmit or receive electromagnetic waves. Engineering systems such as radio communication, radio, television, radar, navigation, electronic countermeasures, remote sensing, radio astronomy, etc., which use electromagnetic waves to transmit information, all rely on antennas to work. In addition, in terms of transmitting energy with electromagnetic waves, non-signal energy radiation also requires an antenna. General antennas are reversible, that is, the same antenna can be used as both a transmitting antenna and a receiving antenna. The basic characteristic parameters of the same antenna as transmitting or receiving are the same. This is the reciprocity theorem for antennas.