An antenna is a converter that transforms a traveling wave traveling along a transmission line into an electromagnetic wave traveling in an unbounded medium (usually free space) , or vice versa.
A component used in radio equipment to transmit or receive electromagnetic waves. Radio Communications, radio, television, radar, navigation, electronic countermeasure, remote sensing, radio astronomy and other engineering systems that use electromagnetic waves to transmit information rely on antennas for their work. In addition, in the use of electromagnetic wave transmission of energy, non-signal Energy Radiation also requires an antenna. General Antennas are reversible, that is, the same antenna can be used as a transmitting antenna, also can be used as 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.