Antenna is a converter that converts the traveling wave propagating on the transmission line into electromagnetic waves propagating in unbounded media (usually free space), or vice versa. It’s a part to transmit or receive in the radio device.
We know that radio equipment,such as communications, radar, navigation, radio and television , transmits information via radio waves, which require radio radiation and reception. In radio device, the equipment of radiation and receive radio waves is called antenna. The antenna provides the required coupling between the transmitter or receiver and the medium for transmitting radio waves. Antenna and transmitter and receiver is at same, is a important part of radio device.
The antenna radiates radio waves and receives radio waves, However, the transmitter through the feeder into the antenna is not a radio wave, the receive antenna can not send the radio wave directly to the receive via the feeder, the processing must has energy exchange.