An antenna is a transducer that transforms a guided wave propagating on a transmission line into an electromagnetic wave propagating in an unbounded medium (usually free space), or vice versa. A component used in a radio device to transmit or receive electromagnetic waves. Engineering systems such as radio communication, broadcasting, television, radar, navigation, electronic countermeasures, remote sensing, radio astronomy, etc., all use electromagnetic waves to transmit information, relying on antennas to work.
In addition, non-signal energy radiation also requires an antenna in terms of transmitting energy using electromagnetic waves. Generally, the antennas are reversible, that is, the same antenna can be used as both a transmitting antenna and a receiving antenna. The same characteristic parameters of the same antenna as transmitting or receiving are the same. This is the reciprocity theorem of the antenna.
Classification of antennas:
1 According to the nature of work can be divided into transmitting antenna and receiving antenna.
2 According to the use, it can be divided into communication antenna, broadcast antenna, TV antenna, radar antenna and so on.
3 According to the working wavelength, it can be divided into ultra long wave antenna, long wave antenna, medium wave antenna, short wave antenna, ultra short wave antenna, microwave antenna and so on.
4 according to the structure and working principle can be divided into line antennas and surface antennas. Describe the characteristic parameters of the antenna with pattern, directivity coefficient, gain, input impedance, radiation efficiency, polarization and bandwidth