Antenna feed

A radio transmitter or receiver is connected to an antenna which emits or receives the radio waves. The antenna feed system or antenna feed is the cable or conductor, and other associated equipment, which connects the transmitter or receiver with the antenna and makes the two devices compatible. In a radio transmitter, the transmitter generates an alternating current of radio frequency, and the feed system feeds the current to the antenna, which converts the power in the current to radio waves. In a radio receiver, the incoming radio waves excite tiny alternating currents in the antenna, and the feed system delivers this current to the receiver, which processes the signal. To transfer radio frequency current efficiently, the feedline connecting the transmitter or receiver to the antenna must be a special type of cable called transmission line. At microwave frequencies, waveguide is often used, which is a hollow metal pipe carrying radio waves. In a parabolic (dish) antenna the feed is usually also defined to include the feed antenna (feed horn) which emits or receives the radio waves. Particularly in transmitters, the feed system is a critical component which impedance matches the antenna, feedline, and transmitter. To accomplish this, the feed system may also include circuits called antenna tuning units or matching networks between the antenna and feedline and the feedline and transmitter. On an antenna the feed point is the point on the driven antenna element at which the feedline is connected. In a transmitter, the antenna feed is considered to be all components between the transmitter's final amplifier and the antenna's feedpoint. In a receiver, it is all components between the antenna and the receiver's input terminals. In some cases such as parabolic dishes it is also defined to include the feed antenna or feed horn. In some radios the antenna is attached directly to the transmitter or receiver, such as the whip antennas mounted on walkie talkies and portable FM radios, the sleeve dipole antennas of wireless routers, and the PIFA antennas inside cellphones.