Antenna feed
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In telecommunications and electronics, an antenna feed refers to the components of an antenna which feed the radio waves to the rest of the antenna structure, or in receiving antennas collect the incoming radio waves, convert them to electric currents and transmit them to the receiver. Antennas typically consist of a feed and additional reflecting or directive structures (such as a parabolic dish or parasitic elements) whose function is to form the radio waves from the feed into a beam or other desired radiation pattern.
In simple antennas the feed usually consists of the feed antenna (driven element), the part of the antenna which actually converts the radio frequency currents to radio waves or vice versa, and the feed line (transmission line), which connects the feed antenna with the receiver or transmitter. For example, in a rooftop Yagi television antenna, the feed consists of a dipole driven element, which converts the radio waves to an electric current, and a coaxial cable or twin lead transmission line which conducts the received signal from the driven element into the house to the television receiver. The rest of the antenna consists of rods called parasitic elements, which strengthen reception from a given direction.
In more complex antenna systems the feed can be more complicated. The antenna feed is usually considered to be all the components between the beam-shaping part of the antenna and the receiver's first amplifier (called the RF front end, the LNB or LNA). For a transmitting antenna, the feed consists of everything after the last power amplifier, and might include an antenna tuner unit and impedance matching sections at the antenna. In a radar or satellite communications antenna the feed might consist of a feed horn, orthomode transducer, polarizer, frequency diplexer, waveguide, waveguide switches, rotary joint, etc.
Particularly with a transmitting antenna, the antenna feed is a critical component that must be adjusted to work correctly with the antenna and transmitter. Each type of transmission line has a specific characteristic impedance. This must be matched to the impedance of the antenna and the transmitter, to transfer power efficiently to the antenna. If these impedances are not matched it can cause a condition called standing waves on the feed line, in which the RF energy is reflected back toward the transmitter, wasting energy and possibly overheating the transmitter. This adjustment is done with a device called an antenna tuner in the transmitter, and sometimes a matching network at the antenna. The degree of mismatch between the feedline and the antenna is measured by an instrument called an SWR meter (standing wave ratio meter), which measures the standing wave ratio (SWR) on the line.
