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. In this case the feed system just consists of an impedance matching circuit (if needed) between the antenna and transmitter or receiver, which matches the impedance of the antenna to the radio.

In other cases the antenna is located separately from the transmitter or receiver, such as broadcast television antennas and satellite dishes mounted on the roofs of residences, the sector antenna on cell towers of cellular base stations, the rotating radar antennas at airports, and the antenna towers of radio and television stations. In this case the antenna is connected to the transmitter or receiver with a cable called a feedline. To carry the radio frequency (RF) current efficiently, the feedline is made of specialized cable called transmission line. The advantage of transmission line is that it has a uniform characteristic impedance to avoid abrupt impedance steps which cause the radio energy to be reflected backward down the line. The main types of transmission line are parallel wire line (Twin lead), coaxial cable, and for microwaves waveguide.

In a radio antenna, the feed line (feedline), or feeder, is the cable or other transmission line that connects the antenna with the radio transmitter or receiver . In a transmitting antenna, it feeds the radio frequency (RF) current from the transmitter to the antenna, where the energy in the current is radiated as radio waves. In a receiving antenna it transfers the tiny RF voltage induced in the antenna by the radio wave to the receiver. In order to carry RF current efficiently, feed lines are made of specialized types of cable called transmission line. The most widely used types of feed line are coaxial cable, twin-lead, ladder line, and at microwave frequencies, waveguide.

Particularly with a transmitting antenna, the feed line 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.

Twin lead is used to connect FM radios and television receivers with their antennas, although it has been largely replaced in the latter application by coaxial cable, and as a feedline for low power transmitters such as amateur radio transmitters. It consists of two wire conductors running parallel to each other with a precisely constant spacing, molded in polyethylene insulating material in a flat ribbon-like cable. The distance between the two wires is small relative to the wavelength of the RF signal carried on the wire. The RF current in one wire is equal in magnitude and opposite in direction to the RF current on the other wire. Thus, far from the line, the radio waves radiated by one wire will be opposite in phase and will cancel the waves radiated by the other wire.