In a radio antenna, the feed line is the cable or other transmission line that connects the antenna with the radio transmitter or receiver. In a transmitter, it feeds the radio frequency (RF) current from the transmitter to the antenna, where it is radiated as radio waves. In a receiver it transfers the RF signal picked up by the antenna 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 equal 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.
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Twin lead is also called parallel wire. It is two conductors molded into an insulating material and running alongside one another in a flat and parallel ribbon-like wire. The distance between the two wires is small relative to the wavelength of the RF signal carried on the wire. Furthermore, the RF current in one wire is equal in magnitude and opposite in direction to the RF current on the other wire (it is inverted). Thus, if both wires radiate energy equally, the radiated energies will cancel each other out and there will be near zero radiation at any distance from the wire. Twin lead is also immune to external noise or RF energies. Any unwanted external noise or unwanted RF energy induced on the wire from external energy sources will be induced in both wires at the same time and equally in magnitude and direction. At the end of the transmission line the inverted signal wire is restored to normal (non-inverted now) and added back to the original non-inverted signal wire by the receiving circuitry. Any noise will now be equal in magnitude and opposite in direction and cancel itself out.
Twin lead is considered a balanced transmission line.
Coaxial cable is a rounded cable with a center conductor and a braided or solid metallic shield, usually copper or aluminum. The center conductor is separated from the outer shield by a dielectric, which is usually foam, air or a compressed gas such as nitrogen. The shield is covered with an outer cable sheath.
Coaxial cable is considered an unbalanced line.
A waveguide is a hollow metallic conductor with a circular or square cross-section. The RF signal travels along the inside of the tube, guided by its size and shape, similar to the way sound travels in a tube. The metallic construction of the wave guide keeps it from radiating energy outwards and also prevents unwanted energy from entering the waveguide.
A waveguide is considered an unbalanced transmission line.
This is a comparison of a few common feed line characteristics. Larger lists are available in other articles, references, and directly from manufacturers.
| type | impedance | velocity factor |
|---|---|---|
| twin-lead | 300 | 82% |
| ladder line | 450, 600 | 95% |
| coax | 50, 75 | 66% |