In a wireless communications or broadcasting antenna system, the feed line connects the antenna to the receiver, transmitter, or transceiver. The line transfers radio-frequency (RF) energy from a transmitter to an antenna, and/or from an antenna to a receiver, but, if operating properly, does not radiate or intercept energy itself. There are three types of antenna feed lines, also called RF transmission lines, commonly used in wireless systems.
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Coaxial line, also called coaxial cable, consists of a wire conductor surrounded by a tubular, braided metallic shield.The conductor is kept at the center of the shield by a dielectric, which is usually solid or foamed polyethylene. The shield is connected to RF ground, while the center conductor carries the signal. The shield,as its name implies, prevents the electromagnetic field (EM field) inside the cable from escaping, and also prevents EM energy from entering the cable from outside. Coaxial cables are used at frequencies below approximately 1 gigahertz.
Parallel-wire line consists of two wires running alongside each other. At each point along the line, the RF current in the two wires are always equal in magnitude but opposite in direction. The two wires are spaced close together in terms of the EM wavelength. Because of this, the EM fields from the two wires practically cancel out each other in the region outside the line.This prevents the line from radiating RF energy. In receiving systems, EM fields from the external environment induce RF currents that flow in the same direction in each conductor. The receiver circuitry cancels out RF currents that flow in the same direction in both conductors, while responding to RF currents that flow in opposite directions. This prevents external EM fields from affecting the line. Parallel-wire line is rarely employed in commercial installations, but a prefabricated form, called TV ribbon, is sometimes used with television receivers in fringe areas for reception of channels 2 through 13. Another type of two-wire line, known as window line, ladder line, or open wire, is popular among amateur radio operators and shortwave listeners.
A waveguide is a hollow, metallic tube or pipe with a circular or rectangular cross section. The diameter of the waveguide is comparable to the wavelength of the EM field. The EM field travels along the inside of the waveguide in a manner somewhat analogous to the way sound waves propagate down a narrow tunnel. The metal structure prevents EM fields inside the waveguide from escaping, and also prevents external EM fields from penetrating to the interior. Waveguides are used at microwave frequencies, that is, at 1 GHz and above.
Because the currents in a parallel-wire line always exactly cancel or balance each other, this type of line constitutes a balanced feed line. Such lines work best with antenna systems that are bilaterally symmetrical; an example is the dipole antenna. Coaxial cables and waveguides are unbalanced feed lines. This type of line will work satisfactorily with antennas that are not symmetrical. With the use of a transformer called a balun (contraction of the words "balanced" and"unbalanced"), coaxial cables and waveguides can be used with symmetrical antennas.