 - In a wireless communications orbroadcasting antenna system, the feed line connects the antenna to the receiver,transmitter, or transceiver. The line transfers radio-frequency (RF) energy from atransmitter 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 antennafeed lines, also called RF transmission lines, commonly used in wireless systems.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 usuallysolid or foamed polyethylene. The shield is connected to RF ground, while the centerconductor carries the signal. The shield,as its name implies, prevents the electromagneticfield (EM field) inside the cable from escaping, and also prevents EM energy from enteringthe 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 currentin the two wires are always equal in magnitude but opposite in direction. The twowires are spaced close together in terms of the EM wavelength. Because of this, the EM fieldsfrom 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, EMfields from the external environment induce RF currents that flow in the same direction ineach conductor. The receiver circuitry cancels out RF currents that flow in thesame direction in both conductors, while responding to RF currents that flow in oppositedirections. This prevents external EM fields from affecting the line. Parallel-wire line is rarely employed in commercial installations, but a prefabricatedform, called TV ribbon, is sometimes used with television receivers in fringeareas for reception of channels 2 through 13. Another type of two-wire line, knownas window line, ladder line, or open wire, is popular amongamateur radio operators and shortwave listeners. A waveguide is a hollow, metallic tube or pipe with a circular or rectangularcross section. The diameter of the waveguide is comparable to the wavelength of theEM field. The EM field travels along the inside of the waveguide in a mannersomewhat analogous to the way sound waves propagate down a narrow tunnel. The metalstructure prevents EM fields inside the waveguide from escaping, and also preventsexternal EM fields from penetrating to the interior. Waveguides are used atmicrowave frequencies, that is, at 1 GHz and above. Because the currents in a parallel-wire line always exactly cancel or balance eachother, this type of line constitutes a balanced feed line. Such lines workbest with antenna systems that are bilaterally symmetrical; an example is the dipole antenna. Coaxial cables andwaveguides are unbalanced feed lines. This type of line will worksatisfactorily with antennas that are not symmetrical. With the use of a transformercalled a balun (contraction of the words "balanced" and"unbalanced"), coaxial cables and waveguides can be used with symmetricalantennas.
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06 Nov 2006
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