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Feed line
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 transmitting antenna, it feeds the radio frequency (RF) current from the transmitter to the antenna, where it 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==
(詳細は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. 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.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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