However, if a piece of metal is located sufficiently close to a twin-lead line, within a distance comparable to the wire spacing, it will be significantly closer to one wire than the other. Thus the interfering currents are canceled out, so twin lead does not tend to pick up radio noise. Since the load at the destination end is connected across the wires, only differential, oppositely-directed currents in the wires create a current in the load. Similarly, any interfering external radio waves will induce equal, in phase RF currents, traveling in the same direction, in the two wires. The result is that almost no net radio energy is radiated by the line. Therefore, in the far field region far from the transmission line, the radio waves radiated by one wire are equal in magnitude but opposite in phase (180° out of phase) to the waves radiated by the other wire, so they superpose and cancel each other. ![]() The RF current in one wire is equal in magnitude and opposite in direction to the RF current in the other wire. The separation between the two wires in twin-lead is small compared to the wavelength of the radio frequency (RF) signal carried on the wire. Twin lead is a form of parallel-wire balanced transmission line. The widely used RG-6 coax has a characteristic impedance of 75 ohms, which requires the use of a balun to match impedance when used with common antenna types.Ī 300-to-75-ohm balun, showing twin-lead on the right hand side Twin-lead generally has higher impedance than the other common transmission wiring, coaxial cable (coax). This is well matched with the natural impedance of a folded dipole antenna, which is normally around 275 ohms. The characteristic impedance of twin-lead is a function of the wire diameter and its spacing in 300 ohm twin-lead, the most common type, the wire is usually 20 or 22 gauge (0.52 or 0.33 mm 2), about 7.5 mm (0.30 inches) apart. Twin-lead is also used in amateur radio stations as a transmission line for balanced transmission of radio frequency signals. 300 ohm twin-lead for television installations has been largely replaced with 75 ohm coaxial cable feedlines. ![]() The most common, 300 ohm twin-lead, was once widely used to connect television sets and FM radios to their receiving antennas. Twin-lead is supplied in several different sizes, with values of 600, 450, 300, and 75 ohm characteristic impedance. It is also common practice to twist the twin lead on long free standing lengths to further reject any induced imbalances to the line. For this reason, when installed along the outside of buildings and on antenna masts, standoff insulators must be used. Its disadvantages are that it is more vulnerable to interference, and must be kept away from metal objects which can cause power losses. Parallel transmission line has the advantage that its losses per unit length are an order of magnitude smaller than that of coaxial cable, the main alternative form of transmission line. Twin lead and other types of parallel-conductor transmission line are mainly used to connect radio transmitters and receivers to their antennas. It therefore requires spacing around rain gutters, and standoff insulators along metal support masts. However, it is more vulnerable to interference proximity to metal objects will inject signals into twin-lead that would be blocked out by coaxial cable. 300 ohm twin lead is widely used to connect FM radios to their antennas, and was previously used to connect television antennas to televisions until it was replaced by coaxial cable. ![]() It can have significantly lower signal loss than miniature flexible coaxial cable, the main alternative type of feedline at these frequencies for example, type RG-58 coaxial cable loses 6.6 dB per 100 m at 30 MHz, while 300 ohm twin-lead loses only 0.55 dB. Twin lead is mainly used as an antenna feedline at shortwave and VHF frequencies, to connect radio receivers and transmitters to their antennas. It is available with several different values of characteristic impedance, the most common type is 300 ohm. The plastic also covers and insulates the wires. The uniform spacing of the wires is the key to the cable's function as a transmission line any abrupt changes in spacing would reflect some of the signal back toward the source. It is constructed of two stranded or solid copper or copper-clad steel wires, held a precise distance apart by a plastic (usually polyethylene) ribbon. Twin-lead cable is a two-conductor flat cable used as a balanced transmission line to carry radio frequency (RF) signals.
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