Twin-lead

Twin-lead cable is a two-conductor flat cable used as a balanced transmission line to carry radio frequency (RF) signals. It is constructed of two stranded copper or copper-clad steel wires, held a precise distance apart by a plastic (usually polyethylene) ribbon. 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. The plastic also covers and insulates the wires. Twin-lead cable is a two-conductor flat cable used as a balanced transmission line to carry radio frequency (RF) signals. It is constructed of two stranded copper or copper-clad steel wires, held a precise distance apart by a plastic (usually polyethylene) ribbon. 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. The plastic also covers and insulates the wires. Twin lead can have significantly lower signal loss than miniature flexible coaxial cable at shortwave and VHF radio 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. However, twin-lead is more vulnerable to interference. Proximity to metal objects will inject signals into twin-lead that would be blocked out by coaxial cable. Twin lead therefore requires careful installation around rain gutters, and standoffs from metal support masts. Twin-lead is also susceptible to significant degradation when wet or ice covered, whereas coax is less or not affected in these conditions. For these reasons, coax has largely replaced twin-lead in most uses, except where maximum signal is required. Twin lead and other types of parallel-conductor transmission line are mainly used to connect radio transmitters and receivers to their antennas. Parallel transmission line has the advantage that its losses are an order of magnitude smaller than that of coaxial cable, the main alternative form of transmission line. Its disadvantages are that it is more vulnerable to interference, and must be kept away from metal objects which can cause power losses. For this reason, when installed along the outside of buildings and on antenna masts, standoff insulators must be used. It is also common practice to twist the twin lead on long free standing lengths to further reject any induced imbalances to the line. Twin-lead is supplied in several different sizes, with values of 600, 450, 300, and 75 ohms characteristic impedance. The most common, 300 ohm twin-lead, was once widely used to connect television sets and FM radios to their receiving antennas. 300 ohm twin-lead for television installations has been largely replaced with 75 ohm coaxial cable feedlines. Twin-lead is also used in amateur radio stations as a transmission line for balanced transmission of radio frequency signals. 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, about 7.5 mm (0.30 inches) apart. This is well matched with the natural impedance of a folded dipole antenna, which is normally around 275 ohms. Twin-lead generally has higher impedance than the other common transmission wiring, coaxial cable (coax). 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. Twin lead is a form of parallel-wire balanced transmission line. 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. The RF current in one wire is equal in magnitude and opposite in direction to the RF current in the other wire. 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 result is that almost no net radio energy is radiated by the line. Similarly, any interfering external radio waves will induce equal, in phase RF currents, traveling in the same direction, in the two wires. 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. Thus the interfering currents are canceled out, so twin lead does not tend to pick up radio noise.