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 also has significantly lower signal losses than coax under most conditions; RG-58 coax loses 6.6 d B per 100 m at 1 GHz, while 300 ohm twin-lead loses only 0.55 d B.
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.
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Twin-lead cable is a two-conductor flat cable used as a transmission line to carry radio frequency (RF) signals.
It is constructed of two multistranded copper or copperclad 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 parallel transmission line; any abrupt changes in spacing would reflect some of the signal back toward the source. 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).