Wikipedia
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 plastic also covers and insulates the wires.
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 also has significantly lower signal losses than coax under most conditions; RG-58 coax loses 6.6 dB per 100 m at 1 GHz, while 300 ohm twin-lead loses only 0.55 dB. However, twin-lead is more vulnerable to interference. For instance, proximity to nearby metal objects will inject signals into twin-lead that would be filtered out by coax, requiring careful installation around rain gutters, and standoffs from 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.