twin-lead

Typical construction materials include copper pipe, ladder line, or twin-lead.

Twin-lead can be connected directly to a suitably designed antenna:

Twin-lead consists of a pair of conductors held apart by a continuous insulator.

There are many formats of balanced lines, amongst the most common are twisted pair, star quad and twin-lead.

Twin-lead is also used in amateur radio stations as a transmission line for balanced transmission of radio frequency signals.

The most common, 300 ohm twin-lead, was once widely used to connect television sets and FM radios to their receiving antennas.

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 constructed of two multistranded copper or copperclad steel wires, held a precise distance apart by a plastic (usually polyethylene) ribbon.

This transformer converts a balanced signal from the antenna (via 300-ohm twin-lead) into an unbalanced signal (75-ohm coaxial cable such as RG-6).

In the 1970s it became commonplace on VHF television antenna connections, as coaxial cables replaced twin-lead, and later for UHF also.

Biaxial cable, biax or Twin-Lead is a figure-8 configuration of two 50 Ω coaxial cables, externally resembling that of lamp cord, or speaker wire.

In general, when used as a feedline, twin-lead (especially ladder line versions) has a higher efficiency than coaxial cable when there is an impedance mismatch between the feedline and the source (or sink).

Its purpose is double: first, it transforms twin-lead's 300-ohm impedance to match the 75-ohm coaxial cable impedance; and second, it transforms the balanced, symmetric transmission line to the asymmetric coax input.

In order to prevent ghosting (the repeat of a faint image displaced slightly from the original), the twin-lead had to be twisted a full turn every 30 centimeters or so in its route from the antenna to the TV set.

For this reason, when attaching a twin-lead line to a coaxial cable connection, such as the 300 ohm twin-lead from a domestic television antenna to the television's 75 ohm coax antenna input, a balun with a 4:1 ratio is commonly used.