Twin-lead

From Wikipedia, the free encyclopedia

This article does not cite any references or sources. (February 2008)
Please help improve this article by adding citations to reliable sources. Unverifiable material may be challenged and removed.

Twin-lead is a two-conductor ribbon cable of 300 ohms characteristic impedance commonly used as a transmission line for balanced connection of televi sion antennas to their receiving antennas.

It has the advantage that its losses are an order of magnitud smaller than the ones of coaxial cable.

A 300-to-75-ohm balun, showing twin-lead on the r ight hand side

Being a transmission line, Transmission efficiency will be maximum when the impedance of the antena, the characteristic impedance of the twin-lead line and the impedance of the equipment are the same. For this reason, in domestic t elevision installations, a balun with a 4:1 ratio is commonly used between the twin-lead and the equipment connection. Its purpose is double: from one side, it matches twin-lead 300-ohm impedance to 75-ohm coaxial cable input of modern equipment; on t he other, it transform a symmetric transmission line into an asymmetric one.

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

a dipole (whose impedance at resonance is ~73 ohm in free space),
a fo lded dipole (a better match, since its characteristic impedance in free space is around 300 ohms),
a Yagi antenna or similar balanced antenna.

In the past, twin-lead was universally used for feedlines to outside antennas. Recently 300 ohm twin lead for television installations has been largely replaced with 75-ohm coaxial cable feedlines.