Twin-lead


Twin-lead

Twin-lead is a two-conductor ribbon cable commonly used as a transmission line for balanced transmission of radio frequency signals. It is made in several different sizes, with values of 450, 300, and 75 ohms characteristic impedance. The most common, 300 ohm twin lead, is widely used to connect television sets and FM radios to their receiving antennas.

Twin lead is constructed of two multistranded copper or copperclad steel wires, held a precise distance apart by a plastic (usually polyethylene) ribbon. The plastic also covers and insulates the wires. In 300 ohm twin lead, the wire is usually 20 or 22 gauge, about 0.30 inches (7.5 mm) apart. In 450 ohm twin lead, which consists of 16 or 18 gauge wire about 0.8 in. (20 mm) apart, the plastic ribbon has rectangular openings between the wires every few inches in order to reduce dielectric signal loss, leaving the wires joined by short plastic 'rungs'. For this reason it is commonly called ladder line. It is widely used as an antenna feedline in amateur radio stations.

Twin lead has the advantage that its losses are an order of magnitude smaller than 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. For this reason, when installed along the outside of buildings and on antenna masts, standoff insulators must be used.

Being a transmission line, transmission efficiency will be maximum when the impedance of the antenna, the characteristic impedance of the twin-lead line and the impedance of the equipment are the same. For this reason, in domestic television 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 the 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 folded 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.

Electrical Properties

Some electrical properties of twin lead: [cite book
last = Kaiser
first = Kenneth L.
authorlink =
coauthors =
title = Transmission Lines, Matching, and CrossTalk
publisher = CRC Press
date = 2005
location = USA
pages = p.2-24
url = http://books.google.com/books?id=QWFd-33xXvgC&pg=PT45&lpg=PT45&dq=specifications+twin+lead+dimensions&source=web&ots=6Vy67VIKB4&sig=2VI56xHd76fqv-GCftVuHxGV7D0&hl=en&sa=X&oi=book_result&resnum=9&ct=result#PPT46,M1
doi =
id =
isbn = 0849363624
, table 2-2
]

References


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