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Twin-lead cable is a two-conductor ribbon cable used as a transmission line to carry radio frequency (RF) signals.

300 ohm Twin-lead

Contents

Characteristics and uses

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

Twin-lead is supplied in several different sizes, with values of 600, 450, 300, and 75 ohms characteristic impedance. The most common, 300 ohm twin-lead, was once widely used to connect television sets and FM radios to their receiving antennas. 300 ohm twin-lead for television installations has been largely replaced with 75 ohm coaxial cable feedlines. Twin-lead is also used in amateur radio stations as a transmission line for balanced transmission of radio frequency signals.

450 Ohm "Ladder line"

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.

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.

Impedance matching

As 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, 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. 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 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). For receive-only use this merely implies that the system can communicate under slightly less optimum conditions; for transmit use, this can often result in significantly less energy lost as heat in the transmission line.

Twin-lead also can serve as a convenient material with which to build a simple folded dipole antenna. Such antennas may be fed either by using a 300 ohm twin-lead feeder or by using a 300-to-75-ohm balun and using coaxial feedline and will usually handle moderate power loads without overheating.

Characteristic Impedance

The characteristic impedance (when used as a transmission line) is given by

Z = \sqrt{{R + j \omega L} \over {G + j \omega C}}

where

R = {R_s \over \pi a}

L = {\mu \over \pi} cosh^{-1}({D \over 2a})

G = {\pi \sigma \over cosh^{-1}({D \over 2a})}

C = {\pi \epsilon \over cosh^{-1}({D \over 2a})}

R_s = \sqrt{\pi f \mu_c / \sigma_c}

and a is the wire radius and D is the separation distance.

Electrical Properties

Some electrical properties of twin-lead cable:[1]
  Characteristic Impedance
300 Ω 75 Ω
Capacitance (pF/m) 11.8 20
Propagation speed (% of light) 80% 71%
Loss (dB/100m) 100 MHz 3.6 3.6
300 MHz 7.2 7.2
500 MHz 10.2 10.2

Antennas

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

  • a dipole (whose impedance at resonance is approximately 73 ohms 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.

References

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