Twin-lead

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

300 ohm Twin-lead

Characteristics and uses

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

Twin-lead 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 radio frequency power back toward the source. The plastic also covers and insulates the wires. In 300 ohm twin-lead, the wire is usually 20 or 22 gauge, about 7.5 mm (0.30 inches) apart.

Twin lead and other types of parallel transmission line are mainly used to connect radio transmitters and receivers to their antennas. Parallel transmission line 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 which can cause power losses. For this reason, when installed along the outside of buildings and on antenna masts, standoff insulators must be used.

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.

Ladder line

450 Ohm "Ladder line"

Commercially manufactured ladder line or "window line" is a type of transmission line similar to twin-lead for balanced connection of antennas. Ladder line is constructed as a pair of evenly spaced wires with supportive plastic webbing holding the wires apart. The plastic webbing has windows cut in it to reduce its dielectric effect and reduce loss in the transmission line. The alternating webbing and windows gives ladder line its characteristic look and name.

600 Ohm "Ladder line" on the outside of a building.

Ladder line may also be manufactured or DIY-constructed as "open wire line" consisting of two parallel wires featuring widely-spaced plastic or ceramic insulating bars and having a characteristic impedance of 600 ohms or more.

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 unbalanced 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 optimal 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 of a parallel-wire transmission line like twin lead or ladder line depends on its dimensions; the diameter of the wires d and their separation D. This is derived below.

The characteristic impedance of any transmission line is given by

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

where for twin-lead line the primary line constants are

$R = 2 {R_s \over \pi d}$

$L = {\mu \over \pi} \, \operatorname{arccosh}\left({D \over d}\right)$

$G = {\pi \sigma \over \operatorname{arccosh}({D \over d})}$

$C = {\pi \epsilon \over \operatorname{arccosh}({D \over d})}$

where the surface resistance of the wires is

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

and where d is the wire diameter and D is the separation of the wires measured between their centrelines.

Neglecting the wire resistance R and the leakage conductance G, this gives

$Z = \frac{Z_0}{\pi \sqrt{\epsilon_r}} \, \operatorname{arccosh}\left(\frac{D}{d}\right)$

where Z₀ is the impedance of free space (approximately 377 Ω), ε_r is the effective dielectric constant (which for air is 1.00054). If the separation D is much greater than the wire diameter d then this is approximately

$Z \approx 276 \log_{10}\left(2\frac{D}{d}\right)$

The separation needed to achieve a given characteristic impedance is therefore

$D = d \cosh \left(\pi\frac{Z\sqrt{\epsilon_r}}{Z_0}\right)$

Typical properties

Some electrical properties of twin-lead cable:

		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.