Twin-lead

The wires are held a fixed distance apart by a plastic ribbon that is a good insulator at radio frequencies (usually polyethylene).

The name twin lead is most often used to refer specifically to 300 Ω (Ohm) ribbon cable, the most common type, but on occasion, twin lead is used to refer to any type of parallel wire line.

It can have significantly lower signal loss than miniature flexible coaxial cable, the main alternative type of feedline at these frequencies; for example, type RG-58 coaxial cable loses 6.6 dB per 100 metres (330 ft) at 30 MHz, while 300 Ω twin-lead loses only 0.55 dB.

However, it is more vulnerable to interference; proximity to metal objects will inject signals into any type of parallel wire line that would be blocked out by more convenient / more popular coaxial cable.

Twin lead and other types of parallel-conductor transmission line are mainly used to connect radio transmitters and receivers to their antennas.

Parallel transmission line has the advantage that its losses per unit length are an order of magnitude smaller than that of 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 and impedance distortion (hence back-reflected waves).

For this reason, when installed along the outside of buildings and on antenna masts, standoff insulators must be used.

It is also common practice to twist the twin lead on long free standing lengths to further average out any imbalance induced on the line.

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

Multiple forms of parallel wire line are used in amateur radio stations as feedline for balanced transmission of radio frequency signals, most often as 450 Ω window line, instead of twin lead ribbon cable.

The characteristic impedance of twin lead is a function of the insulating material and its thickness, and the wire diameter and its spacing; in the most common type, 300 Ω twin-lead ribbon cable, the wire is usually AWG 20 or 22 (0.52 or 0.33 mm²), about 7.5 millimetres (0.30 in) apart.

[2](pp 24⸗16–24⸗17) This is well matched with the natural impedance of a folded dipole antenna, which is normally around 275 Ω. Twin lead generally has higher impedance than the other common transmission wiring, coaxial cable (coax).

Twin lead (in the specific sense of ribbon cable) is a form of parallel wire balanced transmission line.

[2](pp 24⸗16–24⸗17) The result is that almost no net radio energy is radiated by the line.

Similarly, any interfering external radio waves will induce equal, in phase RF currents, traveling in the same direction, in the two wires, as long as they each maintain the same impedance.

Thus the interfering currents are canceled out, so twin lead does not tend to pick up radio noise.

Thus nearby metal objects can cause power losses in twin lead lines, through energy dissipated as heat by induced currents.

Similarly, radio noise originating in cables or metal objects located near the twin-lead line can induce unbalanced currents in the wires, coupling noise into the line.

In order to prevent power from being reflected from the load end of the line, causing high SWR and inefficiency, the load must have an impedance which matches the characteristic impedance of the line.

This causes the load to appear electrically identical to a continuation of the line, preventing reflections.

Similarly, to transfer power efficiently into the line, the source must also match the characteristic impedance.

Window line is a bulkier variety of parallel wire line than twin lead ribbon cable; it is similarly built to a wider size, except that the polyethylene ribbon between them holding the wires apart has regularly spaced rectangular openings ("windows") cut in it.

The windows lighten the line, and reduce the amount of surface on which dirt and moisture can accumulate, making window line somewhat less vulnerable to weather-induced changes in its characteristic impedance.

The "ladder rung" spacers can be made out of any convenient insulating material – at present, usually short pieces cut from plastic plumbing pipe – formerly either long, thin ceramic insulators or dowel stubs of water-proofed wood were used.

To get 600 Ω parallel line, the same wires would be spaced 9 inches (23 cm); 600 Ω is roughly the practical limit for transmission lines made with wire rather than thicker copper-clad steel cable, or with rigid aluminum or copper pipe.

For this reason, when attaching a twin-lead line to a coaxial cable connection, such as the 300 Ω 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 Ω impedance to match the 75 Ω coaxial cable impedance; and second, it transforms the balanced, symmetric transmission line to the nominally unbalanced coax input.

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.