Twin-lead
Encyclopedia
Twin-lead cable is a two-conductor
ribbon cable
used as a transmission line
to carry radio frequency
(RF) signals.
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 antenna
s. 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.
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.
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.
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.
The characteristic impedance of any transmission line is given by
where for twin-lead line the primary line constants
are
where the surface resistance of the wires is
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
where Z0 is the impedance of free space (approximately 377 Ω), εr 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
The separation needed to achieve a given characteristic impedance is therefore
Electrical conductor
In physics and electrical engineering, a conductor is a material which contains movable electric charges. In metallic conductors such as copper or aluminum, the movable charged particles are electrons...
ribbon cable
Ribbon cable
A ribbon cable is a cable with many conducting wires running parallel to each other on the same flat plane. As a result the cable is wide and flat. Its name comes from the resemblance of the cable to a piece of ribbon.Ribbon cables are usually seen for internal peripherals in computers, such as...
used as a transmission line
Transmission line
In communications and electronic engineering, a transmission line is a specialized cable designed to carry alternating current of radio frequency, that is, currents with a frequency high enough that its wave nature must be taken into account...
to carry radio frequency
Radio frequency
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
(RF) signals.
Characteristics and uses
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
or copperclad steel wires, held a precise distance apart by a plastic (usually polyethylene
Polyethylene
Polyethylene or polythene is the most widely used plastic, with an annual production of approximately 80 million metric tons...
) 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
Radio frequency
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
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
Wire gauge
Wire gauge is a measurement of how large a wire is, either in diameter or cross sectional area. This determines the amount of electric current a wire can safely carry, as well as its electrical resistance and weight per unit of length...
, 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 antenna
Antenna (radio)
An antenna is an electrical device which converts electric currents into radio waves, and vice versa. It is usually used with a radio transmitter or radio receiver...
s. Parallel transmission line has the advantage that its losses are an order of magnitude smaller than coaxial cable
Coaxial cable
Coaxial cable, or coax, has an inner conductor surrounded by a flexible, tubular insulating layer, surrounded by a tubular conducting shield. The term coaxial comes from the inner conductor and the outer shield sharing the same geometric axis...
, 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
Television
Television is a telecommunication medium for transmitting and receiving moving images that can be monochrome or colored, with accompanying sound...
sets and FM radios to their receiving antennas. 300 ohm twin-lead for television installations has been largely replaced with 75 ohm coaxial cable
Coaxial cable
Coaxial cable, or coax, has an inner conductor surrounded by a flexible, tubular insulating layer, surrounded by a tubular conducting shield. The term coaxial comes from the inner conductor and the outer shield sharing the same geometric axis...
feedlines. Twin-lead is also used in amateur radio
Amateur radio
Amateur radio is the use of designated radio frequency spectrum for purposes of private recreation, non-commercial exchange of messages, wireless experimentation, self-training, and emergency communication...
stations as a transmission line
Transmission line
In communications and electronic engineering, a transmission line is a specialized cable designed to carry alternating current of radio frequency, that is, currents with a frequency high enough that its wave nature must be taken into account...
for balanced
Balanced line
In telecommunications and professional audio, a balanced line or balanced signal pair is a transmission line consisting of two conductors of the same type, each of which have equal impedances along their lengths and equal impedances to ground and to other circuits. The chief advantage of the...
transmission of radio frequency
Radio frequency
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
signals.
Ladder line
Transmission line
In communications and electronic engineering, a transmission line is a specialized cable designed to carry alternating current of radio frequency, that is, currents with a frequency high enough that its wave nature must be taken into account...
similar to twin-lead for balanced
Balanced line
In telecommunications and professional audio, a balanced line or balanced signal pair is a transmission line consisting of two conductors of the same type, each of which have equal impedances along their lengths and equal impedances to ground and to other circuits. The chief advantage of the...
connection of antennas
Antenna (radio)
An antenna is an electrical device which converts electric currents into radio waves, and vice versa. It is usually used with a radio transmitter or radio receiver...
. 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
Dielectric
A dielectric is an electrical insulator that can be polarized by an applied electric field. When a dielectric is placed in an electric field, electric charges do not flow through the material, as in a conductor, but only slightly shift from their average equilibrium positions causing dielectric...
effect and reduce loss in the transmission line. The alternating webbing and windows gives ladder line its characteristic look and name.
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 impedanceElectrical impedance
Electrical impedance, or simply impedance, is the measure of the opposition that an electrical circuit presents to the passage of a current when a voltage is applied. In quantitative terms, it is the complex ratio of the voltage to the current in an alternating current circuit...
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
Balun
A balun is a type of electrical transformer that can convert electrical signals that are balanced about ground to signals that are unbalanced , and the reverse. They are also often used to connect lines of differing impedance...
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
where for twin-lead line the primary line constants
Primary line constants
The primary line constants are parameters that describe the characteristics of copper transmission lines in terms of the physical electrical properties of the line. The primary line constants are only relevant to copper lines and are to be contrasted with the secondary line constants, which can...
are
where the surface resistance of the wires is
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
where Z0 is the impedance of free space (approximately 377 Ω), εr 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
The separation needed to achieve a given characteristic impedance is therefore
Typical properties
| | Characteristic Impedance | |||
|---|---|---|---|
| 300 Ω | |75 Ω | ||
| Capacitance (pF/m) | 11.8 | 20 | |
| Propagation speed (% of light Speed of light The speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time... ) |
80% | 71% | |
| Loss (dB DB DB may refer to:In science and technology:*Decibel , a logarithmic unit of measurement in acoustics and electronics*Dubnium , a chemical element*DB connector, a size of D-subminiature electrical connector... /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 dipoleDipole antennaA dipole antenna is a radio antenna that can be made of a simple wire, with a center-fed driven element. It consists of two metal conductors of rod or wire, oriented parallel and collinear with each other , with a small space between them. The radio frequency voltage is applied to the antenna at...
(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 antennaYagi antennaA Yagi-Uda array, commonly known simply as a Yagi antenna, is a directional antenna consisting of a driven element and additional parasitic elements...
or similar balanced antenna.