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Transmission line

A transmission line is the material medium or structure that forms all or part of a path from one place to another for directing the transmission of energy, such as electric currents, magnetic fields, acoustic waves, or electromagnetic waves. Examples of transmission lines include wires, optical fibers, coaxial cables, circular or rectangular closed waveguides, and dielectric slabs.

Table of contents
1 Electrical transmission lines
2 Acoustic transmission lines

Electrical transmission lines

Electrical transmission lines can be classified into high-frequency and low-frequency types depending on the type of current they are designed to carry. One feature that is common to both types of line is electrical resistance, which increases with the length of the line and is specified in ohms per metre. At low frequencies this parameter allows one to calculate the amount of power that will be lost in the line.

Low-frequency electrical transmission lines

These structures are most commonly used to transmit mains electrical power, as either direct current or low-frequency alternating current, over long distances. See Electric power transmission for more information.

High-frequency electrical transmission lines

High-frequency transmission lines can be defined as transmission lines that are designed to carry electromagnetic waves whose wavelengths are shorter than or comparable to the length of the line. Under these conditions, the electrical behaviour of the line is more complex than that of a low-frequency transmission line. This often occurs with radio, microwave and optical signals, and with the signals found in high-speed digital circuits.

For the purposes of analysis, an electrical transmission line can be modelled as a two-port network (also called a quadripole network), as follows:

 o---------| Transmission |---------o
Port A     |              |     Port B
 o---------|     Line     |---------o

In the simplest case, the network is assumed to be linear (i.e. the complex voltage across either port is proportional to the complex current flowing into it when there are no reflections), and the two ports are assumed to be interchangeable. If the transmission line is uniform along its length, then its behaviour is largely described by a single parameter called the characteristic impedance, symbol Z0. This is the ratio of the complex voltage to the complex current at any point on the line, and is always a positive real number. Typical values of Z0 are 50 or 75 ohms for a coaxial cable, about 100 ohms for a twisted pair of wires, and about 300 ohms for a common type of untwisted pair used in radio transmission.

When sending power down a transmission line, it is usually desirable that all the power is absorbed by the load and none of it is reflected back to the source. This can be ensured by making the source and load impedances equal to Z0, in which case the transmission line is said to be matched.

As mentioned above, some of the power that is fed into a transmission line is lost because of its resistance. This effect is called ohmic or resistive loss. At high frequencies, another effect called dielectric loss starts to occur, adding to the losses caused by resistance. Dielectric loss is caused when the insulating material inside the transmission line absorbs energy from the alternating electric field and converts it to heat.

The total loss of power in a transmission line is often specified in decibels per metre, and usually depends on the frequency of the signal. The manufacturer often supplies a chart showing the loss in dB/m at a range of frequencies. A loss of 3dB corresponds approximately to a halving of the power.

Acoustic transmission lines

An acoustic transmission line consists of a duct of constant cross-sectional area. Its length is normally of a similar order or longer than the wavelengths of the sound it will be used with, but the dimensions of its cross-section are normally smaller than one quarter of a wavelength: therefore it can be described as long and narrow. The duct must contain some medium that supports sound propogation such as air.

Sound is introduced at one end of the line by forcing the pressure across the whole cross-section to vary with time. A plane wave will travel down the line at the speed of sound, approxomately 330 metres per second in air. When the wave reaches the end of the transmission line, behaviour depends on what is present at the end of the line. There are three possible scenarios:

The impedance of a transmission line is the impedance that the line would exhibit at its input if it was infinitely long. It depends on the crossectional area of the line and the characteristic impedance of the sound propogating meduium within the duct.

Where a transmission line of finite length is mismatched at both ends, there is the potential for a wave to bounce back and forth indefinitely. This phenominon is a kind of resonance and will tend to filter any signal fed into the line.

When this resonance effect is combined with some sort of active feedback mechanism and power input, it is possible to set up an oscillation which can be used to generate periodic acoustic signals such as musical notes, for example.

Part of this article was derived from Federal Standard 1037C.