1. (power line) An electric line, such as an overhead wire, that is used to convey electrical power from a power station or substation to other stations or substations.
2. An electric cable or waveguide that conveys electrical signals from one point to another in a telecommunication system and that forms a continuous path between the two points.
3. (feeder) The one or more conductors that connect an antenna to a transmitter or receiver and that are substantially nonradiative.
Any of the above transmission lines is described as smooth or uniform if its electrical parameters are distributed uniformly along its length. A balanced line is a transmission line that has conductors of the same type, equal values of resistance per unit length, and equal impedances from each conductor to earth and to other electrical circuits. The transmission characteristics of a particular transmission line are usually frequency-dependent and loading is often used to improve the transmission characteristic throughout a given frequency band. Loading is the addition of inductance to a line: in coil loading inductance coils (loading coils) are placed in series with the line conductors at regular intervals; in continuous loading a continuous layer of magnetic material is wrapped along each conductor.
The presence of a discontinuity in a transmission line can cause some signal energy to be reflected back along the line. The presence of such line reflections can lead to the production of standing waves in the transmission line. A transmission line that has no line reflections and therefore no standing waves is a nonresonant line.
The most efficient transfer of signal power from a transmission line to the load occurs when the load is matched to the transmission line, i.e. the load impedance, ZL, is equal to the characteristic impedance, Z0, of the transmission line. Z0 is given by the ratio VI at every point along a transmission line when no standing waves are present in the line. V and I are the complex voltage and current at each point along the line.