Schematic of a wave moving rightward down a lossless two-wire transmission line. Black dots represent electrons, and the arrows show the electric field.
Schematic representation of the elementary components of a transmission line.
A stack of "fishbone" and Yagi–Uda television antennas
One of the most common types of transmission line, coaxial cable.
Schematic showing a wave flowing rightward down a lossless transmission line. Black dots represent electrons, and the arrows show the electric field.
Animation of a half-wave dipole antenna radiating radio waves, showing the electric field lines. The antenna in the center is two vertical metal rods connected to a radio transmitter (not shown). The transmitter applies an alternating electric current to the rods, which charges them alternately positive (+) and negative (−). Loops of electric field leave the antenna and travel away at the speed of light; these are the radio waves. In this animation the action is shown slowed down enormously.
Variations on the schematic electronic symbol for a transmission line.
In the presence of losses the solution of the telegrapher's equation has both damping and dispersion, as visible when compared with the solution of a lossless wave equation.
Electronic symbol for an antenna
Changes of the signal level distribution along the single dimensional transmission medium. Depending on the parameters of the telegraph equation, this equation can reproduce all four patterns.
Antennas of the Atacama Large Millimeter/submillimeter Array.
A transmission line is drawn as two black wires. At a distance x into the line, there is current I(x) travelling through each wire, and there is a voltage difference V(x) between the wires. If the current and voltage come from a single wave (with no reflection), then V(x) / I(x) = Z0, where Z0 is the characteristic impedance of the line.
An automobile's whip antenna, a common example of an omnidirectional antenna.
Standing waves on a transmission line with an open-circuit load (top), and a short-circuit load (bottom). Black dots represent electrons, and the arrows show the electric field.
Half-wave dipole antenna
A type of transmission line called a cage line, used for high power, low frequency applications. It functions similarly to a large coaxial cable. This example is the antenna feed line for a longwave radio transmitter in Poland, which operates at a frequency of 225 kHz and a power of 1200 kW.
Diagram of the electric fields ( blue ) and magnetic fields ( red ) radiated by a dipole antenna ( black rods) during transmission.
A simple example of stepped transmission line consisting of three segments.
Cell phone base station antennas
Standing waves on a half wave dipole driven at its resonant frequency. The waves are shown graphically by bars of color ( red for voltage, V and blue for current, I ) whose width is proportional to the amplitude of the quantity at that point on the antenna.
Typical center-loaded mobile CB antenna with loading coil
Polar plots of the horizontal cross sections of a (virtual) Yagi-Uda-antenna. Outline connects points with 3 dB field power compared to an ISO emitter.
The wave reflected by earth can be considered as emitted by the image antenna.
The currents in an antenna appear as an image in opposite phase when reflected at grazing angles. This causes a phase reversal for waves emitted by a horizontally polarized antenna (center) but not for a vertically polarized antenna (left).
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The telegrapher's equations (or just telegraph equations) are a pair of coupled, linear partial differential equations that describe the voltage and current on an electrical transmission line with distance and time.

- Telegrapher's equations

However, the theory of transmission lines was historically developed to explain phenomena on very long telegraph lines, especially submarine telegraph cables.

- Transmission line

Transmission lines are used for purposes such as connecting radio transmitters and receivers with their antennas (they are then called feed lines or feeders), distributing cable television signals, trunklines routing calls between telephone switching centres, computer network connections and high speed computer data buses.

- Transmission line

It can also be used to electrically model wire radio antennas as truncated single-conductor transmission lines.

- Telegrapher's equations

An antenna lead-in is the transmission line, or feed line, which connects the antenna to a transmitter or receiver.

- Antenna (radio)

The flow of current in wire antennas is identical to the solution of counter-propagating waves in a transmission line, which can be solved using the telegrapher's equations.

- Antenna (radio)
Schematic of a wave moving rightward down a lossless two-wire transmission line. Black dots represent electrons, and the arrows show the electric field.

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In his 1880 British patent, Oliver Heaviside showed how coaxial cable could eliminate signal interference between parallel cables.

Coaxial cable

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RG-59.jpg flexible coaxial cable composed of:1.

RG-59.jpg flexible coaxial cable composed of:1.

In his 1880 British patent, Oliver Heaviside showed how coaxial cable could eliminate signal interference between parallel cables.
Coaxial cable cutaway (not to scale)
Schematic representation of the elementary components of a transmission line
Schematic representation of a coaxial transmission line, showing the characteristic impedance Z_0
RG-6 coaxial cable
RG-142 coaxial cable
RG-405 semi-rigid coaxial cable
High-end coaxial audio cable (S/PDIF)
1+5/8 in flexible line
1-5/8" Heliax coaxial cable
Semi-rigid coax assembly
Semi-rigid coax installed in an Agilent N9344C 20GHz spectrum analyser
Early coaxial antenna feedline of 50 kW radio station WNBC, New York, 1930s
AT&T coaxial cable trunkline installed between East Coast and Midwest in 1948. Each of the 8 coaxial subcables could carry 480 telephone calls or one television channel.

Coaxial cable is a type of transmission line, used to carry high-frequency electrical signals with low losses.

It is used in such applications as telephone trunk lines, broadband internet networking cables, high-speed computer data busses, cable television signals, and connecting radio transmitters and receivers to their antennas.

See Telegrapher's equation.