A stack of "fishbone" and Yagi–Uda television antennas
In his 1880 British patent, Oliver Heaviside showed how coaxial cable could eliminate signal interference between parallel cables.
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.
Coaxial cable cutaway (not to scale)
Electronic symbol for an antenna
Schematic representation of the elementary components of a transmission line
Antennas of the Atacama Large Millimeter/submillimeter Array.
Schematic representation of a coaxial transmission line, showing the characteristic impedance Z_0
An automobile's whip antenna, a common example of an omnidirectional antenna.
RG-6 coaxial cable
Half-wave dipole antenna
RG-142 coaxial cable
Diagram of the electric fields ( blue ) and magnetic fields ( red ) radiated by a dipole antenna ( black rods) during transmission.
RG-405 semi-rigid coaxial cable
Cell phone base station antennas
High-end coaxial audio cable (S/PDIF)
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.
1+5/8 in flexible line
Typical center-loaded mobile CB antenna with loading coil
1-5/8" Heliax coaxial cable
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.
Semi-rigid coax assembly
The wave reflected by earth can be considered as emitted by the image antenna.
Semi-rigid coax installed in an Agilent N9344C 20GHz spectrum analyser
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).
Early coaxial antenna feedline of 50 kW radio station WNBC, New York, 1930s
frame
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.

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.

- Coaxial cable

Such a structure is normally connected to the return connection of an unbalanced transmission line such as the shield of a coaxial cable.

- Antenna (radio)
A stack of "fishbone" and Yagi–Uda television antennas

7 related topics

Alpha

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

Transmission line

Specialized cable or other structure designed to conduct electromagnetic waves in a contained manner.

Specialized cable or other structure designed to conduct electromagnetic waves in a contained manner.

Schematic of a wave moving rightward down a lossless two-wire transmission line. Black dots represent electrons, and the arrows show the electric field.
One of the most common types of transmission line, coaxial cable.
Variations on the schematic electronic symbol for a transmission line.
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.
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.
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.
A simple example of stepped transmission line consisting of three segments.

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.

Types of transmission line include parallel line (ladder line, twisted pair), coaxial cable, and planar transmission lines such as stripline and microstrip.

Coaxial cable feedline emerging from a VHF ground plane antenna.

Feed line

Coaxial cable feedline emerging from a VHF ground plane antenna.
Complicated waveguide feed of a military radar

In a radio antenna, the feed line (feedline), or feeder, is the cable or other transmission line that connects the antenna with the radio transmitter or receiver.

The most widely used types of feed line are coaxial cable, twin-lead, ladder line, and at microwave frequencies, waveguide.

Incident wave (blue) is fully reflected (red wave) out of phase at short-circuited end of transmission line, creating a net voltage (black) standing wave. Γ = −1, SWR = ∞.

Standing wave ratio

Measure of impedance matching of loads to the characteristic impedance of a transmission line or waveguide.

Measure of impedance matching of loads to the characteristic impedance of a transmission line or waveguide.

Incident wave (blue) is fully reflected (red wave) out of phase at short-circuited end of transmission line, creating a net voltage (black) standing wave. Γ = −1, SWR = ∞.
Standing waves on transmission line, net voltage shown in different colors during one period of oscillation. Incoming wave from left (amplitude = 1) is partially reflected with (top to bottom) Γ = 0.6, −0.333, and 0.8 ∠60°. Resulting SWR = 4, 2, 9.
Example of estimated bandwidth of antenna according to the schedule VSWR by the help of the Ansys HFSS
Slotted line. The probe moves along the line to measure the variable voltage. SWR is the maximum divided by the minimum voltage
A directional wattmeter using a rotatable directional coupler element.

In practice most transmission lines used in these applications are coaxial cables with an impedance of either 50 or 75 ohms, so most SWR meters correspond to one of these.

This especially applies to transmission lines connecting radio transmitters and receivers with their antennas, as well as similar uses of RF cables such as cable television connections to TV receivers and distribution amplifiers.

UHF half-wave dipole

Dipole antenna

UHF half-wave dipole
Dipole antenna used by the radar altimeter in an airplane
Animated diagram of a half-wave dipole antenna receiving a radio wave. The antenna consists of two metal rods connected to a receiver R. The electric field ( E, green arrows ) of the incoming wave pushes the electrons in the rods back and forth, charging the ends alternately positive  (+)  and negative  (−) .  Since the length of the antenna is one half the wavelength of the wave, the oscillating field induces standing waves of voltage ( V, represented by red band ) and current in the rods. The oscillating currents (black arrows) flow down the transmission line and through the receiver (represented by the resistance R).
Cage dipole antennas in the Ukrainian UTR-2 radio telescope. The 8 m by 1.8 m diameter galvanized steel wire dipoles have a bandwidth of 8–33 MHz.
Real (black) and imaginary (blue) parts of the dipole feedpoint impedance versus total length in wavelengths, assuming a conductor diameter of 0.001 wavelengths
Feedpoint impedance of (near-) half-wave dipoles versus electrical length in wavelengths. Black: radiation resistance; blue: reactance for 4 different values of conductor diameter
Length reduction factor for a half-wave dipole to achieve electrical resonance (purely resistive feedpoint impedance). Calculated using the Induced EMF method, an approximation that breaks down at larger conductor diameters (dashed portion of graph).
"Rabbit-ears" VHF television antenna (the small loop is a separate UHF antenna).
Collinear folded dipole array
A reflective array antenna for radar consisting of numerous dipoles fed in-phase (thus realizing a broadside array) in front of a large reflector (horizontal wires) to make it uni-directional.

In radio and telecommunications a dipole antenna or doublet is the simplest and most widely used class of antenna.

Many types of coaxial cable (or "coax") have a characteristic impedance of 75 Ω, which would otherwise be a good match for a half-wave dipole.

Pair of AC&E 120 Ω twisted pair (Krone IDC) to 75Ω coaxial cable balun transformers. Actual length is about 3cm.

Balun

Electrical device that allows balanced and unbalanced lines to be interfaced without disturbing the impedance arrangement of either line.

Electrical device that allows balanced and unbalanced lines to be interfaced without disturbing the impedance arrangement of either line.

Pair of AC&E 120 Ω twisted pair (Krone IDC) to 75Ω coaxial cable balun transformers. Actual length is about 3cm.
Isolation transformer
Circuit diagram of a 4:1 autotransformer balun using three taps on a single winding on a ferrite rod.
Picture a 4:1 balun of the same design, wound on ferrite toroid. Notice that the black and red winding wires are joined at the threaded connector.
Homemade 1:1 balun using a toroidal core and coaxial cable. This simple RF choke works as a balun by preventing signals passing along the outside of the braid. Such a device can be used to cure television interference by acting as a braid-breaker.
A 75-to-300-Ω balun built into the antenna plug.
Three audio transformers; two of them baluns.

Something as simple as 10 turns of coaxial cable coiled up on a diameter about the size of a dinner plate makes an effective choke balun for frequencies from about 10 MHz to beyond 30 MHz.

In television, amateur radio, and other antenna installations and connections, baluns convert between impedances and symmetry of feedlines and antennas.

Schematic representation of the elementary components of a transmission line.

Telegrapher's equations

Electrical transmission line with distance and time.

Electrical transmission line with distance and time.

Schematic representation of the elementary components of a transmission line.
Schematic showing a wave flowing rightward down a lossless transmission line. Black dots represent electrons, and the arrows show the electric field.
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.
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.

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

In long distance rigid coaxial cable, to get very low dielectric losses, the solid dielectric may be replaced by air with plastic spacers at intervals to keep the center conductor on axis.

A 100 watt stereo audio amplifier used in home component audio systems in the 1980s.

Low-noise amplifier

Electronic amplifier that amplifies a very low-power signal without significantly degrading its signal-to-noise ratio.

Electronic amplifier that amplifies a very low-power signal without significantly degrading its signal-to-noise ratio.

A 100 watt stereo audio amplifier used in home component audio systems in the 1980s.

Antennas are a common source of weak signals.

An example is a feed line made from 10 feet of RG-174 coaxial cable and used with a global positioning system (GPS) receiver.