A report on Electrical length

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Loading coil in a cellphone antenna mounted on the roof of a car. The coil allows the antenna to be shorter than a quarter wavelength and still be resonant.
Vertical antenna which may be of any desired height : less than about one-half wavelength of the frequency at which the antenna operates. These antennas may operate either as transmitting or receiving antennas
On the left, characteristics plotted from experimentally obtained data on coordinates with logarithmic abscissa. On the right, an antenna with increased effective inductance between the two points in accordance with the well known operation of shunt tuned circuits adjusted somewhat off resonance.

In telecommunications and electrical engineering, electrical length (or phase length)

- Electrical length
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10 related topics with Alpha

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A stack of "fishbone" and Yagi–Uda television antennas

Antenna (radio)

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Antenna or aerial is the interface between radio waves propagating through space and electric currents moving in metal conductors, used with a transmitter or receiver.

Antenna or aerial is the interface between radio waves propagating through space and electric currents moving in metal conductors, used with a transmitter or receiver.

A stack of "fishbone" and Yagi–Uda television antennas
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.
Electronic symbol for an antenna
Antennas of the Atacama Large Millimeter/submillimeter Array.
An automobile's whip antenna, a common example of an omnidirectional antenna.
Half-wave dipole antenna
Diagram of the electric fields ( blue ) and magnetic fields ( red ) radiated by a dipole antenna ( black rods) during transmission.
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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Sometimes the resulting (lower) electrical resonant frequency of such a system (antenna plus matching network) is described using the concept of electrical length, so an antenna used at a lower frequency than its resonant frequency is called an electrically short antenna

UHF half-wave dipole

Dipole antenna

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Simplest and most widely used class of antenna.

Simplest and most widely used class of 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.

The feedpoint impedance of a dipole antenna is sensitive to its electrical length and feedpoint position.

Whip antenna on FM radio receiver

Whip antenna

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Antenna consisting of a straight flexible wire or rod.

Antenna consisting of a straight flexible wire or rod.

Whip antenna on FM radio receiver
Whip antenna on car
A rubber ducky antenna, a common type of electrically short whip, on a handheld UHF CB transceiver. With rubber sheath (left) removed.

The most common type is the quarter-wave whip, which is approximately 1⁄4 wavelength long, but they can be either longer or shorter by design, varying from compact electrically short antennas 1⁄10 wavelength long, up to 5⁄8 wavelength to improve directivity.

A typical mast radiator monopole antenna of an AM radio station in Chapel Hill, North Carolina. The mast itself is connected to the transmitter and radiates the radio waves. It is mounted on a ceramic insulator to isolate it from the ground. The other terminal of the transmitter is connected to a ground system consisting of cables buried under the field.

Monopole antenna

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Class of radio antenna consisting of a straight rod-shaped conductor, often mounted perpendicularly over some type of conductive surface, called a ground plane.

Class of radio antenna consisting of a straight rod-shaped conductor, often mounted perpendicularly over some type of conductive surface, called a ground plane.

A typical mast radiator monopole antenna of an AM radio station in Chapel Hill, North Carolina. The mast itself is connected to the transmitter and radiates the radio waves. It is mounted on a ceramic insulator to isolate it from the ground. The other terminal of the transmitter is connected to a ground system consisting of cables buried under the field.
Showing the monopole antenna has the same radiation pattern over perfect ground as a dipole in free space with twice the voltage
Vertical radiation patterns of ideal monopole antennas over a perfect infinite ground. The distance of the line from the origin at a given elevation angle is proportional to the power density radiated at that angle.
Multi-lobed radiation pattern of 3⁄2 wavelength monopole. Monopole antennas up to 1⁄2 wavelength long have a single "lobe", with field strength declining monotonically from a maximum in the horizontal direction, but longer monopoles have more complicated patterns with several conical "lobes" (radiation maxima) directed at angles into the sky.
VHF ground plane antenna, a type of monopole antenna used at high frequencies. The three conductors projecting downward are the ground plane

At lower frequencies the antenna mast is electrically short giving it a very small radiation resistance, so to increase efficiency and radiated power capacitively toploaded monopoles such as the T-antenna and umbrella antenna are used.

Pupin coils in PTT Museum in Belgrade (Serbia)

Loading coil

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Inductor that is inserted into an electronic circuit to increase its inductance.

Inductor that is inserted into an electronic circuit to increase its inductance.

Pupin coils in PTT Museum in Belgrade (Serbia)
Schematic of a balanced loaded telephone line. The capacitors are not discrete components but represent the distributed capacitance between the closely spaced wire conductors of the line, this is indicated by the dotted lines. The loading coils prevent the audio (voice) signal from being distorted by the line capacitance.  The windings of the loading coil are wound such that the magnetic flux induced in the core is in the same direction for both windings.
A typical mobile antenna with a center-placed loading coil
An enormous antenna loading coil used in a powerful longwave radiotelegraph station in New Jersey in 1912.
Oliver Heaviside
Pupin's design of loading coil
Permalloy cable construction
Mu-metal cable construction

To make an electrically short antenna resonant, a loading coil is inserted in series with the antenna.

Resonant stub tank circuits in vacuum tube backpack UHF transceiver, 1938. About 1/8 wavelength long: (left) 200 MHz stub is 19 cm, (right) 300 MHz stub is 12.5 cm

Stub (electronics)

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Length of transmission line or waveguide that is connected at one end only.

Length of transmission line or waveguide that is connected at one end only.

Resonant stub tank circuits in vacuum tube backpack UHF transceiver, 1938. About 1/8 wavelength long: (left) 200 MHz stub is 19 cm, (right) 300 MHz stub is 12.5 cm
10 kW FM broadcast transmitter from 1947 showing quarter-wave resonant stub plate tank circuit
In a stripline circuit, a stub may be placed just before an output connector to compensate for minor mismatches due to the device's output load or the connector itself.
A microstrip filter using butterfly stubs

Neglecting transmission line losses, the input impedance of the stub is purely reactive; either capacitive or inductive, depending on the electrical length of the stub, and on whether it is open or short circuit.

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

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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.

By charging the transmission line and then discharging it into a resistive load, a rectangular pulse equal in length to twice the electrical length of the line can be obtained, although with half the voltage.

Radio Tower in Yekaterinburg, Lunacharskogo 212

Radio masts and towers

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Radio masts and towers are typically tall structures designed to support antennas for telecommunications and broadcasting, including television.

Radio masts and towers are typically tall structures designed to support antennas for telecommunications and broadcasting, including television.

Radio Tower in Yekaterinburg, Lunacharskogo 212
A radio mast base showing how virtually all lateral support is provided by the guy-wires
The Tokyo Skytree, the tallest freestanding tower in the world, in 2012
Tokyo Tower
Multiwire broadcast T-antenna of early AM station WBZ, Springfield, Massachusetts, 1925.
Masts of the Rugby VLF transmitter near Rugby, England
A 3803 KM-type TV tower located in Penza
Typical 200 foot (61 m) triangular guyed lattice mast of an AM radio station in Mount Vernon, Washington, US
TV Tower in Stuttgart, Germany: the first reinforced-concrete TV tower.
Milad Tower, Tehran, Iran
Kamzík TV Tower, overlooking Bratislava, Slovakia.
This 100 ft tall cross conceals equipment for T-Mobile at Epiphany Lutheran Church in Lake Worth, Florida, US. Completed in December 2009.
Communications tower, at the horizon on the right, camouflaged as a tall tree.
Felsenegg-Girstel TV-tower
Uetliberg TV-tower
A radio amateur's do it yourself steel-lattice tower
Bergwacht antenna with a webcam mounted to aid in weather forecasting and observations of the Großer Feldberg plateau.
Radio tower in Jamshoro
The Desa Coalfields Antenna Tower, behind the trees, is the tallest antenna tower in Malaysia, with the height of 95.3 m, located at the front of the Sungai Buloh Prison

During the first 20 years of radio, long distance radio stations used long wavelengths in the very low frequency band, so even the tallest antennas were electrically short and had very low radiation resistance of 5-25 Ohms, causing excessive power losses in the ground system.

A typical earthing electrode (left of gray pipe), consisting of a conductive rod driven into the ground, at a home in Australia. Most electrical codes specify that the insulation on protective earthing conductors must be a distinctive color (or color combination) not used for any other purpose.

Ground (electricity)

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Reference point in an electrical circuit from which voltages are measured, a common return path for electric current, or a direct physical connection to the Earth.

Reference point in an electrical circuit from which voltages are measured, a common return path for electric current, or a direct physical connection to the Earth.

A typical earthing electrode (left of gray pipe), consisting of a conductive rod driven into the ground, at a home in Australia. Most electrical codes specify that the insulation on protective earthing conductors must be a distinctive color (or color combination) not used for any other purpose.
Metal water pipe used as grounding electrode
Busbars are used for ground conductors in high-current circuits.
3 ply static dissipative vinyl grounding mat shown at macro scale

In the LF and VLF bands, construction height limitations require that electrically short antennas be used, shorter than the fundamental resonant length of one quarter of a wavelength ( the resistance decreases with the square of the ratio of height to wavelength.

Typical mast radiator of a commercial medium wave AM broadcasting station, Chapel Hill, North Carolina, U.S.

Medium wave

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Part of the medium frequency (MF) radio band used mainly for AM radio broadcasting.

Part of the medium frequency (MF) radio band used mainly for AM radio broadcasting.

Typical mast radiator of a commercial medium wave AM broadcasting station, Chapel Hill, North Carolina, U.S.
Realistic TM-152 AM stereo tuner c. 1988
Multiwire T antenna of radio station WBZ, Massachusetts, USA, 1925. T antennas were the first antennas used for medium wave broadcasting, and are still used at lower power
Typical ferrite rod antenna used in AM radio receivers

Because such tall masts can be costly and uneconomic, other types of antennas are often used, which employ capacitive top-loading (electrical lengthening) to achieve equivalent signal strength with vertical masts shorter than a quarter wavelength.