A report on Antenna (radio) and Electrical length

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.

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

Antennas of the Atacama Large Millimeter/submillimeter Array.

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.

An automobile's whip antenna, a common example of an omnidirectional antenna.

Diagram of the electric fields ( blue ) and magnetic fields ( red ) radiated by a dipole antenna ( black rods) during transmission.

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).

In addition to the directive gain in beam antennas suffering away from the design frequency, the antenna feedpoint impedance is very sensitive to frequency offsets.

- Electrical length

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

- Antenna (radio)

6 related topics with Alpha

Monopole antenna

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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

A monopole antenna is a class of radio antenna consisting of a straight rod-shaped conductor, often mounted perpendicularly over some type of conductive surface, called a 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.

Dipole antenna

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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).

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.

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

Whip antenna

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

A whip antenna is an antenna consisting of a straight flexible wire or rod.

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.

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.

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.

The term is also used for inductors in radio antennas, or between the antenna and its feedline, to make an electrically short antenna resonant at its operating frequency.

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

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.

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

Certain types of radio antennas (or their feedlines) require a connection to ground.

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.

Transmission line

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

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.

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.