A report on Dipole antenna and Electrical length

Dipole antenna used by the radar altimeter in an airplane

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.

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).

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

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.

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.

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.

Since resonant antennas are usually specified in terms of the electrical length of their conductors (such as the half wave dipole), the attainment of such an electrical length is loosely equated with electrical resonance, that is, a purely resistive impedance at the antenna's input, as is usually desired. An antenna that has been made slightly too long, for instance, will present an inductive reactance, which can be corrected by physically shortening the antenna. Based on this understanding, a common jargon in the antenna trade refers to the achievement of resonance (cancellation of reactance) at the antenna terminals as electrically shortening that too-long antenna (or electrically lengthening a too-short antenna) when an electrical matching network (or antenna tuner) has performed that task without physically altering the antenna's length. Although the terminology is very inexact, this use is widespread, especially as applied to the use of a loading coil at the bottom of a short monopole (a vertical, or whip antenna) to "electrically lengthen" it and achieve electrical resonance as seen through the loading coil.

- Electrical length

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

- Dipole antenna

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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.

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.

Antennas of the Atacama Large Millimeter/submillimeter Array.

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).

Hertz placed dipole antennas at the focal point of parabolic reflectors for both transmitting and receiving.

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

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.

Monopole and dipole radio antennas are designed to act as resonators for radio waves; the power from the transmitter, applied to the antenna through the antenna's transmission line, excites standing waves of voltage and current in the antenna element.

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

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.

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

This contrasts with a dipole antenna which consists of two identical rod conductors, with the signal from the transmitter applied between the two halves of the antenna.

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.

Medium wave

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

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.

In some rare cases dipole antennas are used, which are slung between two masts or towers.