Antenna (radio)

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

65 related topics

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Line of sight propagation to an antenna

Line-of-sight propagation

Characteristic of electromagnetic radiation or acoustic wave propagation which means waves travel in a direct path from the source to the receiver.

Characteristic of electromagnetic radiation or acoustic wave propagation which means waves travel in a direct path from the source to the receiver.

Line of sight propagation to an antenna
Objects within the Fresnel zone can disturb line of sight propagation even if they don't block the geometric line between antennas.
Two stations not in line-of-sight may be able to communicate through an intermediate radio repeater station.
R is the radius of the Earth, h is the height of the transmitter (exaggerated), d is the line of sight distance

The radio horizon is the locus of points at which direct rays from an antenna are tangential to the surface of the Earth.

UHF television antenna on a residence. This type of antenna, called a Yagi-Uda antenna, is widely used at UHF frequencies.

Ultra high frequency

ITU designation for radio frequencies in the range between 300 megahertz (MHz) and 3 gigahertz (GHz), also known as the decimetre band as the wavelengths range from one meter to one tenth of a meter (one decimeter).

ITU designation for radio frequencies in the range between 300 megahertz (MHz) and 3 gigahertz (GHz), also known as the decimetre band as the wavelengths range from one meter to one tenth of a meter (one decimeter).

UHF television antenna on a residence. This type of antenna, called a Yagi-Uda antenna, is widely used at UHF frequencies.
Corner reflector UHF-TV antenna from 1950s

The length of an antenna is related to the length of the radio waves used.

Animated diagram of waves from an isotropic radiator (red dot). As they travel away from the source, the waves decrease in amplitude by the inverse of distance, shown by the declining contrast of the wavefronts. This diagram only shows the waves in one plane through the source; an isotropic source actually radiates in all three dimensions.

Isotropic radiator

Theoretical point source of electromagnetic or sound waves which radiates the same intensity of radiation in all directions.

Theoretical point source of electromagnetic or sound waves which radiates the same intensity of radiation in all directions.

Animated diagram of waves from an isotropic radiator (red dot). As they travel away from the source, the waves decrease in amplitude by the inverse of distance, shown by the declining contrast of the wavefronts. This diagram only shows the waves in one plane through the source; an isotropic source actually radiates in all three dimensions.
A depiction of an isotropic radiator of sound, published in Popular Science Monthly in 1878. Note how the rings are even and of the same width all the way around each circle, though they fade as they move away from the source.
Diagram of antenna and resistor in cavity

Isotropic radiators are used as reference radiators with which other sources are compared, for example in determining the gain of antennas.

Examples of sound levels in decibels from various sound sources and activities, taken from the "How loud is too loud" screen of the NIOSH Sound Level Meter app

Decibel

Relative unit of measurement equal to one tenth of a bel (B).

Relative unit of measurement equal to one tenth of a bel (B).

Examples of sound levels in decibels from various sound sources and activities, taken from the "How loud is too loud" screen of the NIOSH Sound Level Meter app
A schematic showing the relationship between dBu (the voltage source) and dBm (the power dissipated as heat by the 600 Ω resistor)

dBd: dB(dipole) – the forward gain of an antenna compared with a half-wave dipole antenna. 0 dBd = 2.15 dBi

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Electromagnetic radiation

In physics, electromagnetic radiation (EMR) consists of waves of the electromagnetic (EM) field, propagating through space, carrying electromagnetic radiant energy.

In physics, electromagnetic radiation (EMR) consists of waves of the electromagnetic (EM) field, propagating through space, carrying electromagnetic radiant energy.

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Shows the relative wavelengths of the electromagnetic waves of three different colours of light (blue, green, and red) with a distance scale in micrometers along the x-axis.
In electromagnetic radiation (such as microwaves from an antenna, shown here) the term "radiation" applies only to the parts of the electromagnetic field that radiate into infinite space and decrease in intensity by an inverse-square law of power, so that the total radiation energy that crosses through an imaginary spherical surface is the same, no matter how far away from the antenna the spherical surface is drawn. Electromagnetic radiation thus includes the far field part of the electromagnetic field around a transmitter. A part of the "near-field" close to the transmitter, forms part of the changing electromagnetic field, but does not count as electromagnetic radiation.
Electromagnetic waves can be imagined as a self-propagating transverse oscillating wave of electric and magnetic fields. This 3D animation shows a plane linearly polarized wave propagating from left to right. The electric and magnetic fields in such a wave are in-phase with each other, reaching minima and maxima together.
Representation of the electric field vector of a wave of circularly polarized electromagnetic radiation.
James Clerk Maxwell
Electromagnetic spectrum with visible light highlighted
Rough plot of Earth's atmospheric absorption and scattering (or opacity) of various wavelengths of electromagnetic radiation

When any wire (or other conducting object such as an antenna) conducts alternating current, electromagnetic radiation is propagated at the same frequency as the current.

Coherent waves that travel along two different paths will arrive with phase shift, hence interfering with each other.

Multipath propagation

Coherent waves that travel along two different paths will arrive with phase shift, hence interfering with each other.
A diagram of the ideal situation for TV signals moving through space: The signal leaves the transmitter (TX) and travels through one path to the receiver (the TV set, which is labeled RX)
In this illustration, an object (in this case an aircraft) pollutes the system by adding a second path. The signal arrives at receiver (RX) by means 
of two different paths which have different lengths. The main path is the direct path, while the second is due to a reflection from the plane.
Radar multipath echoes from an actual target cause ghosts to appear.
GPS error due to multipath
Mathematical model of the multipath impulse response.

In radio communication, multipath is the propagation phenomenon that results in radio signals reaching the receiving antenna by two or more paths.

Antenna tuner front view, with partially exposed interior.

Antenna tuner

Antenna tuner front view, with partially exposed interior.
Atmospheric noise as a function of frequency in the LF, MF, and HF radio spectrum according to CCIR 322. The vertical axis is in decibels above the thermal noise floor. It can be seen that as frequency drops atmospheric noise dominates other sources.
A 300-to-75-ohm TV line balun, showing a coaxial connector (balun inside) on the left with twin-lead trailing off to the right.
1:1, 1:4 and 1:9 autotransformer
Basic network
An automatic ATU for amateur transceiver. The columns of white components are relays that switch toroidal coils (red, right-most column) and wafer capacitors (black, central column) into and out of the matching circuit. The large black square at the lower left is the CPU that operates the circuitry.
Two networks in a circuit; both have the same impedance
Three networks in a circuit, all with the same impedance
Six of the eight possible 'L'-network circuits
T-network transmatch
Six balanced tuner schematics
Schematic of Z match antenna tuner
The Z match tuner response
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Cross-needle SWR meter on antenna tuner
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Two possible configurations of a transmitter comprising an antenna, a single-antenna-port antenna tuner (AT), a sensing unit (SU), a control unit (CU) and a transmission and signal processing unit (TSPU).
Circuit as seen by user; parts impedance shown on diagram
After one transformation (unlabeled part impedance is -j 5200Ω)
After two transformations
After three transformations
After four transformations
All eight possible ‘L’-network circuits
Alfred Annecke’s c. 1970 enhanced version of the Johnson Matchbox antenna tuner
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Low impedance 75 Ω type RG-59 coaxial cable: 
 Outer plastic jacket, Woven copper shield, Inner dielectric insulator,  Copper core.

An antenna tuner (and any of the names in the list below) is a device that is inserted between a radio transmitter and its antenna; when properly adjusted (tuned) it improves power transfer by matching the impedance of the radio to the impedance of the antenna, or the feedline which connects the antenna to the transmitter.

Patch antenna gain pattern

Directional antenna

Patch antenna gain pattern
An early example (1922) of a directional AM radio transmitter, built for WOR, then in New Jersey and targeting both New York City and Philadelphia.
Karl Jansky and his rotating directional radio antenna (1932) in Holmdel, New Jersey, which was the world's first radio telescope, discovering radio emissions from the Milky Way.
Grote Reber's homemade antenna in Wheaton, Illinois (1937), world's second radio telescope and first parabolic radio telescope
Holmdel Horn Antenna in Holmdel, New Jersey (1960s). Built to support the Echo satellite communication program,<ref>{{cite journal |author=Crawford, A.B., D.C. Hogg and L.E. Hunt |title=Project Echo: A Horn-Reflector Antenna for Space Communication |journal=The Bell System Technical Journal |date=July 1961 |pages=1095–1099}}</ref> it was later used in experiments that revealed the cosmic background radiation permeating the universe.<ref>{{cite web |url=http://www.nps.gov/history/history/online_books/butowsky5/astro4k.htm |title=National Park Service: Astronomy and Astrophysics (Horn Antenna) |access-date=2008-05-23 |date=2001-11-05 |url-status=dead |archive-url=https://web.archive.org/web/20080512093810/http://www.nps.gov/history/history/online_books/butowsky5/astro4k.htm |archive-date=2008-05-12 }}</ref>
Parabolic antenna – the 70 m antenna at Goldstone Deep Space Communications Complex in the Mojave Desert, California
Voyager 2 spacecraft. The HGA (a parabolic antenna) is the large bowl-shaped object.
A giant phased-array radar in Alaska
A Yagi-Uda antenna. From left to right, the elements mounted on the boom are called the reflector, driven element, and director. The reflector is easily identified as being a bit (5%) longer than the driven element, and the director a bit (5%) shorter.

A directional antenna or beam antenna is an antenna which radiates or receives greater power in specific directions allowing increased performance and reduced interference from unwanted sources.

A common type of array antenna, a reflective array UHF television antenna. This example consists of eight dipole driven elements mounted in front of a wire screen reflector. The X-shaped dipoles give it a wide bandwidth to cover both the VHF (174&ndash;216 MHz) and UHF (470&ndash;700 MHz) TV bands. It has a gain of 5 dB VHF and 12 dB UHF and an 18 dB front-to-back ratio.

Antenna array

A common type of array antenna, a reflective array UHF television antenna. This example consists of eight dipole driven elements mounted in front of a wire screen reflector. The X-shaped dipoles give it a wide bandwidth to cover both the VHF (174&ndash;216 MHz) and UHF (470&ndash;700 MHz) TV bands. It has a gain of 5 dB VHF and 12 dB UHF and an 18 dB front-to-back ratio.
Large planar array antenna of a VHF Russian mobile air defense radar, the Nebo-M. It consists of 175 folded dipole antennas. An early phased array, the antenna radiated a vertical fan-shaped beam which could be swept horizontally across the airspace in front of the antenna.
Animation showing how a phased array works.
A rooftop television antenna, an endfire parasitic array consisting of a combination of a Yagi and log periodic antenna
VHF collinear array of folded dipoles
Sector antennas (white bars) on cell phone tower. Collinear dipole arrays, radiating a flat, fan-shaped beam.
108 MHz reflective array antenna of an SCR-270 radar used during World War II consists of 32 half-wave dipole antennas in front of a reflecting screen.
US Air Force PAVE PAWS phased array 420 - 450 MHz radar antenna for ballistic missile detection, Alaska. The two circular arrays are each composed of 2677 crossed dipole antennas.
Some of the crossed-dipole elements in the PAVE PAWS phased array antenna, left
Batwing VHF television broadcasting antenna
Crossed-dipole FM radio broadcast antenna
Curtain array shortwave transmitting antenna, Austria. Wire dipoles suspended between towers
Turnstile antenna array used for satellite communication
Flat microstrip array antenna for satellite TV reception.
The Very Large Array, a radio telescope made of a Y-shaped array of 27 dish antennas in Socorro, New Mexico
HAARP, a phased array of 180 crossed dipoles in Alaska which can transmit a 3.6 MW beam of 3 - 10 MHz radio waves into the ionosphere for research purposes
Array of four helical antennas used as a satellite tracking antenna, Pleumeur-Bodou, France

An antenna array (or array antenna) is a set of multiple connected antennas which work together as a single antenna, to transmit or receive radio waves.

Aurora at Alaska showing light created by charged particles and magnetism, fundamental concepts to electromagnetism study

Aperture (antenna)

Aurora at Alaska showing light created by charged particles and magnetism, fundamental concepts to electromagnetism study

In electromagnetics and antenna theory, the aperture of an antenna is defined as "A surface, near or on an antenna, on which it is convenient to makeassumptions regarding the field values for the purpose of computing fields at external points. NOTE - The aperture is often taken as that portion of a plane surface near the antenna, perpendicular to the direction of maximum radiation, through which the major part of the radiation passes."