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A directional antenna or beam antenna is an antenna that radiates or receives greater radio wave power in specific directions. Directional antennas can radiate
radio wave Radio waves (formerly called Hertzian waves) are a type of electromagnetic radiation with the lowest frequencies and the longest wavelengths in the electromagnetic spectrum, typically with frequencies below 300 gigahertz (GHz) and wavelengths g ...
s in beams, when greater concentration of radiation in a certain direction is desired, or in receiving antennas receive radio waves from one specific direction only. This can increase the power transmitted to receivers in that direction, or reduce
interference Interference is the act of interfering, invading, or poaching. Interference may also refer to: Communications * Interference (communication), anything which alters, modifies, or disrupts a message * Adjacent-channel interference, caused by extra ...
from unwanted sources. This contrasts with
omnidirectional antenna In radio communication, an omnidirectional antenna is a class of antenna (electronics), antenna which radiates equal radio power in all directions perpendicular to an Cartesian coordinate system, axis (azimuthal directions), with power varying wi ...
s such as
dipole antenna In radio and telecommunications a dipole antenna or doublet is one of the two simplest and most widely used antenna types, types of antenna; the other is the monopole antenna, monopole. The dipole is any one of a class of antennas producin ...
s which radiate radio waves over a wide angle, or receive from a wide angle. The extent to which an antenna's angular distribution of radiated power, its
radiation pattern In the field of antenna design the term radiation pattern (or antenna pattern or far-field pattern) refers to the ''directional'' (angular) dependence of the strength of the radio waves from the antenna or other source.Constantine A. Balanis: " ...
, is concentrated in one direction is measured by a parameter called
antenna gain In electromagnetics, an antenna's gain is a key performance parameter which combines the antenna's directivity and radiation efficiency. The term ''power gain'' has been deprecated by IEEE. In a transmitting antenna, the gain describes how ...
. A high-gain antenna (HGA) is a directional antenna with a focused, narrow beam width, permitting more precise targeting of the radio signals. Most commonly referred to during space missions, these antennas are also in use all over
Earth Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...
, most successfully in flat, open areas where there are no mountains to disrupt radiowaves. In contrast, a low-gain antenna (LGA) is an
omnidirectional antenna In radio communication, an omnidirectional antenna is a class of antenna (electronics), antenna which radiates equal radio power in all directions perpendicular to an Cartesian coordinate system, axis (azimuthal directions), with power varying wi ...
, with a broad radiowave beam width, that allows the signal to propagate reasonably well even in mountainous regions and is thus more reliable regardless of terrain. Low-gain antennas are often used in
spacecraft A spacecraft is a vehicle that is designed spaceflight, to fly and operate in outer space. Spacecraft are used for a variety of purposes, including Telecommunications, communications, Earth observation satellite, Earth observation, Weather s ...
as a backup to the ''high-gain antenna'', which transmits a much narrower beam and is therefore susceptible to loss of signal. All practical antennas are at least somewhat directional, although usually only the direction in the plane parallel to the earth is considered, and practical antennas can easily be omnidirectional in one plane. The most common directional antenna types are * the
Yagi–Uda antenna A Yagi–Uda antenna, or simply Yagi antenna, is a directional antenna consisting of two or more parallel Antenna (radio)#Resonant antennas, resonant antenna elements in an Antenna array#Types, end-fire array; these elements are most often metal ...
, * the log-periodic antenna, and * the
corner reflector antenna A corner reflector antenna is a type of directional antenna used at VHF and UHF frequencies. It was invented by John D. Kraus in 1938.Kraus, John D., US patent 2270314 Corner reflector antenna filed January 31, 1940; granted January 20, 1942. ...
. These antenna types, or combinations of several single-frequency versions of one type or (rarely) a combination of two different types, are frequently sold commercially as residential TV antennas.
Cellular repeater A cellular repeater (also known as cell phone signal booster or cell phone signal amplifier) is a type of bi-directional amplifier used to improve cell phone reception. A cellular repeater system commonly consists of a donor Antenna (radio), anten ...
s often make use of external directional antennas to give a far greater signal than can be obtained on a standard
cell phone A mobile phone or cell phone is a portable telephone that allows users to make and receive calls over a radio frequency link while moving within a designated telephone service area, unlike fixed-location phones ( landline phones). This radio ...
.
Satellite television Satellite television is a service that delivers television programming to viewers by relaying it from a communications satellite orbiting the Earth directly to the viewer's location.ITU Radio Regulations, Section IV. Radio Stations and Systems ...
receivers usually use
parabolic antenna A parabolic antenna is an antenna that uses a parabolic reflector, a curved surface with the cross-sectional shape of a parabola, to direct the radio waves. The most common form is shaped like a dish and is popularly called a dish antenna or p ...
s. For long and medium
wavelength In physics and mathematics, wavelength or spatial period of a wave or periodic function is the distance over which the wave's shape repeats. In other words, it is the distance between consecutive corresponding points of the same ''phase (waves ...
frequencies Frequency is the number of occurrences of a repeating event per unit of time. Frequency is an important parameter used in science and engineering to specify the rate of oscillatory and vibratory phenomena, such as mechanical vibrations, audio ...
,
tower array A tower array is an arrangement of multiple radio towers which are mast radiators in a phased array. They were originally developed as ground-based tracking radars. Tower arrays can consist of free-standing or guyed towers or a mix of them. Tower ...
s are used in most cases as directional antennas.


Principle of operation

When transmitting, a ''high-gain antenna'' allows more of the transmitted power to be sent in the direction of the receiver, increasing the received signal strength. When receiving, a high gain antenna captures more of the signal, again increasing signal strength. Due to reciprocity, these two effects are equal—an antenna that makes a transmitted signal 100 times stronger (compared to an isotropic radiator) will also capture 100 times as much energy as the isotropic antenna when used as a receiving antenna. As a consequence of their directivity, directional antennas also send less (and receive less) signal from directions other than the main beam. This property may avoid interference from other out-of-beam transmitters, and always reduces antenna noise. (Noise comes from every direction, but a desired signal will only come from one approximate direction, so the narrower the antenna's beam, the better the crucial
signal-to-noise ratio Signal-to-noise ratio (SNR or S/N) is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. SNR is defined as the ratio of signal power to noise power, often expressed in deci ...
.) There are many ways to make a high-gain antenna; the most common are
parabolic antenna A parabolic antenna is an antenna that uses a parabolic reflector, a curved surface with the cross-sectional shape of a parabola, to direct the radio waves. The most common form is shaped like a dish and is popularly called a dish antenna or p ...
s, helical antennas, Yagi-Uda antennas, and
phased array In antenna (radio), antenna theory, a phased array usually means an electronically scanned array, a computer-controlled Antenna array, array of antennas which creates a radio beam, beam of radio waves that can be electronically steered to point ...
s of smaller antennas of any kind.
Horn antenna A horn antenna or microwave horn is an antenna (radio), antenna that consists of a flaring metal waveguide shaped like a horn (acoustic), horn to direct radio waves in a beam. Horns are widely used as antennas at Ultrahigh frequency, UHF and m ...
s can also be constructed with high gain, but are less commonly seen. Still other configurations are possible—the
Arecibo Observatory The Arecibo Observatory, also known as the National Astronomy and Ionosphere Center (NAIC) and formerly known as the Arecibo Ionosphere Observatory, is an observatory in Barrio Esperanza, Arecibo, Puerto Rico owned by the US National Science F ...
used a combination of a ''line feed'' with an enormous spherical reflector (as opposed to a more usual parabolic reflector), to achieve extremely high gains at specific frequencies.


Antenna gain

Antenna gain In electromagnetics, an antenna's gain is a key performance parameter which combines the antenna's directivity and radiation efficiency. The term ''power gain'' has been deprecated by IEEE. In a transmitting antenna, the gain describes how ...
is often quoted with respect to a hypothetical antenna that radiates equally in all directions, an isotropic radiator. This gain, when measured in
decibel The decibel (symbol: dB) is a relative unit of measurement equal to one tenth of a bel (B). It expresses the ratio of two values of a Power, root-power, and field quantities, power or root-power quantity on a logarithmic scale. Two signals whos ...
s, is called dBi. Conservation of energy dictates that high gain antennas must have narrow beams. For example, if a high gain antenna makes a 1 
Watt The watt (symbol: W) is the unit of Power (physics), power or radiant flux in the International System of Units (SI), equal to 1 joule per second or 1 kg⋅m2⋅s−3. It is used to quantification (science), quantify the rate of Work ...
transmitter look like a 100 Watt transmitter, then the beam can cover at most of the sky (otherwise the total amount of energy radiated in all directions would sum to more than the transmitter power, which is not possible). In turn this implies that high-gain antennas must be physically large, since according to the
diffraction limit In optics, any optical instrument or systema microscope, telescope, or camerahas a principal limit to its resolution due to the physics of diffraction. An optical instrument is said to be diffraction-limited if it has reached this limit of res ...
, the narrower the beam desired, the larger the antenna must be (measured in wavelengths). Antenna gain can also be measured in dBd, which is gain in
decibel The decibel (symbol: dB) is a relative unit of measurement equal to one tenth of a bel (B). It expresses the ratio of two values of a Power, root-power, and field quantities, power or root-power quantity on a logarithmic scale. Two signals whos ...
s compared to the maximum intensity direction of a half wave dipole. In the case of Yagi-type aerials this more or less equates to the gain one would expect from the aerial under test minus all its directors and reflector. It is important not to confuse dB and dB; the two differ by 2.15 dB, with the dBi figure being higher, since a dipole has 2.15 dB of gain with respect to an isotropic antenna. Gain is also dependent on the number of elements and the tuning of those elements. Antennas can be tuned to be resonant over a wider spread of frequencies but, all other things being equal, this will mean the gain of the aerial is lower than one tuned for a single frequency or a group of frequencies. For example, in the case of wideband TV antennas the fall off in gain is particularly large at the bottom of the TV transmitting band. In the UK this bottom third of the TV band is known as group A. Other factors may also affect gain such as aperture (the area the antenna collects signal from, almost entirely related to the size of the antenna but for small antennas can be increased by adding a ferrite rod), and efficiency (again, affected by size, but also resistivity of the materials used and impedance matching). These factors are easy to improve without adjusting other features of the antennas or coincidentally improved by the same factors that increase directivity, and so are typically not emphasized.


Applications

High gain antennas are typically the largest component of deep space probes, and the highest gain radio antennas are physically enormous structures, such as the
Arecibo Observatory The Arecibo Observatory, also known as the National Astronomy and Ionosphere Center (NAIC) and formerly known as the Arecibo Ionosphere Observatory, is an observatory in Barrio Esperanza, Arecibo, Puerto Rico owned by the US National Science F ...
. The
Deep Space Network The NASA Deep Space Network (DSN) is a worldwide Telecommunications network, network of spacecraft communication ground segment facilities, located in the United States (California), Spain (Madrid), and Australia (Canberra), that supports NASA' ...
uses 35 m dishes at about 1 cm wavelengths. This combination gives the antenna gain of about 100,000,000 (or 80 dB, as normally measured), making the transmitter appear about 100 million times stronger, and a receiver about 100 million times more sensitive, ''provided the target is within the beam''. This beam can cover at most one hundred millionth () of the sky, so very accurate pointing is required. Use of high gain and millimeter-wave communication in WPAN gaining increases the probability of concurrent scheduling of non‐interfering transmissions in a localized area, which results in an immense increase in network throughput. However, the optimum scheduling of concurrent transmission is an
NP-hard In computational complexity theory, a computational problem ''H'' is called NP-hard if, for every problem ''L'' which can be solved in non-deterministic polynomial-time, there is a polynomial-time reduction from ''L'' to ''H''. That is, assumi ...
problem.


Gallery

Image:BambergersWOR1922.jpg, An early example (1922) of a directional AM radio transmitter using a long wire antenna, built for WOR, then in Newark, New Jersey and targeting both New York City and Philadelphia in addition to Newark. File:JanskyatAntenna hi.tif,
Karl Jansky Karl Guthe Jansky (October 22, 1905 – February 14, 1950) was an American physicist and radio engineer who in April 1933 first announced his discovery of radio waves emanating from the Milky Way in the constellation Sagittarius. He is consider ...
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 The Milky Way or Milky Way Galaxy is the galaxy that includes the Solar System, with the name describing the #Appearance, galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars in other arms of the galax ...
. File:Grote Antenna Wheaton.gif, Grote Reber's homemade antenna in Wheaton, Illinois (1937), world's second radio telescope and first parabolic radio telescope Image:Horn Antenna-in Holmdel, New Jersey - restoration1.jpg, Holmdel Horn Antenna in Holmdel, New Jersey (1960s). Built to support the Echo satellite communication program, it was later used in experiments that revealed the
Cosmic microwave background The cosmic microwave background (CMB, CMBR), or relic radiation, is microwave radiation that fills all space in the observable universe. With a standard optical telescope, the background space between stars and galaxies is almost completely dar ...
permeating the universe. Image:Goldstone DSN antenna.jpg,
Parabolic antenna A parabolic antenna is an antenna that uses a parabolic reflector, a curved surface with the cross-sectional shape of a parabola, to direct the radio waves. The most common form is shaped like a dish and is popularly called a dish antenna or p ...
– the 70 m antenna at Goldstone Deep Space Communications Complex in the Mojave Desert,
California California () is a U.S. state, state in the Western United States that lies on the West Coast of the United States, Pacific Coast. It borders Oregon to the north, Nevada and Arizona to the east, and shares Mexico–United States border, an ...
Image:Voyager_Spacecraft_During_Vibration_Testing_-_GPN-2003-000008.jpg,
Voyager 2 ''Voyager 2'' is a space probe launched by NASA on August 20, 1977, as a part of the Voyager program. It was launched on a trajectory towards the gas giants (Jupiter and Saturn) and enabled further encounters with the ice giants (Uranus and ...
spacecraft. The HGA (a parabolic antenna) is the large bowl-shaped object. Image:PAVE PAWS Radar Clear AFS Alaska.jpg, A giant phased-array radar in Alaska Image:Montreal-tower-top.thumb2.jpg, 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 longer (5% or more) than all the other elements, and the director(s) a bit shorter (5% or more).


See also

* Amateur radio direction finding * Antenna boresight *
Antenna gain In electromagnetics, an antenna's gain is a key performance parameter which combines the antenna's directivity and radiation efficiency. The term ''power gain'' has been deprecated by IEEE. In a transmitting antenna, the gain describes how ...
* Cantenna * Cardioid *
Cassegrain antenna In telecommunications and radar, a Cassegrain antenna is a parabolic antenna in which the Antenna feed, feed antenna is mounted at or behind the surface of the concave main parabolic reflector dish and is aimed at a smaller Convex mirror, convex ...
*
Cassegrain reflector The Cassegrain reflector is a combination of a primary concave mirror and a secondary convex mirror, often used in optical telescopes and Antenna (radio), radio antennas, the main characteristic being that the optical path folds back onto itself, ...
*
Directivity In electromagnetics, directivity is a parameter of an antenna or optical system which measures the degree to which the radiation emitted is concentrated in a single direction. It is the ratio of the radiation intensity in a given direction f ...
*
Loop antenna A loop antenna is a antenna (radio), radio antenna consisting of a loop or coil of wire, tubing, or other electrical conductor, that for transmitting is usually fed by a balanced power source or for receiving feeds a balanced load. Within this p ...
*
Omnidirectional antenna In radio communication, an omnidirectional antenna is a class of antenna (electronics), antenna which radiates equal radio power in all directions perpendicular to an Cartesian coordinate system, axis (azimuthal directions), with power varying wi ...
*
Parabolic antenna A parabolic antenna is an antenna that uses a parabolic reflector, a curved surface with the cross-sectional shape of a parabola, to direct the radio waves. The most common form is shaped like a dish and is popularly called a dish antenna or p ...
*
Phased array In antenna (radio), antenna theory, a phased array usually means an electronically scanned array, a computer-controlled Antenna array, array of antennas which creates a radio beam, beam of radio waves that can be electronically steered to point ...
*
Radio direction finder Direction finding (DF), radio direction finding (RDF), or radiogoniometry is the use of radio waves to determine the direction to a radio source. The source may be a cooperating radio transmitter or may be an inadvertent source, a natural ...
*
Radio propagation model Radio propagation is the behavior of radio waves as they travel, or are propagated, from one point to another in vacuum, or into various parts of the atmosphere. As a form of electromagnetic radiation, like light waves, radio waves are affect ...
, Antenna subsection *
Radiation pattern In the field of antenna design the term radiation pattern (or antenna pattern or far-field pattern) refers to the ''directional'' (angular) dependence of the strength of the radio waves from the antenna or other source.Constantine A. Balanis: " ...


References


External links

* {{DEFAULTSORT:Directional Antenna Radio frequency antenna types Radio frequency propagation Antennas (radio)