Array antennas
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An antenna array (or array antenna) is a set of multiple connected
antenna Antenna ( antennas or antennae) may refer to: Science and engineering * Antenna (radio), also known as an aerial, a transducer designed to transmit or receive electromagnetic (e.g., TV or radio) waves * Antennae Galaxies, the name of two collid ...
s which work together as a single antenna, to transmit or receive
radio wave Radio waves are a type of electromagnetic radiation with the longest wavelengths in the electromagnetic spectrum, typically with frequencies of 300 gigahertz (GHz) and below. At 300 GHz, the corresponding wavelength is 1 mm (short ...
s. The individual antennas (called ''elements'') are usually connected to a single receiver or
transmitter In electronics and telecommunications, a radio transmitter or just transmitter is an electronic device which produces radio waves with an antenna (radio), antenna. The transmitter itself generates a radio frequency alternating current, which i ...
by
feedline In a radio antenna, the feed line (feedline), or feeder, is the cable or other transmission line that connects the antenna with the radio transmitter or receiver. In a transmitting antenna, it feeds the radio frequency (RF) current from the ...
s that feed the power to the elements in a specific
phase Phase or phases may refer to: Science *State of matter, or phase, one of the distinct forms in which matter can exist *Phase (matter), a region of space throughout which all physical properties are essentially uniform * Phase space, a mathematic ...
relationship. The radio waves radiated by each individual antenna combine and superpose, adding together ( interfering constructively) to enhance the power radiated in desired directions, and cancelling ( interfering destructively) to reduce the power radiated in other directions. Similarly, when used for receiving, the separate radio frequency currents from the individual antennas combine in the receiver with the correct phase relationship to enhance signals received from the desired directions and cancel signals from undesired directions. More sophisticated array antennas may have multiple transmitter or receiver modules, each connected to a separate antenna element or group of elements. An antenna array can achieve higher
gain Gain or GAIN may refer to: Science and technology * Gain (electronics), an electronics and signal processing term * Antenna gain * Gain (laser), the amplification involved in laser emission * Gain (projection screens) * Information gain in de ...
(
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 from ...
), that is a narrower beam of radio waves, than could be achieved by a single element. In general, the larger the number of individual antenna elements used, the higher the gain and the narrower the beam. Some antenna arrays (such as military
phased array In antenna theory, a phased array usually means an electronically scanned array, a computer-controlled array of antennas which creates a beam of radio waves that can be electronically steered to point in different directions without moving th ...
radars) are composed of thousands of individual antennas. Arrays can be used to achieve higher gain, to give path diversity (also called MIMO) which increases communication reliability, to
cancel Cancel or cancellation may refer to: *Flight cancellation and delay, not operating a scheduled flight Sociology * Cancel culture, boycott and ostracism calling out offensive behavior on social media or in real life Technology and science ...
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 specific directions, to steer the radio beam electronically to point in different directions, and for
radio direction finding Direction finding (DF), or radio direction finding (RDF), isin accordance with International Telecommunication Union (ITU)defined as radio location that uses the reception of radio waves to determine the direction in which a radio station ...
(RDF). The term antenna array most commonly means a ''driven array'' consisting of multiple identical driven elements all connected to the receiver or transmitter. A ''parasitic array'' consists of a single driven element connected to the feedline, and other elements which are not, called parasitic elements. It is usually another name for a
Yagi–Uda antenna A Yagi–Uda antenna or simply Yagi antenna, is a directional antenna consisting of two or more parallel resonant antenna elements in an end-fire array; these elements are most often metal rods acting as half-wave dipoles. Yagi–Uda a ...
. A ''
phased array In antenna theory, a phased array usually means an electronically scanned array, a computer-controlled array of antennas which creates a beam of radio waves that can be electronically steered to point in different directions without moving th ...
'' usually means an ''electronically scanned array''; a driven array antenna in which each individual element is connected to the transmitter or receiver through a
phase shifter A phase shift module is a microwave network module which provides a controllable phase shift of the RF signal. Phase shifters are used in phased arrays. Classification Active versus passive Active phase shifters provide gain, while passive ...
controlled by a computer. The beam of radio waves can be steered electronically to point instantly in any direction over a wide angle, without moving the antennas. However the term "phased array" is sometimes used to mean an ordinary array antenna.


How arrays work

From the Rayleigh criterion, the
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 from ...
of an antenna, the angular width of the beam of radio waves it emits, is proportional to the wavelength of the radio waves divided by the width of the antenna. Small antennas around one wavelength in size, such as quarter-wave monopoles and half-wave dipoles, don't have much directivity (
gain Gain or GAIN may refer to: Science and technology * Gain (electronics), an electronics and signal processing term * Antenna gain * Gain (laser), the amplification involved in laser emission * Gain (projection screens) * Information gain in de ...
); they are omnidirectional antennas which radiate radio waves over a wide angle. To create a
directional antenna 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. Directional antennas provide increased performance ...
( high gain antenna), which radiates radio waves in a narrow beam, two general techniques can be used. One technique is to use reflection by large metal surfaces such as
parabolic reflectors A parabolic (or paraboloid or paraboloidal) reflector (or dish or mirror) is a reflective surface used to collect or project energy such as light, sound, or radio waves. Its shape is part of a circular paraboloid, that is, the surface generated ...
or
horns Horns or The Horns may refer to: * Plural of Horn (instrument), a group of musical instruments all with a horn-shaped bells * The Horns (Colorado), a summit on Cheyenne Mountain * ''Horns'' (novel), a dark fantasy novel written in 2010 by Joe Hill ...
, or
refraction In physics, refraction is the redirection of a wave as it passes from one medium to another. The redirection can be caused by the wave's change in speed or by a change in the medium. Refraction of light is the most commonly observed phenomeno ...
by dielectric
lenses A lens is a transmissive optical device which focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements''), ...
to change the direction of the radio waves, to focus the radio waves from a single low gain antenna into a beam. This type is called an ''aperture antenna''. A
parabolic dish A parabolic (or paraboloid or paraboloidal) reflector (or dish or mirror) is a reflective surface used to collect or project energy such as light, sound, or radio waves. Its shape is part of a circular paraboloid, that is, the surface generated ...
is an example of this type of antenna. A second technique is to use multiple antennas which are fed from the same transmitter or receiver; this is called an array antenna, or antenna array. If the currents are fed to the antennas with the proper
phase Phase or phases may refer to: Science *State of matter, or phase, one of the distinct forms in which matter can exist *Phase (matter), a region of space throughout which all physical properties are essentially uniform * Phase space, a mathematic ...
, due to the phenomenon of
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 ...
the spherical waves from the individual antennas combine (superpose) in front of the array to create
plane wave In physics, a plane wave is a special case of wave or field: a physical quantity whose value, at any moment, is constant through any plane that is perpendicular to a fixed direction in space. For any position \vec x in space and any time t, th ...
s, a beam of radio waves traveling in a specific direction. In directions in which the waves from the individual antennas arrive in phase, the waves add together ( constructive interference) to enhance the power radiated. In directions in which the individual waves arrive out of phase, with the peak of one wave coinciding with the valley of another, the waves cancel ( destructive interference) reducing the power radiated in that direction. Similarly, when receiving, the oscillating currents received by the separate antennas from radio waves received from desired directions are in phase and when combined in the receiver reinforce each other, while currents from radio waves received from other directions are out of phase and when combined in the receiver cancel each other. The radiation pattern of such an antenna consists of a strong beam in one direction, the
main lobe In a radio antenna's radiation pattern, the main lobe, or main beam, is the lobe containing the higher power. This is the lobe that exhibits the greater field strength. The radiation pattern of most antennas shows a pattern of "''lobes''" at v ...
, plus a series of weaker beams at different angles called
sidelobe In antenna engineering, sidelobes are the lobes (local maxima) of the far field radiation pattern of an antenna or other radiation source, that are not the ''main lobe''. The radiation pattern of most antennas shows a pattern of "''lobes''" ...
s, usually representing residual radiation in unwanted directions. The larger the width of the antenna and the greater the number of component antenna elements, the narrower the main lobe, and the higher the gain which can be achieved, and the smaller the sidelobes will be. Arrays in which the antenna elements are fed in phase are broadside arrays; the main lobe is emitted perpendicular to the plane of the elements. The largest array antennas are radio interferometers used in the field of
radio astronomy Radio astronomy is a subfield of astronomy that studies celestial objects at radio frequencies. The first detection of radio waves from an astronomical object was in 1933, when Karl Jansky at Bell Telephone Laboratories reported radiation coming f ...
, in which multiple
radio telescope A radio telescope is a specialized antenna and radio receiver used to detect radio waves from astronomical radio sources in the sky. Radio telescopes are the main observing instrument used in radio astronomy, which studies the radio frequency ...
s consisting of large
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 pa ...
s are linked together into an antenna array, to achieve higher resolution. Using the technique called aperture synthesis such an array can have the resolution of an antenna with a diameter equal to the distance between the antennas. In the technique called Very Long Baseline Interferometry (VLBI) dishes on separate continents have been linked, creating "array antennas" thousands of miles in size.


Gallery

File:Collinear folded dipole antenna array.jpg, VHF collinear array of folded dipoles File:6 sector site in CDMA.jpg,
Sector antenna A sector antenna is a type of directional microwave antenna with a sector-shaped radiation pattern. The word "sector" is used in the geometric sense; some portion of the circumference of a circle measured in degrees of arc. 60°, 90° and 120° ...
s ''(white bars)'' on
cell phone tower A cell site, cell tower, or cellular base station is a cellular-enabled mobile device site where antennas and electronic communications equipment are placed (typically on a radio mast, tower, or other raised structure) to create a cell, or adjac ...
. Collinear dipole arrays, radiating a flat, fan-shaped beam. File:SCR-270 Radar Antenna2.jpg, 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. File:PAVE PAWS Radar Clear AFS Alaska.jpg, US Air Force
PAVE PAWS PAVE PAWS (PAVE Phased Array Warning System) is a complex Cold War early warning radar and computer system developed in 1980 to "detect and characterize a sea-launched ballistic missile attack against the United States". With the first solid-st ...
phased array In antenna theory, a phased array usually means an electronically scanned array, a computer-controlled array of antennas which creates a beam of radio waves that can be electronically steered to point in different directions without moving th ...
420–450 MHz radar antenna for ballistic missile detection, Alaska. The two circular arrays are each composed of 2677 crossed dipole antennas. File:Cape Cod Air Station - HAER MA-151-A - 384568pu.jpg, Some of the crossed-dipole elements in the PAVE PAWS phased array antenna, left File:Superturnstile Tx Muehlacker.JPG, Batwing VHF television broadcasting antenna File:FM broadcasting antenna Willans Hill.jpg, Crossed-dipole FM radio broadcast antenna File:Moosbrunn SW Antenna.jpg,
Curtain array Curtain arrays are a class of large multielement directional radio transmitting wire antennas, used in the shortwave radio bands. They are a type of reflective array antenna, consisting of multiple wire dipole antennas, suspended in a vert ...
shortwave transmitting antenna, Austria. Wire dipoles suspended between towers File:Kreuzdipolarp.jpg, Turnstile antenna array used for satellite communication File:Feedingstrips.jpg, Flat microstrip array antenna for satellite TV reception. File:USA.NM.VeryLargeArray.02.jpg, The
Very Large Array The Karl G. Jansky Very Large Array (VLA) is a centimeter-wavelength radio astronomy observatory located in central New Mexico on the Plains of San Agustin, between the towns of Magdalena and Datil, ~ west of Socorro. The VLA comprises twen ...
, a
radio telescope A radio telescope is a specialized antenna and radio receiver used to detect radio waves from astronomical radio sources in the sky. Radio telescopes are the main observing instrument used in radio astronomy, which studies the radio frequency ...
made of a Y-shaped array of 27 dish antennas in Socorro, New Mexico File:HAARP Antenna Grid with Electrical Transformers.jpg,
HAARP The High-frequency Active Auroral Research Program (HAARP) was initiated as an ionospheric research program jointly funded by the U.S. Air Force, the U.S. Navy, the University of Alaska Fairbanks, and the Defense Advanced Research Projects Ag ...
, a
phased array In antenna theory, a phased array usually means an electronically scanned array, a computer-controlled array of antennas which creates a beam of radio waves that can be electronically steered to point in different directions without moving th ...
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 File:Traqueur acquisition.JPG, Array of four helical antennas used as a satellite tracking antenna,
Pleumeur-Bodou Pleumeur-Bodou (; ) is a commune in the Côtes-d'Armor department of Brittany in northwestern France. Population Inhabitants of Pleumeur-Bodou are called ''pleumeurois'' in French. Sister town Pleuveur-Bodoù is twinned with Crosshaven, a vil ...
, France


Types

Most array antennas can be divided into two classes based on how the component antennas' axis is related to the direction of radiation. *A ''broadside array'' is a one or two dimensional array in which the direction of radiation (
main lobe In a radio antenna's radiation pattern, the main lobe, or main beam, is the lobe containing the higher power. This is the lobe that exhibits the greater field strength. The radiation pattern of most antennas shows a pattern of "''lobes''" at v ...
) of the radio waves is perpendicular to the plane of the antennas. To radiate perpendicularly, the antennas must be fed in phase. *An ''endfire array'' is a linear array in which the direction of radiation is along the line of the antennas. The antennas must be fed with a phase difference equal to the separation of adjacent antennas. There are also arrays (such as
phased array In antenna theory, a phased array usually means an electronically scanned array, a computer-controlled array of antennas which creates a beam of radio waves that can be electronically steered to point in different directions without moving th ...
s) which don't belong to either of these categories, in which the direction of radiation is at some other angle to the antenna axis. * ''Driven array'' - This is an array in which the individual component antennas are all "driven" - connected to the transmitter or receiver. The individual antennas, which are usually identical, often consist of single '' driven element''s, such as half-wave dipoles, but may also be composite antennas such as
Yagi antenna Yagi may refer to: Places *Yagi, Kyoto, in Japan *Yagi (Kashihara), in Nara Prefecture, Japan *Yagi-nishiguchi Station, in Kashihara, Nara, Japan *Kami-Yagi Station, a JR-West Kabe Line station located in 3-chōme, Yagi, Asaminami-ku, Hiroshima, Hi ...
s or turnstile antennas. ** '' Collinear array'' - a broadside array consisting of multiple identical dipole antennas oriented vertically in a line. This is a high gain omnidirectional antenna, often used in the
VHF Very high frequency (VHF) is the ITU designation for the range of radio frequency electromagnetic waves (radio waves) from 30 to 300 megahertz (MHz), with corresponding wavelengths of ten meters to one meter. Frequencies immediately below VHF ...
band as broadcasting antennas for
television station A television station is a set of equipment managed by a business, organisation or other entity, such as an amateur television (ATV) operator, that transmits video content and audio content via radio waves directly from a transmitter on the earth ...
s and base station antennas for land mobile two-way radios. *** '' Superturnstile'' or ''Batwing'' array - specialized vertical antenna used for television broadcasting consisting of multiple crossed-dipole antennas mounted collinearly on a mast. High gain omnidirectional radiation pattern with wide bandwidth. ** ''Planar array'' - a flat two-dimensional array of antennas. Since an array of omnidirectional antennas radiates two beams 180° apart broadside from both sides of the antenna, it often is either mounted in front of a reflector, or is composed of directive antennas such as Yagi or helical antennas, to give a unidirectional beam. ** '' Reflective array'' - a planar array of antennas, often half-wave dipoles fed in phase, in front of a flat reflector such as a metal plate or wire screen. This radiates a single beam of radio waves perpendicular (broadside) to the array. Used as UHF television antennas and radar antennas. *** ''
Curtain array Curtain arrays are a class of large multielement directional radio transmitting wire antennas, used in the shortwave radio bands. They are a type of reflective array antenna, consisting of multiple wire dipole antennas, suspended in a vert ...
'' - an outdoor wire shortwave transmitting antenna consisting of a planar array of wire dipoles suspended in front of a vertical reflector made of a "curtain" of parallel wires. Used on HF band as long distance transmitting antenna for shortwave broadcasting stations. May be steered as phased array. *** ''
Microstrip antenna In telecommunication, a microstrip antenna (also known as a printed antenna) usually means an antenna fabricated using photolithographic techniques on a printed circuit board (PCB). It is a kind of internal antenna. They are mostly used at micro ...
'' - an array of patch antennas fabricated on a
printed circuit board A printed circuit board (PCB; also printed wiring board or PWB) is a medium used in Electrical engineering, electrical and electronic engineering to connect electronic components to one another in a controlled manner. It takes the form of a L ...
with copper foil on the reverse side functioning as a reflector. The elements are fed through
stripline Stripline is a transverse electromagnetic (TEM) transmission line medium invented by Robert M. Barrett of the Air Force Cambridge Research Centre in the 1950s. Stripline is the earliest form of planar transmission line. Description A strip ...
s made of copper foil. Used as UHF and satellite television antennas. ** ''
Phased array In antenna theory, a phased array usually means an electronically scanned array, a computer-controlled array of antennas which creates a beam of radio waves that can be electronically steered to point in different directions without moving th ...
'' or ''electronically scanned array'' - A planar array in which the beam can be steered electronically to point in any direction over a wide angle in front of the array, without physically moving the antenna. The current from the transmitter is fed to each component antenna through a
phase shifter A phase shift module is a microwave network module which provides a controllable phase shift of the RF signal. Phase shifters are used in phased arrays. Classification Active versus passive Active phase shifters provide gain, while passive ...
, controlled by a computer. By changing the relative phase of the feed currents, the beam can instantly be pointed in different directions. Widely used in military radars, this technique is rapidly spreading to civilian applications. *** '' Passive Electronically Scanned Array'' (PESA) - A phased array as described above, in which the antenna elements are fed from a single transmitter or receiver through phase shifters. *** ''
Active Electronically Scanned Array An active electronically scanned array (AESA) is a type of phased array antenna, which is a computer-controlled array antenna in which the beam of radio waves can be electronically steered to point in different directions without moving the an ...
'' (AESA) - A phased array in which each antenna element has its own transmitter and/or receiver module, controlled by a central computer. This second generation phased array technology can radiate multiple beams at multiple frequencies simultaneously, and is mostly used in sophisticated military radars. ** '' Conformal array'' - a two-dimensional phased array which is not flat, but conforms to some curved surface. The individual elements are driven by
phase shifter A phase shift module is a microwave network module which provides a controllable phase shift of the RF signal. Phase shifters are used in phased arrays. Classification Active versus passive Active phase shifters provide gain, while passive ...
s which compensate for the varying path lengths, allowing the antenna to radiate a plane wave beam. Conformal antennas are often integrated into the curving skin of aircraft and missiles, to reduce aerodynamic drag. ** '' Smart antenna'', ''
reconfigurable antenna A reconfigurable antenna is an antenna capable of modifying its frequency and radiation properties dynamically, in a controlled and reversible manner. In order to provide a dynamic response, reconfigurable antennas integrate an inner mechanism (su ...
'' or ''adaptive array'' - a receiving array that estimates the direction of arrival of the radio waves and electronically optimizes the radiation pattern adaptively to receive it, synthesizing a
main lobe In a radio antenna's radiation pattern, the main lobe, or main beam, is the lobe containing the higher power. This is the lobe that exhibits the greater field strength. The radiation pattern of most antennas shows a pattern of "''lobes''" at v ...
in that direction. Like a phased array it consists of multiple identical elements with
phase shifter A phase shift module is a microwave network module which provides a controllable phase shift of the RF signal. Phase shifters are used in phased arrays. Classification Active versus passive Active phase shifters provide gain, while passive ...
s in the feed lines, controlled by a computer. * '' Log-periodic dipole array'' (LPDA) - an endfire array consisting of many dipole driven elements in a line, with gradually increasing length. It acts as a high gain broadband antenna. Used as television reception antennas and for shortwave communication. * ''Parasitic array'' - This is an endfire array which consist of multiple antenna elements in a line of which only one, the driven element, is connected to the transmitter or receiver, while the other elements, called parasitic elements, are not. The parasitic elements function as resonators, absorbing radio waves from the driven element and reradiating them with a different phase, to modify the radiation pattern of the antenna, increasing the power radiated in the desired direction. Since these have only one driven element they are often called "antennas" instead of "arrays". ** ''
Yagi–Uda antenna A Yagi–Uda antenna or simply Yagi antenna, is a directional antenna consisting of two or more parallel resonant antenna elements in an end-fire array; these elements are most often metal rods acting as half-wave dipoles. Yagi–Uda a ...
'' or Yagi antenna - this endfire array consists of multiple half-wave dipole elements in a line. It consists of a single driven element with multiple "director" parasitic elements in the direction of radiation, and usually a single "reflector" parasitic element behind it. They are widely used on the HF,
VHF Very high frequency (VHF) is the ITU designation for the range of radio frequency electromagnetic waves (radio waves) from 30 to 300 megahertz (MHz), with corresponding wavelengths of ten meters to one meter. Frequencies immediately below VHF ...
, and
UHF Ultra high frequency (UHF) is the 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 (on ...
bands as television antennas, shortwave communication antennas, and in radar arrays. ** ''
Quad antenna A quad antenna is a type of directional wire radio antenna used on the HF and VHF bands. Like a Yagi–Uda antenna ("Yagi"), a quad consists of a driven element and one or more parasitic elements; however in a quad, each of these elements is ...
'' - This consists of multiple
loop antenna A loop antenna is a radio antenna consisting of a loop or coil of wire, tubing, or other electrical conductor, that is usually fed by a balanced source or feeding a balanced load. Within this physical description there are two (possibly three) di ...
s in a line, with one driven loop and the others parasitic. Functions similarly to the Yagi antenna.


Design of antenna arrays

In an antenna array providing a fixed radiation pattern, we may consider that the feed network is a part of the antenna array. Thus, the antenna array has a single port. Narrow beams can be formed, provided the phasing of each element of the array is appropriate. If, in addition, the amplitude of the excitation received by each element (during emission) is also well chosen, it is possible to synthesize a single-port array having a radiation pattern that closely approximates a specified pattern. Changes in current distribution are known to affect the radiation pattern when applied to arrays. Many methods have been developed for array pattern synthesis. Additional issues to be considered are matching, radiation efficiency and bandwidth. The design of an electronically steerable antenna array is different, because the phasing of each element can be varied, and possibly also the relative amplitude for each element. Here, the antenna array has multiple ports, so that the subject matters of matching and efficiency are more involved than in the single-port case. Moreover, matching and efficiency depend on the excitation, except when the interactions between the antennas can be ignored. An antenna array used for spatial diversity and/or spatial multiplexing (which are different types of MIMO radio communication) always has multiple ports. It is intended to receive independent excitations during emission, and to deliver more or less independent signals during reception. Here also, the subject matters of matching and efficiency are involved, especially in the case of an antenna array of a mobile device (see chapter 10 of ), since, in this case, the surroundings of the antenna array influence its behavior, and vary over time. Suitable matching metrics and efficiency metrics take into account the worst possible excitations.


See also

*
Total active reflection coefficient The total active reflection coefficient (TARC) within mathematics and physics scattering theory, relates the total incident power to the total outgoing power in an N-port microwave component. The TARC is mainly used for multiple-input multiple-o ...


References

{{reflist Radio frequency antenna types Microwave transmission Microwave technology Antennas (radio)