- Phased array
"This article is about general theory and electromagnetic phased array.""For the ultrasonic and medical imaging application, see
phased array ultrasonics."
wave theory, a phased array is a group of antennas in which the relative phases of the respective signals feeding the antennas are varied in such a way that the effective radiation patternof the arrayis reinforced in a desired direction and suppressed in undesired directions. [ Federal Standard 1037C. [http://glossary.its.bldrdoc.gov/fs-1037/dir-027/_3979.htm Definition of Phased Array] . Accessed April 27, 2006.] This technology was originally developed by future Nobel Laureate Luis Alvarezduring World War IIas a rapidly-steerable radar system for " ground-controlled approach", a system to aid in the landing of airplanes in England. GEMA in Germany built at the same time the PESA Mammut 1. [http://www.100jahreradar.de/index.html?/gdr_5_deutschefunkmesstechnikim2wk.html Mamut1 first early waring PESA Radar] It was later adapted for radio astronomy, leading to Physics Nobel Prizes for Antony Hewishand Martin Ryleafter several large phased arrays were developed at Cambridge University. The design is also used in radar, and is generalized in interferometric radio antennas. Recently, DARPAresearchers announced a 16 element phased array integrated with all necessary circuits to send at 30-50 GHz on a single silicon chip for military purposes. [ [http://ucsdnews.ucsd.edu/newsrel/science/10-07PhasedArrayChipDK-L.asp World’s Most Complex Silicon Phased Array Chip Developed at UC San Diego] in UCSD News (reviewed 02. November 2007)]
An antenna array is a multiple of active antennas coupled to a common source or load to produce a directive radiation pattern. Usually the spatial relationship also contributes to the directivity of the antenna. Use of the term "active antennas" is intended to describe elements whose energy output is modified due to the presence of a source of energy in the element (other than the mere signal energy which passes through the circuit) or an element in which the energy output from a source of energy is controlled by the signal input.
amplitudes of — and constructive and destructive interferenceeffects among — the signals radiated by the individual antennas determine the effective radiation patternof the array. A phased array may be used to point a fixed radiation pattern, or to scanrapidly in azimuthor elevation. Simultaneous electrical scanning in both azimuth and elevation was first demonstrated in a phased array antenna at Hughes Aircraft Company, Culver City, CA, in 1957 (see Joseph Spradley, “A Volumetric Electrically Scanned Two-Dimensional Microwave Antenna Array,” IRE National Convention Record, Part I - Antennas and Propagation; Microwaves, New York: The Institute of Radio Engineers, 1958, 204-212). When phased arrays are used in sonar, it is called beamforming.
The phased array is used for instance in
optical communicationas a wavelength-selective splitter.
For information about active as well as passive phased array radars, see also
active electronically scanned array.
broadcast engineering, phased arrays are required to be used by many AM broadcast radio stationsto enhance signal strengthand therefore coverage in the city of license, while minimizing interference to other areas. Due to the differences between daytime and nighttime ionospheric propagation at mediumwavefrequencies, it is common for AM broadcast stations to change between day ( groundwave) and night ( skywave) radiation patterns by switching the phase and power levels supplied to the individual antenna elements ( mast radiators) daily at sunriseand sunset. More modest phased array longwire antenna systems may be employed by private radio enthusiasts to receive longwave, mediumwave (AM) and shortwave radio broadcasts from great distances.
VHF, phased arrays are used extensively for FM broadcasting. These greatly increase the antenna gain, magnifying the emitted RF energy toward the horizon, which in turn greatly increases a station's broadcast range. In these situations, the distance to each element from the transmitter is identical, or is one (or other integer) wavelength apart. Phasing the array such that the lower elements are slightly delayed (by making the distance to them longer) causes a downward beam tilt, which is very useful if the antenna is quite high on a radio tower.
Other phasing adjustments can increase the downward radiation in the
far fieldwithout tiliting the main lobe, creating null fillto compensate for extremely high mountaintop locations, or decrease it in the near field, to prevent excessive exposure to those workers or even nearby homeowners on the ground. The latter effect is also achieved by half-wave spacing – inserting additional elements halfway between existing elements with full-wave spacing. This phasing achieves roughly the same horizontal gain as the full-wave spacing; that is, a five-element full-wave-spaced array equals a nine- or ten-element half-wave-spaced array.
Phased array radar systems are also used by
warshipsof several navies including the Chinese, Japanese, Norwegian, Spanish, Korean and United States' navies in the Aegis combat system. Phased array radars allow a warship to use one radarsystem for surface detection and tracking (finding ships), air detection and tracking (finding aircraft and missiles) and missile uplink capabilities. Prior to using these systems, each surface-to-air missilein flight required a dedicated fire-control radar, which meant that ships could only engage a small number of simultaneous targets. Phased array systems can be used to control missiles during the mid-course phase of the missile's flight. During the terminal portion of the flight, continuous-wavefire control directors provide the final guidance to the target. Because the radar beam is electronically steered, phased array systems can direct radar beams fast enough to maintain a fire control quality track on many targets simultaneously while also controlling several in-flight missiles. The AN/SPY-1phased array radar, part of the Aegis combat systemdeployed on modern U.S. cruisersand destroyers, "is able to perform search, track and missile guidance functions simultaneously with a capability of over 100 targets." [cite web | last = | first = | authorlink = http://www.janes.com/company/about/| coauthors = | title = AEGIS Weapon System MK-7| work = | publisher = Jane's Information Group| date = 2001-04-25| url = http://www.janes.com/defence/naval_forces/news/misc/aegis010425.shtml| format = | doi = | accessdate = 2006-08-10 .] Likewise, the Thales Herakles phased array multi-function radar onboard the Formidable class frigates of the Republic of Singapore Navyhas a track capacity of 200 targets and is able to achieve automatic target detection, confirmation and track initiation in a single scan, while simultaneously providing mid-course guidance updates to the MBDA Astermissiles launched from the ship. [cite journal |last=Scott |first=Richard |year=2006 |month=April |title=Singapore Moves to Realise Its Formidable Ambitions |journal=Jane's Navy International |volume=111 |issue=4 |pages=42–49] The German Navyand the Dutch Navyhave developed the Active Phased Array RadarSystem (APAR).:"See also: Active Electronically Scanned Array, Aegis combat systemand AN/SPY-1"
pace probe communication
MESSENGERspacecraft is a mission to the planet Mercury (arrival 18 March 2011). This spacecraft is the first deep-space mission to use a phased-array antenna for communications. The radiating elements are linearly-polarized, slotted waveguides. The antenna, which uses the X band, uses 26 radiative elements but can gracefully downgrade. [ [http://ieeexplore.ieee.org/iel5/7416/20159/00931694.pdf Phased-Array Antenna for the MESSENGER Deep Space Mission] ]
Weather research usage
National Severe Storms Laboratoryhas been using a SPY-1A phased array antenna, provided by the US Navy, for weather research at its Norman, Oklahomafacility since April 23, 2003. It is hoped that research will lead to a better understanding of thunderstorms and tornadoes, eventually leading to increased warning times and enhanced prediction of tornadoes. Project participants include the National Severe Storms Laboratory and National Weather Service Radar Operations Center, Lockheed Martin, United States Navy, University of OklahomaSchool of Meteorology and School of Electrical and Computer Engineering, Oklahoma State Regents for Higher Education, the Federal Aviation Administration, and [http://www.bcisse.com/ Basic Commerce and Industries] . The project includes research and development, future technology transferand potential deployment of the system throughout the United States. It is expected to take 10 to 15 years to complete and initial construction was approximately $25 million. [ National Oceanic and Atmospheric Administration. [http://www.norman.noaa.gov/publicaffairs/backgrounders/backgrounder_par.html PAR Backgrounder] . Accessed April 6, 2006.]
Within the visible or infrared spectrum of electromagnetic waves it is also possible to construct optical phased arrays. They are used in wavelength multiplexers and filters for telecommunication purposes, [P. D. Trinh, S. Yegnanarayanan, F. Coppinger and B. Jalali [http://www.ee.ucla.edu/~oecs/comp_pub/intr_opt/Optics23.pdf Silicon-on-Insulator (SOI) Phased-Array Wavelength Multi/Demultiplexer with Extremely Low-Polarization Sensitivity] IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 9, NO. 7, JULY 1997] laser beam steering, and holography.
Recently phased array antennas have been included in
RFIDsystems in order to significantly boost the reading capability of passive UHF tags passing from 30 feet to 600 feet. [ [http://www.rfidradio.com/?p=25 Mojix Redefines the Range for Passive RFID] , RFID Radio]
Mathematical perspective and formulas
A phased array is an example of "N"-slit
diffraction. It may also be viewed as the coherent addition of "N" line sources. Since each individual antenna acts as a slit, emitting radio waves, their diffraction pattern can be calculated by adding the phase shift φ to the fringing term.
We will begin from the "N"-slit diffraction pattern derived on the
Now, adding a φ term to the fringe effect in the second term yields:
Taking the square of the wave function gives us the intensity of the wave.
Now space the emitters a distance apart. This distance is chosen for simplicity of calculation but can be adjusted as any scalar fraction of the wavelength.
Sin achieves its maximum at so we set the numerator of the second term = 1.
Thus as "N" gets large, the term will be dominated by the term. As sin can oscillate between −1 and 1, we can see that setting will send the maximum energy on an angle given by :Additionally, we can see that if we wish to adjust the angle at which the maximum energy is emitted, we need only to adjust the phase shift φ between successive antennas. Indeed the phase shift corresponds to the negative angle of maximum signal.
A similar calculation will show that the denominator is minimized by the same factor.
Different types of phased arrays
There are two main different types of phased arrays, also called beamformers. There are
time domainbeamformers and frequency domainbeamformers.
A time domain beamformer works, as the name says, by doing time-based operations. The basic operation is called "delay and sum". It delays the incoming signal from each array element by a certain amount of time, and then adds them together. Sometimes a multiplication with a window across the array is done to increase the mainlobe/sidelobe ratio, and to insert zeroes in the characteristic.
There are two different types of frequency domain beamformers. The first type separates the different frequency components that are present in the received signal into different frequency bins (using either an
FFTor a filterbank). When different delay and sum beamformers are applied to each frequency bin, it is possible to point the main lobe to different directions for different frequencies. This can be an advantage for communication links.
The other type of frequency domain beamformers makes use of so called Spatial Frequency. This means that an FFT is taken across the different array elements, not in time. The output of the N point FFT are N channels, which are evenly divided in space. This approach makes a simple implementation of several beamformers at the same time possible, but this approach is not flexible, because the different directions are fixed.
Active Phased Array Radar
Synthetic aperture radar
Synthetic aperture sonar
Inverse synthetic aperture radar(ISAR)
Active Electronically Scanned Array
Phased array optics
Phased array ultrasonics
Thinned array curse
* NIIP and NIIR,
Russian developers of phased arrays
Optical heterodyne detection
* [http://www.nssl.noaa.gov/par/ Radar Research and Development - Phased Array Radar] -
National Severe Storms Laboratoryexperimental phased array radar, using a former US Navy SPY-1A antenna.
* [http://www.harpoonhq.com/waypoint/articles/Article_044.pdf Shipboard Phased Array Radars]
* [http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19870018450_1987018450.pdf NASA Report: MMICs For Multiple Scanning Beam Antennas for Space Applications]
* [http://americas.kyocera.com/kai/semiparts/products/trmods.cfm?page=28 Kyocera America, Inc. TR Modules for Phased Array Radars]
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