- Phased array
"This article is about general theory and electromagnetic phased array.""For the ultrasonic and medical imaging application, see
phased array ultrasonics ."In
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 effectiveradiation pattern of thearray is 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] . AccessedApril 27 ,2006 .] This technology was originally developed by future Nobel LaureateLuis Alvarez duringWorld War II as 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 forradio astronomy , leading to Physics Nobel Prizes forAntony Hewish andMartin Ryle after several large phased arrays were developed at Cambridge University. The design is also used inradar , and is generalized in interferometric radio antennas. Recently,DARPA researchers 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.
Usage
The relative
amplitude s of — and constructive and destructiveinterference effects among — the signals radiated by the individual antennas determine the effectiveradiation pattern of the array. A phased array may be used to point a fixed radiation pattern, or toscan rapidly inazimuth or 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 calledbeamforming .The phased array is used for instance in
optical communication as awavelength -selective splitter.For information about active as well as passive phased array radars, see also
active electronically scanned array .Broadcasting
In
broadcast engineering , phased arrays are required to be used by many AM broadcastradio stations to enhancesignal strength and therefore coverage in the city of license, while minimizing interference to other areas. Due to the differences between daytime and nighttime ionospheric propagation atmediumwave frequencies, 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 radiator s) daily atsunrise andsunset . 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.On
VHF , phased arrays are used extensively forFM broadcasting . These greatly increase theantenna gain , magnifying the emitted RF energy toward thehorizon , which in turn greatly increases a station'sbroadcast range . In these situations, the distance to each element from the transmitter is identical, or is one (or otherinteger ) wavelength apart. Phasing the array such that the lower elements are slightly delayed (by making the distance to them longer) causes a downwardbeam tilt , which is very useful if the antenna is quite high on aradio tower .Other phasing adjustments can increase the downward radiation in the
far field without tiliting the mainlobe , creatingnull fill to compensate for extremely highmountain top locations, or decrease it in thenear 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.Naval usage
Phased array radar systems are also used by
warships of several navies including the Chinese, Japanese, Norwegian, Spanish, Korean and United States' navies in theAegis combat system . Phased array radars allow a warship to use oneradar system for surface detection and tracking (finding ships), air detection and tracking (finding aircraft and missiles) and missile uplink capabilities. Prior to using these systems, eachsurface-to-air missile in flight required a dedicatedfire-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-wave fire 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. TheAN/SPY-1 phased array radar, part of theAegis combat system deployed on modern U.S.cruisers anddestroyers , "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 theFormidable class frigate s of theRepublic of Singapore Navy has 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 theMBDA Aster missiles 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] TheGerman Navy and theDutch Navy have developed theActive Phased Array Radar System (APAR).:"See also:Active Electronically Scanned Array ,Aegis combat system andAN/SPY-1 "pace probe communication
The
MESSENGER spacecraft is a mission to the planet Mercury (arrival18 March 2011 ). This spacecraft is the first deep-space mission to use a phased-array antenna for communications. The radiating elements are linearly-polarized, slottedwaveguide s. The antenna, which uses theX 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
The
National Severe Storms Laboratory has been using a SPY-1A phased array antenna, provided by the US Navy, for weather research at itsNorman, Oklahoma facility sinceApril 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 Oklahoma School of Meteorology and School of Electrical and Computer Engineering, Oklahoma State Regents for Higher Education, theFederal Aviation Administration , and [http://www.bcisse.com/ Basic Commerce and Industries] . The project includesresearch and development , futuretechnology transfer and 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] . AccessedApril 6 ,2006 .]Optics
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.
Radio-Frequency Identification
Recently phased array antennas have been included in
RFID systems 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 source s. 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
diffraction page.:
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 domain beamformers andfrequency domain beamformers.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
FFT or afilterbank ). 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.
See also
*
Active Phased Array Radar
*Aperture synthesis
*Beamforming
*Synthetic aperture radar
*Synthetic aperture sonar
*Inverse synthetic aperture radar (ISAR)
*Side-scan sonar
*Smart antenna
*Active Electronically Scanned Array
*MIL-STD-188
*Phased array optics
*Phased array ultrasonics
*Radar MASINT
*Thinned array curse
* NIIP and NIIR,Russia n developers of phased arrays
*Optical heterodyne detection References
External links
* [http://www.nssl.noaa.gov/par/ Radar Research and Development - Phased Array Radar] -
National Severe Storms Laboratory experimental 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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