# Phased array

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Phased array

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 effective radiation pattern of the array 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] . Accessed April 27, 2006.] This technology was originally developed by future Nobel Laureate Luis Alvarez during World 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 for radio astronomy, leading to Physics Nobel Prizes for Antony Hewish and Martin Ryle after several large phased arrays were developed at Cambridge University. The design is also used in radar, 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 amplitudes of — and constructive and destructive interference effects among — the signals radiated by the individual antennas determine the effective radiation pattern of the array. A phased array may be used to point a fixed radiation pattern, or to scan rapidly in azimuth 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 called beamforming.

The phased array is used for instance in optical communication as a wavelength-selective splitter.

On 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 field without tiliting the main lobe, creating null fill to 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 &ndash; 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

pace probe communication

The MESSENGER spacecraft 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

The National Severe Storms Laboratory has been using a SPY-1A phased array antenna, provided by the US Navy, for weather research at its Norman, Oklahoma facility 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 Oklahoma School 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 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] . Accessed April 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.

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 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 diffraction page.

:$psi =$psi }_0}left [frac{sin left(fracpi a{lambda }sin heta ight)}{fracpi a{lambda }sin heta} ight] left [frac{sinleft(frac{N}{2}{kd}sin heta ight)}{sin left(frackd{2}sin heta ight)} ight]

Now, adding a φ term to the fringe effect in the second term yields:

:$psi =$psi }_0}left [frac{sin left(fracpi a{lambda }sin heta ight)}{fracpi a{lambda }sin heta} ight] left [frac{sinleft(frac{N}{2}ig(frac2pi d{lambda }sin heta + phi ig) ight)}{sin left(fracpi d{lambda }sin heta +phi ight)} ight]

Taking the square of the wave function gives us the intensity of the wave.

:$I = I_0$left [frac{sin left(frac{pi a}{lambda }sin heta ight)}{fracpi a{lambda } sin [ heta] } ight] }^2}left [frac{sin left(frac{N}{2}(frac{2pi d}{lambda} sin heta+phi ) ight)}{sin left(fracpi d{lambda} sin heta+phi ight)} ight] }^2}

:$I =I_0$left [frac{sin left(fracpi a{lambda } sin heta ight)}{fracpi a{lambda }sin heta} ight] }^2}left [frac{sin left(frac{pi }{lambda } N d sin heta+frac{N}{2} phi ight)}{sinleft(fracpi d{lambda } sin heta+phi ight)} ight] }^2}

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.

:$I =I_0$left [frac{sin left(frac{pi }{lambda } a heta ight)}{frac{pi }{lambda } a heta } ight] }^2}left [frac{sin left(frac{pi }{4} N sin heta+frac{N}{2} phi ight)}{sin left(frac{pi }{4}sin heta+ phi ight)} ight] }^2}

Sin achieves its maximum at so we set the numerator of the second term = 1.

:$frac\left\{pi \right\}\left\{4\right\} N sin heta+frac\left\{N\right\}\left\{2\right\} phi = frac\left\{pi \right\}\left\{2\right\}$

:$sin heta=Big\left(frac\left\{pi \right\}\left\{2\right\} - frac\left\{N\right\}\left\{2\right\} phi Big\right)frac\left\{4\right\}\left\{N pi \right\}$

:$sin heta=frac\left\{2\right\}\left\{N\right\}-frac\left\{2phi \right\}\left\{pi \right\}$

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 and frequency 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 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.

* Aperture synthesis
* Beamforming
* 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
* Thinned array curse
* NIIP and NIIR, Russian developers of phased arrays
* Optical heterodyne detection

References

* [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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