In antenna theory, a phased array is an array 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. An antenna array is a group of multiple active
antennas coupled to a common source or load to produce a directive radiation pattern.
Usually, the spatial relationship of the individual antennas also contributes to the directivity
of the antenna array. 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
or an element in which the energy output from a source of energy is controlled by the signal
input. One common application of this is with a standard multiband television antenna, which
has multiple elements coupled together.
History Phased array transmission was originally developed
in 1905 by Nobel laureate Karl Ferdinand Braun who demonstrated enhanced transmission of
radio waves in one direction. During World War II, Nobel laureate Luis Alvarez used phased
array transmission in a rapidly steerable radar system for "ground-controlled approach",
a system to aid in the landing of aircraft. At the same time, the GEMA in Germany built
the PESA Mammut 1. It was later adapted for radio astronomy leading to Nobel Prizes for
Physics for Antony Hewish and Martin Ryle after several large phased arrays were developed
at the University of Cambridge. The design is also used in radar, and is generalized
in interferometric radio antennas. In 2007, 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. 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, California
in 1957. The phased array is used for instance in optical
communication as a wavelength-selective splitter. For information about active as well as passive
phased array radars, see also active electronically scanned array.
Broadcasting In broadcast engineering, it is required that
phased arrays be used by many AM broadcast radio stations to enhance signal 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 at mediumwave
frequencies, it is common for AM broadcast stations to change between day and night radiation
patterns by switching the phase and power levels supplied to the individual antenna
elements daily at sunrise and sunset. More modest phased array longwire antenna systems
may be employed by private radio enthusiasts to receive longwave, mediumwave and shortwave
radio broadcasts from great distances. 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
wavelength apart. Phasing the array such that the lower elements are slightly delayed 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
tilting 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 – 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 many navies. Because of the rapidity
with which the beam can be steered, phased array radars allow a warship to use one radar
system for surface detection and tracking, air detection and tracking and missile uplink
capabilities. Before using these systems, each surface-to-air missile in 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-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. The AN/SPY-1 phased array radar, part of the
Aegis combat system deployed on modern U.S. cruisers and destroyers, "is able to perform
search, track and missile guidance functions simultaneously with a capability of over 100
targets." Likewise, the Thales Herakles phased array multi-function radar on board the Formidable
class frigates of the Republic 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 the MBDA Aster missiles
launched from the ship. The German Navy and the Royal Dutch Navy have developed the Active
Phased Array Radar System.
See also: Active Phased Array Radar, SMART-L, Active Electronically Scanned Array, Aegis
combat system and AN/SPY-1 Phased arrays are used in naval sonar, in
active and passive and hull-mounted and towed array sonar.
Space probe communication The MESSENGER spacecraft is a mission to the
planet Mercury. 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.
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.
Current 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, School of Electrical and Computer Engineering, and Atmospheric
Radar Research Center, Oklahoma State Regents for Higher Education, the Federal Aviation
Administration, and 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. Optics
Within the visible or infrared spectrum of electromagnetic waves it is possible to construct
optical phased arrays. They are used in wavelength multiplexers and filters for telecommunication
purposes, laser beam steering, and holography. Synthetic array heterodyne detection is an
efficient method for multiplexing an entire phased array onto a single element photodetector.
Radio-frequency identification Recently, phased array antennas have been
implemented into RFID systems to significantly increase the operable range while still using
traditional passive UHF tags. Human-machine interfaces
A phased array of acoustic transducers, denominated airborne ultrasound tactile display, was developed
at the University of Tokyo's Shinoda Lab to induce tactile feedback. This system was demonstrated
to enable a user to interactively manipulate virtual holographic objects.
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 formalism 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.
As sine achieves its maximum at , we set the numerator of the second term = 1.
Thus as N gets large, the term will be dominated by the term. As sine 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 types of beamformers. These are time domain beamformers and frequency
domain beamformers. A graduated attenuation window is sometimes
applied across the face of the array to improve side-lobe suppression performance, in addition
to the phase shift. Time domain beamformer works by introducing
time delays. 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. The
most common kind of time domain beam former is serpentine waveguide. Active phase array
uses individual delay lines that are switched on and off. Yttrium iron garnet phase shifters
vary the phase delay using the strength of a magnetic field.
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 multiple frequency bins or a filterbank). When different delay and sum beamformers are
applied to each frequency bin, the result is that the main lobe simultaneously points
in multiple different directions at each of the different frequencies. This can be an
advantage for communication links, and is used with the SPS-48 radar.
The other type of frequency domain beamformer makes use of Spatial Frequency. Discrete samples
are taken from each of the individual array elements. The samples are processed using
a Discrete Fourier Transform. The DFT introduces multiple different discrete phase shifts during
processing. The outputs of the DFT are individual channels that correspond with evenly spaced
beams formed simultaneously. A 1-dimensional DFT produces a fan of different beams. A 2-dimensional
DFT produces beams with a pineapple configuration. These techniques are used to create two kinds
of phase array.
Dynamic - an array of variable phase shifters are used to move the beam
Fixed - the beam position is stationary with respect to the array face and the whole antenna
is moved
There are two further sub-categories that modify the kind of dynamic array or fixed
array.
Active - amplifiers or processors in each phase shifter element
Passive - large central amplifier with attenuating phase shifters
Dynamic Phased Array Each array element incorporates an adjustable
phase shifter that are collectively used to move the beam with respect to the array face.
Dynamic phase array require no physical movement to aim the beam. The beam is moved electronically.
This can produce antenna motion fast enough to use a small pencil-beam to simultaneously
track multiple targets while searching for new targets using just one radar set.
As an example, an antenna with a 2 degree beam with a pulse rate of 1 kHz will require
approximately 8 seconds to cover an entire a hemisphere consisting of 8,000 pointing
positions. This configuration provides 12 opportunities to detect a 1,000 metres per
second vehicle over a range of 100 km, which is suitable for military applications.
The position of mechanically steered antennas can be predicted, which can be used to create
electronic countermeasures that interfere with radar operation. The flexibility resulting
from phase array operation allows beams to be aimed at random locations, which eliminates
this vulnerability. This is also desirable for military applications.
Fixed Phase Array Fixed phase array antennas are typically used
to create an antenna with a more desirable form factor than the conventional parabolic
reflector or cassegrain reflector. Fixed phased array radar incorporate fixed phase shifters.
This kind of phase array is physically moved during the track and scan process. There are
two configurations.
Multiple frequencies with a delay-line Multiple adjacent beams
The SPS-48 radar uses multiple transmit frequencies with a serpentine delay line along the left
side of the array to produce vertical fan of stacked beams. Each frequency experiences
a different phase shift as it propagates down the serpentine delay line, which forms different
beams. A filter bank is used to split apart the individual receive beams. The antenna
is mechanically rotated. Semi-active radar homing uses monopulse radar
that relies on a fixed phase array to produce multiple adjacent beams that measure angle
errors. This form factor is suitable for gimbal mounting in missile seekers.
Active Phase Array Active phased arrays elements incorporate
transmit amplification with phase shift in each antenna element. Each element also includes
receive pre-amplification. The phase shifter setting is the same for transmit and receive.
Active phase array do not require phase reset after the end of the transmit pulse, which
is compatible with Doppler radar and Pulse-Doppler radar.
Passive Phase Array Passive phased arrays typically use large
amplifiers that produce all of the microwave transmit signal for the antenna. Phase shifters
typically consist of waveguide elements that contain phase shifters controlled by magnetic
field, voltage gradient, or equivalent technology. The phase shift process used with passive
phase array typically puts the receive beam and transmit beam into diagonally opposite
quadrants. The sign of the phase shift must be inverted after the transmit pulse is finished
and before the receive period begins to place the receive beam into the same location as
the transmit beam. That requires a phase impulse that degrades sub-clutter visibility performance
on Doppler radar and Pulse-Doppler radar. As an example, Yttrium iron garnet phase shifters
must be changed after transmit pulse quench and before receiver processing starts to align
transmit and receive beams. That impulse introduces FM noise that degrades clutter performance.
Passive phase array is used with AEGIS. See also
Active Electronically Scanned Array Antenna array
Aperture synthesis Beamforming
Interferometric synthetic aperture radar Inverse synthetic aperture radar
Optical heterodyne detection Phased array ultrasonics
Phased-array optics Radar MASINT
Rajendra Radar, India Side-scan sonar
Single frequency network Smart antenna
Synthetic aperture radar Synthetic aperture sonar
Synthetically thinned aperture radar Thinned array curse
Tikhomirov Scientific Research Institute of Instrument Design and Phazotron, Russian developers
of phased arrays Wave field synthesis
History of smart antennas References
External links Radar Research and Development - Phased Array
Radar—National Severe Storms Laboratory Shipboard Phased Array Radars
NASA Report: MMICs For Multiple Scanning Beam Antennas for Space Applications
Principle of Phased Array @ www.radartutorial.eu 'Phased Array' microphone system of Tony Faulkner
Software tool to predict the radiation pattern of an antenna array