Broadcasting

In broadcast engineering, the term 'phased array' has a meaning different from its normal meaning, it means an ordinary array antenna, an array of multiple mast radiators designed to radiate a directional radiation pattern, as opposed to a single mast which radiates an omnidirectional pattern. Broadcast phased arrays have fixed radiation patterns and are not 'steered' during operation as are other phased arrays.

Phased arrays are 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 (groundwave) and night (skywave) radiation patterns by switching the phase and power levels supplied to the individual antenna elements (mast radiators) daily at sunrise and sunset. For shortwave broadcasts many stations use arrays of horizontal dipoles. A common arrangement uses 16 dipoles in a 4×4 array. Usually this is in front of a wire grid reflector. The phasing is often switchable to allow beam steering in azimuth and sometimes elevation.

Radar

Phased arrays were invented for radar tracking of ballistic missiles, and because of their fast tracking abilities phased array radars are widely used in military applications. For example, 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 (finding ships), air detection and tracking (finding aircraft and missiles) and missile uplink capabilities. Before using these systems, each surface-to-air missile in flight required a dedicated fire-control radar, which meant that radar-guided weapons 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 antenna pattern 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.

Active Phased Array Radar mounted on top of Sachsen-class frigate F220 Hamburg's superstructure of the German Navy

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."^[24] Likewise, the Thales Herakles phased array multi-function radar used in service with France and Singapore 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.^[25] The German Navy and the Royal Dutch Navy have developed the Active Phased Array Radar System (APAR). The MIM-104 Patriot and other ground-based antiaircraft systems use phased array radar for similar benefits.

Phased arrays are used in naval sonar, in active (transmit and receive) and passive (receive only) and hull-mounted and towed array sonar.

Space probe communication

The MESSENGER spacecraft was a space probe mission to the planet Mercury (2011–2015^[26]). This was the first deep-space mission to use a phased-array antenna for communications. The radiating elements are circularly-polarized, slotted waveguides. The antenna, which uses the X band, used 26 radiative elements and can gracefully degrade.^[27]

Weather research usage

AN/SPY-1A radar installation at National Severe Storms Laboratory, Norman, Oklahoma. The enclosing radome provides weather protection.

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.^[28] A team from Japan's RIKEN Advanced Institute for Computational Science (AICS) has begun experimental work on using phased-array radar with a new algorithm for instant weather forecasts.^[29]

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,^[30] laser beam steering, and holography. Synthetic array heterodyne detection is an efficient method for multiplexing an entire phased array onto a single element photodetector. The dynamic beam forming in an optical phased array transmitter can be used to electronically raster or vector scan images without using lenses or mechanically moving parts in a lensless projector.^[31] Optical phased array receivers have been demonstrated to be able to act as lensless cameras by selectively looking at different directions.^[32][33]

Satellite broadband internet transceivers

Starlink is a low Earth orbit satellite constellation that is under construction as of 2021. It is designed to provide broadband internet connectivity to consumers; the user terminals of the system will use phased array antennas.^[34]

Radio-frequency identification (RFID)

By 2014, phased array antennas were integrated into RFID systems to increase the area of coverage of a single system by 100% to 76,200 m² (820,000 sq ft) while still using traditional passive UHF tags.^[35]

Human-machine interfaces (HMI)

A phased array of acoustic transducers, denominated airborne ultrasound tactile display (AUTD), was developed in 2008 at the University of Tokyo's Shinoda Lab to induce tactile feedback.^[36] This system was demonstrated to enable a user to interactively manipulate virtual holographic objects.^[37]

Radio astronomy

Phased Array Feeds (PAF)^[38] have recently been used at the focus of radio telescopes to provide many beams, giving the radio telescope a very wide field of view. Two examples are the ASKAP telescope in Australia and the Apertif upgrade to the Westerbork Synthesis Radio Telescope in The Netherlands.

Array factor

The total directivity of a phased array will be a result of the gain of the individual array elements, and the directivity due their positioning in an array. This latter component is closely tied (but not equal to^[39]) to the array factor.^[40][39] In a (rectangular) planar phased array, of dimensions ${\displaystyle M\times N}$, with inter-element spacing ${\displaystyle d_{x}}$ and ${\displaystyle d_{y}}$, respectively, the array factor can be calculated accordingly^[4][40]:

$${\displaystyle AF=\sum _{n=1}^{N}I_{n1}\left[\sum _{m=1}^{M}I_{m1}\mathrm {e} ^{j\left(m-1\right)\left(kd_{x}\sin \theta \cos \phi +\beta _{x}\right)}\right]\mathrm {e} ^{j\left(n-1\right)\left(kd_{y}\sin \theta \sin \phi +\beta _{y}\right)}}$$

Here, ${\displaystyle \theta }$ and ${\displaystyle \phi }$ are the directions which we are taking the array factor in, in the coordinate frame depicted to the right. The factors ${\displaystyle \beta _{x}}$ and ${\displaystyle \beta _{y}}$ are the progressive phase shift that is used to steer the beam electronically. The factors ${\displaystyle I_{n1}}$ and ${\displaystyle I_{m1}}$ are the excitation coefficients of the individual elements.

Beam steering is indicated in the same coordinate frame, however the direction of steering is indicated with ${\displaystyle \theta _{0}}$ and ${\displaystyle \phi _{0}}$, which is used in calculation of progressive phase:

${\displaystyle \beta _{x}=-kd_{x}\sin \theta _{0}\cos \phi _{0}}$

${\displaystyle \beta _{y}=-kd_{y}\sin \theta _{0}\sin \phi _{0}}$

In all above equations, the value ${\displaystyle k}$ describes the wavenumber of the frequency used in transmission.

These equations can be solved to predict the nulls, main lobe, and grating lobes of the array. Referring to the exponents in the array factor equation, we can say that major and grating lobes will occur at integer ${\displaystyle m,n=0,1,2,\dots }$ solutions to the following equations:^[4][40]

${\displaystyle kd_{x}\sin \theta \cos \phi +\beta _{x}=\pm 2m\pi }$

${\displaystyle kd_{y}\sin \theta \sin \phi +\beta _{y}=\pm 2n\pi }$

Worked example

It is common in engineering to provide phased array ${\displaystyle AF}$ values in decibels through ${\displaystyle AF_{dB}=10\log _{10}AF}$. Recalling the complex exponential in the array factor equation above, often, what is really meant by array factor is the magnitude of the summed phasor produced at the end of array factor calculation. With this, we can produce the following equation:

$${\displaystyle AF_{dB}=10\log _{10}{\Bigg \|}\sum _{n=1}^{N}I_{1n}\left[\sum _{m=1}^{M}I_{m1}\mathrm {e} ^{j\left(m-1\right)\left(kd_{x}\sin \theta \cos \phi +\beta _{x}\right)}\right]\mathrm {e} ^{j\left(n-1\right)\left(kd_{y}\sin \theta \sin \phi +\beta _{y}\right)}{\Bigg \|}}$$

For the ease of visualization, we will analyze array factor given an input azimuth and elevation, which we will map to the array frame ${\displaystyle \theta }$ and ${\displaystyle \phi }$ through the following conversion:

${\displaystyle \theta =\arccos \left(\cos \left(\theta _{az}\right)\sin \left(\theta _{el}\right)\right)}$

${\displaystyle \phi =\arctan 2\left(\sin \left(\theta _{el}\right),\sin \left(\theta _{az}\cos \left(\theta _{el}\right)\right)\right)}$

This represents a coordinate frame whose ${\displaystyle \mathbf {x} }$ axis is aligned with the array ${\displaystyle \mathbf {z} }$ axis, and whose ${\displaystyle \mathbf {y} }$ axis is aligned with the array ${\displaystyle \mathbf {x} }$ axis.

If we consider a ${\displaystyle 16\times 16}$ phased array, this process provides the following values for ${\displaystyle AF_{dB}}$, when steering to bore-sight (${\displaystyle \theta _{0}=0^{\circ }}$,${\displaystyle \phi _{0}=0^{\circ }}$):

These values have been clipped to have a minimum ${\displaystyle AF}$ of -50 dB, however, in reality, null points in the array factor pattern will have values significantly small than this.

Dynamic phased array

Each array element incorporates an adjustable phase shifter. These are collectively used to move the beam with respect to the array face.

Dynamic phased arrays 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, a capability known as track while search.

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 hemisphere consisting of 8,000 pointing positions. This configuration provides 12 opportunities to detect a 1,000 m/s (2,200 mph; 3,600 km/h) vehicle over a range of 100 km (62 mi), 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 phased array operation allows beams to be aimed at random locations, which eliminates this vulnerability. This is also desirable for military applications.

Fixed phased array

An antenna tower consisting of a fixed phase collinear antenna array with four elements

Fixed phased 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 arrays incorporate fixed phase shifters. For example, most commercial FM Radio and TV antenna towers use a collinear antenna array, which is a fixed phased array of dipole elements.

In radar applications, this kind of phased 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 phased array to produce multiple adjacent beams that measure angle errors. This form factor is suitable for gimbal mounting in missile seekers.

Active phased array

Active electronically-scanned arrays (AESA) elements incorporate transmit amplification with phase shift in each antenna element (or group of elements). Each element also includes receive pre-amplification. The phase shifter setting is the same for transmit and receive.^[41]

Active phased arrays do not require phase reset after the end of the transmit pulse, which is compatible with Doppler radar and pulse-Doppler radar.

Passive phased 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 controlled by magnetic field, voltage gradient, or equivalent technology.^[42][43]

The phase shift process used with passive phased arrays 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 phased array design is used in the AEGIS Combat System.^[44] for direction-of-arrival estimation.