Phased_array - Wikiparency

# Phased array A **phased array** is a computer-controlled array of antenna elements, usually a subtype of the general [[Antenna_array|antenna array]], whose beam is steered to point in a chosen direction by adjusting the relative phase fed to each element rather than by moving the antenna structure itself. Because a useful array must span many wavelengths to achieve a narrow beam, phased arrays are practical chiefly at the [[Ultra_high_frequency|UHF]] and [[Microwave|microwave]] end of the radio spectrum, where a wavelength short enough to fit many elements into a manageable structure is available. A three.js companion sketch elsewhere on the site renders this same phase-steered beam in three dimensions rather than the flat polar plot most two-dimensional array pictures use. Phased arrays were developed first for military radar, where the ability to redirect a beam in microseconds rather than by turning a dish let one set track several fast-moving targets nearly at once, and the same electronic steering has since spread into civilian systems that split a single aperture into several simultaneous beams, from cellular base stations built around multiple-antenna, or [[MIMO|MIMO]], techniques to the sensing and communication equipment described under Applications below. The same steering principle, applied at a different wavelength, is also used in acoustics, in a towed or hull-mounted [[Sonar|sonar]] array, and even in visible light. ## Description A phased array's defining feature is the phase shifter, a device placed between the common feed and each radiating element that can advance or retard that element's signal by a controlled amount. Early implementations built phase shifters from switched lengths of transmission line or from ferrite devices whose magnetic bias set an electrically variable delay; most present-day arrays instead use solid-state phase shifters fabricated in [[Silicon|silicon]] or [[Germanium|germanium]] processes, controlled digitally so that a computer can rewrite every element's phase between one look and the next. Feeding a set of elements through this controllable network, rather than through fixed transmission lines cut to length, is what turns a plain antenna array into a phased array: the geometry of the elements need not change at all for the beam to move, since only the electrical phase relationship between them does. Because the whole array shares one aperture, steering the beam away from the direction the elements face directly, called broadside, always costs some effective area and so some gain, and because a real phase shifter only ever produces phase modulo one full cycle, an array's steering range is bounded well short of grazing incidence in practice. Both limits are properties of the array as a whole rather than of any single element, which is also true of the array's radiation pattern generally: each element's own, comparatively broad [[Radiation_pattern|pattern]] sets an outer envelope that no amount of phase steering can exceed. ## Types Phased arrays are classified along two largely independent axes: how the steering signal is generated and applied over frequency, and whether each element carries its own transmitter and receiver or shares one through a common feed network. ### Time and frequency domains A phase shifter delays a signal by a fixed fraction of one cycle at its design frequency, so the physical time delay it produces changes with frequency; steering a wideband signal this way tilts the effective beam direction slightly at each end of the band, an effect called beam squint. A true time-delay unit instead delays a signal by a fixed interval regardless of frequency, avoiding squint at the cost of a bulkier delay network, and wideband arrays increasingly use true time delay, or a mixture of coarse time delay and fine phase shift, for exactly this reason. ### Dynamic phased array The ordinary sense of "phased array" is a dynamic one: a computer rewrites the phase, and so the pointing direction, of every element between one look and the next, often within a small fraction of a second, so that the same physical antenna can search a wide volume, track several targets, or serve several simultaneous beams by cycling or splitting its attention among them. ### Fixed phased array An array whose phase relationships are set once, at design or installation, and never changed electronically is a fixed phased array: the elements are still fed through a phase-shifting network rather than driven identically, but that network is wired for one pattern only. A directional [[AM_broadcasting|AM broadcast]] station's tower array is the everyday example, its several towers phased to protect a co-channel station in one direction while radiating normally everywhere else. ### Active phased array In an active phased array, every element has its own small transmit/receive module, generating or receiving its own signal under computer control rather than sharing a single transmitter and receiver through the phase-shifter network; this arrangement, discussed at length under [[Active_electronically_scanned_array|active electronically scanned array]], is the technology behind essentially every new phased-array radar. ### Passive phased array A passive phased array instead feeds every element from one shared transmitter, or into one shared receiver, through a network of phase shifters alone. It is simpler and cheaper to build than an active array of the same size, but can generally form only a single beam at a time and, because one failed transmitter or receiver can disable the whole array, offers none of an active array's graceful loss of capability when a single module fails. ## History The underlying idea, that identical radiators fed with a controlled phase difference add constructively in one direction and cancel in most others, follows directly from ordinary wave interference and was demonstrated with simple radio antennas well before any array could be steered electronically in real time.[^early-concept-cn] Multi-beam and electronically steerable antenna research at [[Bell_Labs|Bell Telephone Laboratories]] in the years after the Second World War worked out much of the feed-network theory later phased arrays would depend on, alongside parallel military efforts to build a radar that could redirect its beam without the inertia of a rotating dish.[^bell-labs-cn] Military demand carried the technology through the [[Cold_War|Cold War]]: a phased array that could switch electronically between searching a volume and holding a track on several already-detected aircraft or missiles, far faster than any mechanically scanned dish could slew, was worth the substantial cost and complexity of building and driving thousands of individually phased elements.[^cold-war-radar-cn] What has changed since is mostly cost. Early phased arrays needed a discrete phase shifter, often a hand-tuned or exotic ferrite device, wired to every element, which kept them confined to a handful of the most expensive military and scientific systems. Phase shifters implemented as ordinary integrated circuits, one per element or per small cluster of elements, brought the per-element cost down by orders of magnitude, and it is this shift, more than any change in the underlying array theory, that carried phased arrays out of dedicated radar rooms and into commercial base stations, satellite terminals and the other civilian applications described below.[^civilian-spread-cn] ## Formulation The mathematics of a phased array is the same array-factor construction used for any [[Antenna_array|antenna array]]; what a phased array's phase shifters add is the ability to set the steering term electronically, look to look, rather than leaving it fixed by the elements' physical arrangement. ### Array factor For N identical elements spaced along a line, each fed with a progressive phase increment δ relative to its neighbour, the far-field pattern the elements produce together, independent of any single element's own pattern, is the array factor `AF(ψ) = Σ e^(jnψ)` summed over the elements, where `ψ = k·d·sinθ + δ` combines the geometric phase difference across the aperture with the phase the feed network adds deliberately. This sum peaks sharply wherever ψ equals zero, so choosing `δ = −k·d·sinθ0` points that peak at any desired angle θ0 without moving a single element; the array factor is itself a discrete, spatial analogue of a [[Discrete_Fourier_transform|Fourier transform]], the aperture's excitation pattern mapped into an angular spectrum, which is why a longer aperture, like a longer time record in any other Fourier analysis, always buys a narrower main lobe. ### Worked example Consider eight elements spaced half a wavelength apart and fed in phase, aimed broadside. The array factor's first nulls, on either side of the main lobe, fall where the summed phasors first complete a full extra turn among themselves, which for this spacing and element count works out to roughly ±14.5° off broadside, for a beam about 29° wide between nulls; doubling the element count to sixteen at the same half-wavelength spacing halves that width to around 14.5°, since a longer aperture always narrows the main lobe. Now widen the spacing to a full wavelength instead of half, still with eight elements: a second, equally strong lobe appears exactly at endfire, 90° from broadside, because at that spacing the array factor repeats itself once within the range of physically real angles. Steering the main lobe even slightly off broadside at that same wide spacing brings the grating lobe in from endfire into ordinary visible space, which is the practical reason a working phased array normally holds its element spacing close to half a wavelength rather than the full wavelength a simpler feed network would allow. ## Applications Once confined to specialised radar, phased-array steering now appears wherever a beam needs to move faster than a motor can turn it, at whatever wavelength the application calls for. ### Radar Radar remains the application phased arrays were built for: a search-and-track [[Radar|radar]] built as a phased array can switch its beam among a search pattern and several already-detected targets almost instantly, the basis of the [[Active_electronically_scanned_array|active electronically scanned array]] radars now standard on many military and some civilian aircraft, and a natural fit for a [[Track_while_scan|track-while-scan]] radar that must revisit many targets without ever pausing its search. ### Sonar The same phased steering works underwater at acoustic frequencies: a towed or hull-mounted [[Sonar|sonar]] array of hydrophones combines its elements' signals with the right relative delay to steer a receive beam electronically, searching a wide arc or holding a bearing on a contact without physically training the array. ### Space probe communication Large ground antenna arrays built from many smaller, cheaper elements rather than one huge dish can be phased together to communicate with a distant spacecraft, giving an agency such as [[NASA|NASA]] a way to add capacity or cover a busy period by adding elements rather than building an entirely new dish. ### Weather research usage Research meteorologists have built experimental phased-array weather radars that scan an entire storm volume in a fraction of the time a mechanically rotating dish needs, letting forecasters watch a fast-evolving severe storm's structure update far more often than a conventional weather radar's slower rotation allows. ### Optics The same beam-forming mathematics applies at optical wavelengths: an optical phased array steers a laser beam by controlling the phase of light from many tiny emitters on a chip, aiming to give a solid-state alternative to a mechanically scanned mirror in applications such as automotive [[Laser|lidar]]. ### Satellite broadband internet transceivers A flat, electronically steered phased-array antenna lets a satellite-internet ground terminal track a fast-moving low-orbit satellite, or hand off smoothly from one satellite to the next, without the motorised gimbal a mechanically pointed dish would need. ### Radio-frequency identification (RFID) A phased-array reader antenna can steer or shape its interrogation beam electronically to control which tags in a crowded field respond, or estimate a responding tag's direction from the phase measured across the reader's own elements. ### Human-machine interfaces (HMI) A small, short-range phased array operating at millimetre-wave frequencies can sense the phase and Doppler signature of a moving hand well enough to recognise a gesture, giving a touchless control surface no camera or physical button is needed for. ### Radio astronomy A radio telescope built as a phased array of many small, fixed antennas, combined electronically rather than by moving one giant dish, can steer its effective beam across the sky purely in software and, unlike a single dish, form several independent beams over different parts of the sky at once. ### Broadcasting Terrestrial [[Broadcast_engineering|broadcast]] engineering uses fixed phased arrays for exactly the directional protection described under Fixed phased array above: an [[AM_broadcasting|AM broadcast]] station with several towers, each fed at a set phase and amplitude, can shape a pattern that reaches its own listeners while staying weak enough in another station's direction to avoid interference, all without a single moving part. ## Microsims This article carries no p5.js sketch of its own. A three.js companion, built as a variant of the Antenna_array sketch, instead renders this same phase-steered beam in three dimensions, letting the reader watch the main lobe swing and a grating lobe form exactly as the worked example above describes. *Try:* in the [[Antenna_array]] sketch, push the spacing control out past one wavelength and watch a second, full-strength lobe appear on the polar plot, the same grating lobe the worked example above predicts once element spacing passes half a wavelength. *Try:* in the [[Radar]] sketch, freeze the sweep with the space bar and picture the same rotating beam instead jumping instantly between bearings under electronic control; that instant retargeting, with no motor and no inertia to overcome, is what a phased array buys a search-and-track radar over the mechanically scanned antenna the sketch itself animates. ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Phased_array) : [Wikitube](https://en.wikitube.io/wiki/Phased_array) Skeleton mirrored at revision 1368849163. Prose, emphasis and the microsims are Wikitube's own. ## See also - [[Antenna_array]] - [[Active_electronically_scanned_array]] - [[Beamforming]] - [[Monopulse_radar]] - [[Radar]] - [[Sonar]] - [[MIMO]] ## References The array-factor construction, its steering by progressive phase, the beamwidth-versus-aperture relationship and the grating-lobe condition are standard antenna theory used throughout radio engineering and are not separately footnoted here, per the Wikitube style guide's §6.1; a fuller treatment of the same formalism is under [[Antenna_array]]. This topic's planned source, sub-manual 06 §6.4, treats antenna temperature rather than array-factor theory, so no claim below leans on a mismatched citation; the footnotes here mark the specific historical claims this article cannot yet pin to a source. [^early-concept-cn]: Citation needed: the earliest documented demonstration or patent of steering a radio beam by controlled phase difference between fixed radiators, with its inventor and date. [^bell-labs-cn]: Citation needed: the specific Bell Telephone Laboratories multi-beam or electronically steerable antenna research programme this claim describes, with its date and the researchers credited. [^cold-war-radar-cn]: Citation needed: a named Cold War-era military phased-array radar system and its in-service date would support the claim made here. [^civilian-spread-cn]: Citation needed: the specific product, standard or paper marking integrated-circuit phase shifters bringing phased arrays into civilian communication systems. **Further reading** — the open textbooks this article draws on, since the pair carries no separate Further reading heading of its own: - Steven Ellingson. *Radio Systems Engineering, Revised First Edition*. 2023. Open Textbook Library: https://open.umn.edu/opentextbooks/textbooks/radio-systems-engineering . CC BY-NC. - Christian Tiberius; Max Mulder. *Engineering Signal Analysis: From Fourier to filtering: Theory*. 2026. Open Textbook Library: https://open.umn.edu/opentextbooks/textbooks/engineering-signal-analysis-from-fourier-to-filtering-theory . CC BY. - Don Johnson. *Fundamentals of Electrical Engineering I*. 2014. Open Textbook Library: https://open.umn.edu/opentextbooks/textbooks/fundamentals-of-electrical-engineering-1 . CC BY. - Michael Stiber; Bilin Stiber; Eric Larson. *Signal Computing: Digital Signals in the Software Domain*. 2020. Open Textbook Library: https://open.umn.edu/opentextbooks/textbooks/signal-computing-digital-signals-in-the-software-domain . CC BY-SA. - Allen Downey. *Think DSP: Digital Signal Processing in Python*. 2012. Open Textbook Library: https://open.umn.edu/opentextbooks/textbooks/think-dsp-digital-signal-processing-in-python . CC BY-NC. - John Dyer; Chad Davis. *Measurement and Instrumentation: An Introduction to Concepts and Methods, 1st Edition*. 2020. Open Textbook Library: https://open.umn.edu/opentextbooks/textbooks/measurement-and-instrumentation-an-introduction-to-concepts-and-methods . CC BY-NC-SA. ## External links This article's Microsims section points outward to sketches carried by neighbouring articles; it carries none of its own to list here.