Yagi–Uda_antenna - Wikiparency

# Yagi–Uda antenna The **Yagi–Uda antenna**, generally called simply a Yagi antenna, is a directional radio antenna built from a single powered dipole and a line of unpowered, unconnected rods that reradiate its field so that the combined pattern concentrates power into one direction instead of spreading it broadly. One rod behind the powered element, the reflector, and one or more rods ahead of it, the directors, do all of the shaping: neither is wired to the transmitter or receiver at all, and the whole effect follows purely from how each rod's own length detunes it and how far it sits from its neighbours. Among the many designs surveyed in the general article on the [[Antenna_(radio)|radio antenna]], the Yagi–Uda is one of the most recognizable: the row of parallel rods on a supporting boom, familiar from countless rooftop television installations and amateur-radio masts, is a Yagi–Uda array in essentially every case. Its appeal is a rare combination of high directional gain, a single feed point, and a design built entirely from plain conductors with no matching network more complicated than the driven element itself. Elsewhere on this page, a three.js companion sketch builds a driven element, a reflector and a line of directors in three dimensions and shows the beam take shape, element by element, as the array is assembled; the sketches carried by the neighbouring [[Radar]] and [[Sonar]] articles show a comparably narrow beam doing its work in a pulsed ranging system rather than in a fixed television or amateur-radio link. ## Origins The Yagi–Uda antenna originated at Tohoku Imperial University in Sendai, Japan, in the mid-1920s, out of research into [[Radio_propagation|short-wave radio propagation]] led by professor Hidetsugu Yagi together with his research assistant Shintaro Uda. Uda's experiments with a driven dipole surrounded by tuned but unfed parallel rods showed that such rods, left electrically open with no connection to the transmitter, could still reradiate the driven element's field with a phase shift set purely by their own length and spacing, reinforcing the total field in one direction and cancelling it in the opposite one. Yagi and Uda published this design, which they called a projector of the sharpest beam of electric waves, in a short joint paper in February 1926.[^cite-yagi-uda-1926] Yagi followed two years later with a longer paper written in English for an international readership, describing the same reflector-and-director geometry in the form that most later antenna engineers came to know it by.[^cite-yagi-1928] ## Description A Yagi–Uda antenna is built along a single support boom, usually metal or an insulating rod, that carries every element at right angles to its own length and contributes essentially nothing to the electrical behaviour itself. One element only, the driven element, connects to the transmitter or receiver through a [[Transmission_line|feed line]]; it is ordinarily a half-wave [[Dipole_antenna|dipole]], sometimes folded to raise its feedpoint impedance to a more convenient value once it is loaded by the parasitic elements around it. Every other rod is a parasitic element: a solid, unbroken length of conductor with no feed connection of its own, mounted at its centre to the same boom. Exactly one reflector sits behind the driven element, on the side opposite the intended beam direction, cut a few percent longer than the driven element so that the current it picks up lags in exactly the phase needed to reinforce radiation forward and cancel it backward. Ahead of the driven element, in the beam direction, one or more directors are cut a few percent shorter than the driven element and spaced roughly a tenth to a quarter of a wavelength apart; adding directors lengthens the boom and narrows the beam further, at a rate of diminishing return per element added. The whole assembly forms an [[Antenna_array|antenna array]] in the general sense, but an end-fire one, radiating along the boom's own axis rather than broadside to it as a side-by-side array of fed elements would. ## Theory of operation Each parasitic element intercepts part of the driven element's near field and, because it is a resonant conductor left open at no point but tuned off its own natural resonance by being slightly long or slightly short, it re-radiates that intercepted energy with a phase shift that the element's own length and the free-space travel time to its neighbours together set. A reflector, tuned below its own resonant frequency by being a little too long, presents an inductive reactance to the current it picks up and reradiates with a phase lag that, combined with the extra travel time back to the driven element, reinforces the driven element's own forward radiation and works against its backward radiation. A director, tuned above resonance by being a little too short, presents a capacitive reactance and reradiates with a phase lead that has the same reinforcing effect further along the beam direction, so that adding one director after another approximates the fixed-geometry analogue of the electronic [[Beamforming|beamforming]] used in a modern [[Phased_array|phased array]], without ever wiring a phase shifter to any element: the geometry alone sets each element's phase relative to its neighbours. The resulting current along the row of elements behaves loosely like a slow travelling wave running from reflector to the last director, and the array radiates most strongly along that same axis, an end-fire pattern rather than the broadside pattern a side-by-side row of identically fed elements produces. Because every parasitic element also loads the driven element electromagnetically through this same coupling, the driven element's own feedpoint impedance drops well below an isolated dipole's, which is the practical reason a folded dipole, with its inherently higher impedance, is so often chosen as the driven element instead of a plain one. ## Analysis Adding parasitic elements raises the antenna's [[Directivity|directivity]], the concentration of radiated power into the main beam relative to a hypothetical antenna radiating equally in all directions, measured in [[Decibel|decibels]] as dBi. A single half-wave dipole already has a directivity of about 2.15 dBi; a well-built three-element Yagi–Uda antenna, one reflector plus one director, typically reaches roughly 7 to 9 dBi, and a long-boom design with many directors can reach into the mid-teens of dBi, though each additional director adds progressively less gain than the one before it, so a doubled boom length does not translate into a doubled beam concentration. A second figure, the front-to-back ratio, measures how much less strongly the antenna radiates directly behind itself than directly ahead; it is set largely by the single reflector and is far more sensitive to small changes in reflector spacing and length than the forward gain is, so a design optimized purely for maximum forward gain often trades away several decibels of front-to-back rejection. Concentrating received power from one direction, rather than picking up interference from every direction equally, also improves the [[Signal-to-noise_ratio|signal-to-noise ratio]] the antenna delivers to a receiver, which is a large part of why the design remains popular wherever a fixed link direction is known in advance. Because the parasitic elements' reactances are tuned for a single design frequency, both the gain and the input impedance change quickly as the operating frequency moves away from that design point, so a Yagi–Uda antenna is inherently a comparatively narrowband design next to a simple dipole. The idealized picture above, of parasitic elements carrying a purely sinusoidal current set by their own length alone, is the standard simplified model used to explain the antenna's behaviour; a real design's element currents also depend on the mutual coupling between every pair of elements at once, which is why practical designs are optimized by computer modelling rather than by the simple phase argument alone. ## Design Because a parasitic element's exact length and spacing set both its reactance and its coupling to its neighbours, a Yagi–Uda antenna's design has traditionally leaned on tabulated, computer-optimized dimensions rather than on the simple phase argument alone: the United States National Bureau of Standards published one influential set of such tables in 1976, giving director spacings and lengths that maximize gain for a chosen boom length from computer models of the whole coupled array.[^cite-nbs-viezbicke] Because the driven element's feedpoint impedance falls well below a plain dipole's once it is loaded by the parasitic elements, most practical designs feed a folded dipole instead of a plain one, or add a separate matching section such as a gamma match, a short shorting strap and series capacitor built from a length of rod parallel to the driven element, to bring the impedance back up to a standard 50 ohm [[Coaxial_cable|coaxial]] feed line. The boom itself, if conductive, slightly detunes every element mounted through it and is usually accounted for in the same computer optimization rather than in the simple hand formulas; an insulating boom avoids the effect entirely, at some cost in mechanical strength for long designs. Because a well-optimized design concentrates most of its gain improvement in the first few directors, most commercial and amateur antennas use anywhere from three to a dozen or so elements, matching the boom length available on a rooftop or a mast to the [[Very_high_frequency|VHF]] or [[Ultra_high_frequency|UHF]] television and amateur bands where the antenna's physical size, a few tenths of a metre to a few metres depending on frequency, remains convenient to mount. ## History For years after publication, the Yagi–Uda antenna found comparatively little practical use in Japan itself, while abroad Yagi's 1928 English-language paper carried the design into the wider international radio-engineering literature, where the reflector-and-director geometry was picked up and adapted by other laboratories working on directional short-wave and, later, very-high-frequency antennas.[^cite-yagi-1928] The design's low cost, light weight and single feed point suited it especially well to two tasks that both grew rapidly in the following decades: land-based television reception, where huge numbers of rooftop antennas across the VHF and UHF [[Radio_spectrum|broadcast bands]] settled on some form of Yagi–Uda geometry as the standard low-cost directional design, and amateur radio, where operators building their own directional antennas for [[Shortwave_radio|shortwave]] and VHF bands found a geometry that could be scaled and duplicated into stacked [[Antenna_array|arrays]] using the same simple element-count and boom-length trade-offs described above. The antenna community's later, more deliberate use of the compound name Yagi–Uda antenna, rather than plain Yagi antenna, reflects a retrospective recognition of Uda's larger share of the original experimental work, alongside Yagi's role in carrying the design to an international audience through the paper that bore his name alone.[^cite-ethw-yagi] ## Microsims This article carries no p5.js sketch of its own. A three.js companion elsewhere on this page instead builds a driven element, a reflector and a line of directors in three dimensions, adding one director at a time so the beam's growing concentration in one direction can be watched forming rather than only read off a finished diagram. *Try:* in the [[Radar]] sketch, press space to freeze the sweeping beam and compare the single narrow wedge it draws at that instant with the full circle an isotropic source's [[Radiation_pattern|radiation pattern]] would fill — the same forward concentration a Yagi–Uda array reaches by adding a reflector and directors to a plain dipole, rather than by mechanically narrowing a rotating beam. ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Yagi–Uda_antenna) : [Wikitube](https://en.wikitube.io/wiki/Yagi–Uda_antenna) Skeleton mirrored at revision 1375091506. Prose, emphasis and the microsims are Wikitube's own. ## See also - [[Directivity]] - [[Antenna_(radio)]] - [[Antenna_array]] - [[Dipole_antenna]] - [[Phased_array]] - [[Radiation_pattern]] - [[Radar]] ## Notes The element-current picture in Theory of operation and Analysis — a purely sinusoidal current on each parasitic element, set only by its own length and spacing — is the standard illustrative simplification used throughout antenna-engineering teaching; a real design's currents also depend on the mutual coupling among every element at once, which is why practical antennas are optimized by computer modelling rather than by this simple phase argument alone. ## References Standard antenna-array theory — end-fire radiation from parasitically coupled elements, the reflector/director reactance argument, and the general relationship between element count, boom length and directivity — is covered in essentially every antenna-engineering textbook and is not separately footnoted here, per the Wikitube style guide §6.1. [^cite-yagi-uda-1926]: Yagi, H.; Uda, S. "Projector of the Sharpest Beam of Electric Waves." *Proceedings of the Imperial Academy*, vol. 2, no. 2 (1926), pp. 49-52. doi:10.2183/pjab1912.2.49. [^cite-yagi-1928]: Yagi, H. "Beam Transmission of Ultra Short Waves." *Proceedings of the Institute of Radio Engineers*, vol. 16, no. 6 (June 1928), pp. 715-740. [^cite-ethw-yagi]: "Yagi Antenna." Engineering and Technology History Wiki (ETHW), IEEE. https://ethw.org/Yagi_Antenna . [^cite-nbs-viezbicke]: Viezbicke, P. P. *Yagi Antenna Design*. NBS Technical Note 688. National Bureau of Standards, U.S. Department of Commerce, December 1976 (public domain, US government work). https://archive.org/details/yagiantennadesig688viez . **Further reading** — the open textbook this article draws general antenna vocabulary from, 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 ## External links This article carries no sketch of its own yet. The three.js companion named in the lead is placed on the live page by the site's build process and will list its own link here once published.