Antenna_(radio) - Wikiparency

# Antenna (radio) An **antenna**, called an aerial in British usage, is a structure that converts an alternating electric current into [[Radio_wave|radio waves]] for transmission, and converts radio waves back into an electric current for reception. It is the interface between currents guided along a conductor and waves propagating freely through space, and every piece of [[Radio|radio]] equipment, from a broadcast transmitter to a handheld receiver, depends on one. This article's three.js companion sketch lets the reader orbit the three-dimensional radiation pattern of several antenna types side by side, from a simple dipole and monopole to a multi-element array. An antenna's usefulness is set by a handful of properties considered together rather than any one alone: how efficiently it turns current into radiated power, how that power is distributed in direction, over how wide a range of frequencies it performs acceptably, and how cleanly it can be matched to the feedline that connects it to a transmitter or receiver. The same physical structure very often serves equally well as a transmitting or a receiving element, a property called reciprocity that is central enough to warrant its own section below. ## Terminology "Antenna" is the term standard in American usage; "aerial" is preferred in British usage, and the two refer to the same structure with no difference in meaning. The word entered English by way of Italian, where *antenna* denotes a ship's slender yardarm and, in an older sense, an insect's feeler; [[Guglielmo_Marconi]] is widely credited with carrying the word into wireless engineering from his own early experiments, after which English-speaking engineers adopted it largely unchanged.[^cn-etymology] Every antenna, however it is built, is treated in a circuit diagram as a single component and given its own reference designator, regardless of how many individual conductor elements it actually contains. ## Overview Functionally, an antenna is a transducer between two very different domains: a guided electric current, confined to a conductor and described by ordinary circuit quantities of voltage and current, and a free electromagnetic wave, spreading through space at the speed of light and described instead by field strength and polarization. A transmitting antenna is fed by a [[Transmission_line|transmission line]] from a transmitter and launches a wave outward; a receiving antenna intercepts a passing wave and delivers a current to a transmission line running the other way, into a receiver. Physically an antenna is an arrangement of one or more conductor elements, sized and shaped relative to the signal's wavelength, and the same underlying physics governs designs as different as a short whip on a vehicle and a large dish reflector, even though the two look nothing alike and serve very different frequency ranges. Whether an antenna favours one direction over others, and by how much, is the property considered next, once reciprocity is out of the way. ## Reciprocity An antenna's impedance, its gain and the shape of its radiation pattern are identical whether it is transmitting or receiving, a consequence of the reciprocity that [[Maxwell's_equations|Maxwell's equations]] impose on any passive, linear structure. Because of this, a pattern measured by feeding an antenna and recording the radiated field in every direction is exactly the pattern the same antenna would show if it were instead used to receive a wave arriving from each of those directions in turn, which is why radiation patterns are conventionally measured in transmitting mode even for antennas that will only ever receive. Reciprocity is also what allows a single number, an antenna's gain, to describe its performance in either role without further qualification. ## Resonant antennas Most practical antennas are operated at or near a resonance of their own conductor length, where the standing wave of current and voltage that forms on the structure settles into its simplest, most efficient pattern. ### Current and voltage distribution On a resonant linear element such as a [[Dipole_antenna|dipole]] or a [[Monopole_antenna|monopole]], current and voltage form a standing wave rather than travelling smoothly along the conductor: current peaks where the conductor is electrically longest and falls to zero at a free end, while voltage does the opposite, reaching its maximum at the free end and its minimum where current peaks. A half-wave dipole fed at its centre therefore sees a current maximum and a voltage minimum exactly at the feed point, which is why its feed-point impedance is comparatively low and largely resistive at resonance. ### Electrically short antennas When an antenna's physical length is well under a quarter of a wavelength, its radiation resistance falls off sharply, roughly as the square of the length-to-wavelength ratio, while its reactance becomes strongly capacitive. Both effects make an electrically short antenna hard to feed efficiently: the small radiation resistance is easily swamped by ordinary conductor loss, and the large reactance must be cancelled with a loading coil or a similar matching structure before any power can be delivered to it at all. The shorter an antenna is made relative to the wavelength it must handle, the narrower the range of frequencies over which that cancellation stays good enough to use, a limit that returns below under Bandwidth. ### Arrays and reflectors Combining several elements lets an antenna trade an omnidirectional pattern for a directional one. A driven [[Antenna_array|array]] feeds every element with a controlled amplitude and phase, steering or shaping the combined pattern by [[Beamforming|beamforming]]; a simpler alternative feeds only one element directly and lets nearby parasitic elements, tuned to reflect or reinforce the driven element's field, shape the pattern instead, the arrangement used in a [[Yagi–Uda_antenna|Yagi-Uda antenna]]'s single reflector and one or more directors, a design invented in Japan in the 1920s by Shintaro Uda under Hidetsugu Yagi, whose later English-language paper gave the antenna its Western name.[^cn-yagi] Either approach concentrates radiated power into a narrower range of directions, at the cost of the omnidirectional coverage a single simple element provides for free. ## Characteristics An antenna's suitability for a given job is judged against several distinct properties, each considered in turn below. ### Bandwidth Bandwidth is the range of frequencies over which an antenna's impedance match and radiation pattern both stay within an acceptable limit, commonly stated as a maximum [[Standing_wave_ratio|standing wave ratio]] on its feedline. Electrically small antennas, as noted above, are inherently narrowband, because the same short length that limits radiation resistance also stores a large amount of reactive energy relative to the power radiated each cycle; a physically larger antenna, closer to a full wavelength, need not make that trade and can be built for a wide bandwidth from its geometry alone. ### Gain Gain measures how strongly an antenna concentrates power in its favoured direction compared with a hypothetical antenna that radiates equally in every direction, expressed in [[Decibel|decibels]] relative to that isotropic reference as dBi. It is closely related to, but not identical with, [[Directivity|directivity]], which considers only the shape of the pattern; gain also accounts for power the antenna simply fails to radiate, lost instead to conductor and dielectric heating, so gain never exceeds directivity and equals it only for a lossless antenna. An ideal half-wave dipole has a directivity of about 1.64, or 2.15 dBi, one of the few round numbers in antenna engineering worth memorising outright. ### Effective area or aperture A receiving antenna can equally well be described by its effective area, the equivalent area from which it appears to draw power out of a passing wave; effective area and gain are two views of the same property, related by a fixed factor of the wavelength squared, and the [[Friis_transmission_equation|Friis transmission equation]] is built directly on that relationship. Because it has an effective area, an antenna also collects incidental power from whatever its surroundings radiate into its pattern even when no signal of interest is present, quantified as an antenna temperature: galactic background radiation alone contributes several hundred kelvin near the low end of the shortwave bands, falling off quickly as frequency rises,[^antennatemp-galactic] while man-made noise near the ground in a city can run to over a million kelvin at 30 MHz, dwarfing both the sky and the antenna's own thermal noise combined.[^antennatemp-manmade] ### Radiation pattern A [[Radiation_pattern|radiation pattern]] is a plot of an antenna's gain as a function of direction, typically drawn as one or more lobes surrounding the antenna: a main lobe in the favoured direction, smaller side lobes elsewhere, and often a back lobe opposite the main one. A simple dipole's pattern is a single doughnut-shaped lobe, a [[Torus|torus]] of revolution around the antenna's own axis, with no radiation at all directly off its two ends; the three-dimensional shape of patterns like this one, and how they differ between antenna types, is what this article's three.js companion lets the reader turn and inspect directly. ### Field regions Close to an antenna, the fields it produces are complicated and do not yet resemble a travelling wave; only beyond a distance of roughly twice the square of the antenna's largest dimension divided by the wavelength does the field settle into the simple, radially spreading pattern assumed everywhere else in this article, called the far field. Between the immediate reactive near field and the far field lies a transitional radiating near field, in which the angular shape of the pattern has not yet fully formed. Measurements and calculations of gain, pattern and polarization are all implicitly far-field quantities unless stated otherwise. ### Efficiency Radiation efficiency is the fraction of the power delivered to an antenna's terminals that actually leaves as a radiated wave, rather than being dissipated as heat in the conductor, in nearby lossy dielectric, or in a poor ground connection. It is the factor that separates gain from directivity, and it is usually the first place lost performance shows up in an electrically small antenna, whose already low radiation resistance can be comparable to, or even smaller than, the ordinary resistance of the conductor and matching network placed in series with it. ### Polarization Polarization describes the orientation of the wave's electric field as it leaves the antenna: linear if the field stays along a single line, and circular or elliptical if it instead rotates as the wave travels, tracing a circle or an [[Ellipse|ellipse]] when viewed along the direction of travel. A transmitting and a receiving antenna work best when their polarizations match; a receiving antenna cross-polarized with the incoming wave, linear at right angles to linear, or circular of the opposite handedness, can lose most of the available power to the mismatch alone, a loss distinct from and additional to anything the path itself introduces. ### Impedance matching Whatever an antenna's own impedance turns out to be, it rarely equals the standard impedance of the feedline and radio it must connect to, and the mismatch has to be corrected somewhere along the line. #### Antenna tuning at the antenna One option places a matching network, often no more than an inductor or a capacitor, directly at the antenna's own feed point, cancelling reactance right where it arises. This is the usual approach for an electrically short [[Monopole_antenna|monopole]], whose large capacitive reactance is most efficiently cancelled with a loading coil at or near the base rather than partway down a long feedline. #### Line matching at the radio The alternative places the matching network at the radio end of the [[Transmission_line|transmission line]] instead, adjusting it to present the line's characteristic impedance looking back toward the antenna regardless of what the antenna itself actually is; a [[Smith_chart|Smith chart]] is the classical graphical tool for designing such a network by hand. Matching at the radio end is simpler to adjust and to automate, at the cost of leaving a standing wave, and its associated extra loss, on the line between the mismatch and the antenna. #### Extreme examples of loaded small antennas The most extreme cases are antennas built for wavelengths many times longer than the structure that must radiate them, such as those used for very low frequency transmission: the physical structure covers only a minute fraction of a wavelength, and the loading and matching needed to make it resonate at all cost most of the antenna's efficiency and nearly all of its bandwidth. ## Effect of ground An antenna operated near the Earth's surface has its pattern and impedance modified by reflections from the ground beneath it, an effect strong enough that a [[Monopole_antenna|monopole]] is normally analysed by replacing the ground with an image of the antenna itself: a conductor above a perfectly conducting ground plane behaves exactly as if a mirror-image conductor extended the same distance below it, the pair together forming the equivalent of a longer antenna in free space. Real ground is a far poorer conductor than the ideal image theory assumes, and how much it falls short matters a great deal in practice: sea water behaves almost like the ideal case, while dry, rocky soil absorbs a significant share of the power that would otherwise have been radiated, turning it to heat in the ground itself rather than returning it as a useful wave. Broadcasters at the lower end of the radio spectrum go to considerable lengths to improve on bare soil, burying a system of radial ground wires beneath and around the base of a tower to give the induced return currents a low-loss path to flow through instead of the earth itself; the same problem, solved on a much smaller scale, is why a portable antenna often does noticeably better once it is given an artificial ground plane of its own rather than relying on whatever conductive surface happens to be nearby. ## Modeling antennas with line equations Because a resonant antenna element supports the same kind of standing wave as an open-circuited stub of [[Transmission_line|transmission line]], its input impedance can be estimated by treating it as one: the impedance of an open-circuited line swings between very low and very high values as its electrical length passes successive quarter-wavelength points, and a real antenna's impedance follows roughly the same swing as its own length is tuned through resonance and antiresonance. The analogy is only approximate, since a transmission line does not radiate and an antenna does, but it is close enough to give a useful first estimate before turning to a full electromagnetic calculation. It is also a genuinely old technique rather than a modern shortcut: antenna engineers were estimating input impedance this way well before general-purpose electromagnetic simulation existed, and the line-equation picture remains useful today chiefly because it gives a quick, physically motivated first answer that a numerical model can then refine, rather than requiring a full simulation to be run just to see whether a proposed length is even in the right neighbourhood. ## Mutual impedance and interaction between antennas Antenna elements placed close enough together, as in an array or a Yagi-Uda design, induce currents in one another in addition to whatever current their own feed supplies, an effect described by a mutual impedance between each pair of elements, analogous to the mutual inductance between two coupled coils. This coupling is not a nuisance to be designed away in every case: it is the entire mechanism by which a parasitic element with no feed of its own, a reflector or a director in a [[Yagi–Uda_antenna|Yagi-Uda]] array, shapes the pattern of the one element that is actually driven. In a driven array, by contrast, mutual impedance changes the impedance each element presents to its own feed from what it would show in isolation, a correction an array's feed network must account for rather than ignore. The closer elements are spaced, the stronger this coupling becomes, which is why an array's element spacing is chosen as a balance: close enough together to fit the physical structure and to avoid unwanted grating lobes, but not so close that mutual impedance makes every element's feed impedance impractically sensitive to its neighbours' exact position and current. ## Antenna types The properties described above are shared by every antenna, but the physical forms that realise them are numerous: the [[Dipole_antenna|dipole]] and the [[Monopole_antenna|monopole]] are the simplest resonant elements and the building blocks for most others; the [[Yagi–Uda_antenna|Yagi-Uda antenna]] adds parasitic directors and a reflector to a single driven element for gain in one direction; loop antennas trade the straight element for a closed conductor loop; and a large reflector antenna, typically a parabolic dish, uses a separate small feed element together with a much larger passive reflecting surface to reach a gain no single element could achieve on its own. Each type earns a separate article of its own; this one is the shared vocabulary all of them draw on. ## Microsims A three.js companion sketch lets the reader orbit the three-dimensional radiation-pattern lobes of several of the antenna types introduced above, dipole, monopole, loop and Yagi-Uda alike, side by side rather than one at a time; it is presented separately from this text. *Try:* in the [[Radar]] sketch, compare its narrow rotating beam, aimed to search every bearing in turn, with the fixed, all-round azimuth pattern a simple vertical antenna radiates without needing to turn at all. *Try:* in the [[Sonar]] sketch, watch the ping's wavefront expand as a widening arc, the same spherical spreading this article's Effective area or aperture section uses to describe how power density falls with distance from any antenna.  **Microsim — three.js (Wikitube framework), pending deploy:** *Antenna radiation pattern: a lobe you orbit* will play here once `https://wikitube-3d-microsims.netlify.app/radio/Antenna_(radio).html` is live.  *Built from `MICROSIM_GUIDE/specs/sims/Antenna_(radio).json`; part of the [[PORTAL_Radio|Radio]] set.*  ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Antenna_(radio)) : [Wikitube](https://en.wikitube.io/wiki/Antenna_(radio)) Skeleton mirrored at revision 1373727230. Prose, emphasis and the microsims are Wikitube's own. ## See also - [[Monopole_antenna]] - [[Yagi–Uda_antenna]] - [[Radiation_pattern]] - [[Directivity]] - [[Standing_wave_ratio]] - [[Dipole_antenna]] - [[Radio_wave]] ## Footnotes Standard antenna and electromagnetic theory used throughout this article, including reciprocity, effective aperture and the transmission-line analogy for input impedance, is not separately footnoted, per Wikitube style guide §6.1. The antenna-temperature figures under Characteristics are cited to Ellingson by page below; the etymology and Yagi-Uda dating claims are flagged citation needed rather than guessed. Page numbers are PDF pages of the open editions. ## References [^cn-etymology]: Citation needed: a lexicographic or historical source confirming when and by whom the Italian nautical sense of *antenna* entered English-language wireless engineering in connection with [[Guglielmo_Marconi]]'s experiments. [^antennatemp-galactic]: Ellingson, S. *Radio Systems Engineering - Revised First Edition*. 2023, p. 105 (PDF page). Open Textbook Library: https://open.umn.edu/opentextbooks/textbooks/radio-systems-engineering . CC BY-NC. [^antennatemp-manmade]: Ellingson, S. *Radio Systems Engineering - Revised First Edition*. 2023, pp. 106-107 (PDF pages). Open Textbook Library: https://open.umn.edu/opentextbooks/textbooks/radio-systems-engineering . CC BY-NC. [^cn-yagi]: Citation needed: the original 1920s Japanese-language paper by Shintaro Uda, or Hidetsugu Yagi's later English-language paper, that would fix the exact year the design was first published.