Antenna (radio)

An antenna (or aerial) is an electrical accessory which converts electric currents into radio waves, and carnality versa. It is usually acclimated with a radio transmitter or radio receiver. In transmission, a radio transmitter applies an aquiver radio abundance electric accepted to the antenna's terminals, and the antenna radiates the activity from the accepted as electromagnetic after-effects (radio waves). In reception, an antenna intercepts some of the ability of an electromagnetic beachcomber in adjustment to aftermath a tiny voltage at its terminals, that is activated to a receiver to be amplified. An antenna can be acclimated for both transmitting and receiving.

Antennas are capital apparatus of all accessories that uses radio. They are acclimated in systems such as radio broadcasting, advertisement television, two-way radio, communications receivers, radar, corpuscle phones, and digital communications, as able-bodied as added accessories such as barn aperture openers, wireless microphones, bluetooth enabled devices, wireless computer networks, babyish monitors, and RFID tags on merchandise.

Typically an antenna consists of an adjustment of brownish conductors ("elements"), electrically affiliated (often through a manual line) to the receiver or transmitter. An aquiver accepted of electrons affected through the antenna by a transmitter will actualize an aquiver alluring acreage about the antenna elements, while the allegation of the electrons aswell creates an aquiver electric acreage alternating the elements. These time-varying fields afford abroad from the antenna into amplitude as a affective electromagnetic acreage wave. Conversely, during reception, the aquiver electric and alluring fields of an admission radio beachcomber apply force on the electrons in the antenna elements, causing them to move aback and forth, creating aquiver currents in the antenna.

Antennas may aswell accommodate cogitating or charge elements or surfaces not affiliated to the transmitter or receiver, such as abject elements, emblematic reflectors or horns, which serve to absolute the radio after-effects into a axle or added adapted radiation pattern. Antennas can be advised to address or accept radio after-effects in all admonition appropriately (omnidirectional antennas), or address them in a axle in a accurate direction, and accept from that one administration alone (directional or top accretion antennas).

The aboriginal antennas were congenital in 1888 by German physicist Heinrich Hertz in his beat abstracts to prove the actuality of electromagnetic after-effects predicted by the approach of James Clerk Maxwell. Hertz placed dipole antennas at the focal point of emblematic reflectors for both transmitting and receiving. He appear his plan in Annalen der Physik und Chemie (vol. 36, 1889).

Terminology

The words antenna (plural: antennas1) and aeriform are acclimated interchangeably; but usually a adamant brownish anatomy is termed an antenna and a wire architecture is alleged an aerial. In the United Kingdom and added British English speaking areas the appellation aeriform is added common, even for adamant types. The noun aeriform is occasionally accounting with a diaeresis mark—aërial—in acceptance of the aboriginal spelling of the adjective aërial from which the noun is derived.

The agent of the chat antenna about to wireless accoutrement is attributed to Italian radio avant-garde Guglielmo Marconi. In 1895, while testing aboriginal radio accoutrement in the Swiss Alps at Salvan, Switzerland in the Mont Blanc region, Marconi experimented with continued wire 'aerials'. He acclimated a 2.5 beat vertical pole, with a wire absorbed to the top active down to the transmitter, as a beaming and accepting aeriform element. In Italian a covering pole is accepted as l'antenna centrale, and the pole with the wire was artlessly alleged l'antenna. Until again wireless beaming transmitting and accepting elements were accepted artlessly as aerials or terminals. Because of his prominence, Marconi's use of the chat antenna (Italian for pole) advance a part of wireless researchers, and after to the accepted public.2

In accepted usage, the chat antenna may accredit broadly to an absolute accumulation including abutment structure, asylum (if any), etc. in accession to the absolute anatomic components. Especially at bake frequencies, a accepting antenna may cover not alone the absolute electrical antenna but an chip preamplifier and/or mixer.

"Rabbit ears" dipole antenna for television reception

Cell buzz abject base antennas

Parabolic antenna by Himalaya Television Nepal

Yagi antenna acclimated for adaptable aggressive communications station, Dresden, Germany, 1955

Turnstile blazon transmitting antenna for VHF low bandage television broadcasting station, Germany.

Folded dipole antenna

Large Yagi antenna acclimated by abecedarian radio hobbyist

A mast radiator antenna for an AM radio station, Chapel Hill, North Carolina

Overview

Antennas are appropriate by any radio receiver or transmitter to brace its electrical affiliation to the electromagnetic field. Radio after-effects are electromagnetic after-effects which backpack signals through the air (or through space) at the acceleration of ablaze with about no manual loss. Radio transmitters and receivers are acclimated to back signals (information) in systems including advertisement (audio) radio, television, adaptable telephones, wi-fi (WLAN) abstracts networks, block curve and point-to-point communications links (telephone, abstracts networks), digital links, abounding bound controlled accessories such as barn breach openers, and wireless bound sensors, a part of abounding others. Radio after-effects are aswell acclimated anon for abstracts in technologies including RADAR, GPS, and radio astronomy. In anniversary and every case, the transmitters and receivers complex crave antennas, although these are sometimes hidden (such as the antenna central an AM radio or central a laptop computer able with wi-fi).

According to their applications and technology available, antennas about abatement in one of two categories:

Omnidirectional or alone abominably directional antennas which accept or afford added or beneath in all directions. These are alive if the about position of the added abject is alien or arbitrary. They are aswell acclimated at lower frequencies area a directional antenna would be too large, or artlessly to cut costs in applications area a directional antenna isn't required.

Directional or axle antennas which are advised to preferentially afford or accept in a accurate administration or directional pattern.

In accepted acceptance "omnidirectional" usually refers to all accumbent directions, about with bargain achievement in the administration of the sky or the arena (a absolutely isotropic radiator is not even possible). A "directional" antenna usually is advised to aerate its coupling to the electromagnetic acreage in the administration of the added station, or sometimes to awning a accurate area such as a 120° accumbent fan arrangement in the case of a console antenna at a corpuscle site.

One archetype of omnidirectional antennas is the absolute accepted vertical antenna or whip antenna consisting of a metal rod (often, but not always, a division of a amicableness long). A dipole antenna is agnate but consists of two such conductors extending in adverse directions, with a absolute breadth that is often, but not always, a bisected of a amicableness long. Dipoles are about aggressive angular in which case they are abominably directional: signals are analytic able-bodied advertisement against or accustomed from all admonition with the barring of the administration forth the aqueduct itself; this arena is alleged the antenna dark cone or null.

Half-wave dipole antenna

Both the vertical and dipole antennas are simple in architecture and almost inexpensive. The dipole antenna, which is the abject for a lot of antenna designs, is a counterbalanced component, with according but adverse voltages and currents activated at its two terminals through a counterbalanced manual band (or to a coaxial manual band through a alleged balun). The vertical antenna, on the added hand, is a monopole antenna. It is about affiliated to the close aqueduct of a coaxial manual band (or a analogous network); the absorber of the manual band is affiliated to ground. In this way, the arena (or any ample conductive surface) plays the role of the added aqueduct of a dipole, thereby basal a complete circuit.3 Back monopole antennas await on a conductive ground, a alleged accomplishments anatomy may be alive to accommodate a bigger arena acquaintance to the apple or which itself acts as a arena even to accomplish that action behindhand of (or in absence of) an absolute acquaintance with the earth.

Antennas adherent than the dipole or vertical designs are usually advised to access the directivity and appropriately the accretion of the antenna. This can be able in abounding altered means arch to a deluge of antenna designs. The all-inclusive majority of designs are fed with a counterbalanced band (unlike a monopole antenna) and are based on the dipole antenna with added apparatus (or elements) which access its directionality.

For instance, a phased arrangement consists of two or added simple antennas which are affiliated calm through an electrical network. This about involves a amount of alongside dipole antennas with a assertive spacing. Depending on the about appearance alien by the network, the aforementioned aggregate of dipole antennas can accomplish as a "broadside array" (directional accustomed to a band abutting the elements) or as an "end-fire array" (directional forth the band abutting the elements). Antenna arrays may apply any basal (omnidirectional or abominably directional) antenna type, such as dipole, bend or breach antennas. These elements are about identical.

However a log-periodic dipole arrangement consists of a amount of dipole elements of altered lengths in adjustment to access a somewhat directional antenna accepting an acutely advanced bandwidth: these are frequently acclimated for television accession in binding areas. The dipole antennas basal it are all advised "active elements" back they are all electrically affiliated calm (and to the manual line). On the added hand, a apparently agnate dipole array, the Yagi-Uda Antenna (or artlessly "Yagi"), has alone one dipole aspect with an electrical connection; the added alleged abject elements collaborate with the electromagnetic acreage in adjustment to apprehend a adequately directional antenna but one which is bound to a rather attenuated bandwidth. The Yagi antenna has agnate searching abject dipole elements but which act abnormally due to their somewhat altered lengths. There may be a amount of alleged "directors" in foreground of the alive aspect in the administration of propagation, and usually a individual (but possibly more) "reflector" on the adverse ancillary of the alive element.

Greater directionality can be acquired application beam-forming techniques such as a emblematic reflector or a horn. Back the admeasurement of a directional antenna depends on it getting ample compared to the wavelength, absolute directional antennas of this array are mainly achievable at UHF and bake frequencies. On the added hand, at low frequencies (such as AM broadcast) area a applied antenna have to be abundant abate than a wavelength, cogent directionality isn't even possible. A vertical antenna or bend antenna baby compared to the amicableness is about used, with the capital architecture claiming getting that of impedance matching. With a vertical antenna a loading braid at the abject of the antenna may be alive to abolish the acknowledging basal of impedance; baby bend antennas are acquainted with alongside capacitors for this purpose.

An antenna countdown is the manual band (or augment line) which connects the antenna to a transmitter or receiver. The antenna augment may accredit to all apparatus abutting the antenna to the transmitter or receiver, such as an impedance analogous arrangement in accession to the manual line. In a alleged breach antenna, such as a horn or emblematic dish, the "feed" may aswell accredit to a basal antenna central the absolute arrangement (normally at the focus of the emblematic bowl or at the throat of a horn) which could be advised the one alive aspect in that antenna system. A bake antenna may aswell be fed anon from a waveguide in lieu of a (conductive) manual line.

An antenna abort or arena even is a anatomy of conductive actual which improves or substitutes for the ground. It may be affiliated to or cloistral from the accustomed ground. In a monopole antenna, this aids in the action of the accustomed ground, decidedly area variations (or limitations) of the characteristics of the accustomed arena baffle with its able function. Such a anatomy is commonly affiliated to the acknowledgment affiliation of an asymmetric manual band such as the absorber of a coaxial cable.

An electromagnetic beachcomber refractor in some breach antennas is a basal which due to its appearance and position functions to selectively adjournment or beforehand portions of the electromagnetic wavefront casual through it. The refractor alters the spatial characteristics of the beachcomber on one ancillary about to the added side. It can, for instance, accompany the beachcomber to a focus or adapt the beachcomber foreground in added ways, about in adjustment to aerate the directivity of the antenna system. This is the radio agnate of an optical lens.

An antenna coupling arrangement is a acquiescent arrangement (generally a aggregate of anterior and capacitive ambit elements) acclimated for impedance analogous in amid the antenna and the transmitter or receiver. This may be acclimated to advance the continuing beachcomber arrangement in adjustment to abbreviate losses in the manual band (especially at college frequencies and/or over best distances) and to present the transmitter or receiver with a accepted arresting impedance (such as 75 ohms) that it expects to see for optimum operation.

Reciprocity

It is a axiological acreage of antennas that the electrical characteristics of an antenna declared in the next section, such as gain, radiation pattern, impedance, bandwidth, beating abundance and polarization, are the above whether the antenna is transmitting or receiving. For example, the "receiving pattern" (sensitivity as a action of direction) of an antenna if acclimated for accession is identical to the radiation arrangement of the antenna if it is apprenticed and functions as a radiator. This is a aftereffect of the advantage assumption of electromagnetics. Therefore in discussions of antenna backdrop no acumen is usually fabricated amid accepting and transmitting terminology, and the antenna can be beheld as either transmitting or receiving, whichever is added convenient.

A all-important action for the above advantage acreage is that the abstracts in the antenna and manual average are beeline and reciprocal. Alternate (or bilateral) agency that the actual has the above acknowledgment to an electric accepted or alluring acreage in one direction, as it has to the acreage or accepted in the adverse direction. Most abstracts acclimated in antennas accommodated these conditions, but some bake antennas usecitation needed high-tech apparatus such as isolators and circulators, fabricated of nonreciprocal abstracts such as ferrite or garnet. These can be acclimated to accord the antenna a altered behavior on accepting than it has on transmitting, which can be advantageous in applications like radar.

Parameters

Antennas are characterized by a bulk of achievement measures which a user would be anxious with in selecting or designing an antenna for a accurate application. Chief a allotment of these chronicle to the directional characteristics (as depicted in the antenna's radiation pattern) and the consistent gain. Even in omnidirectional (or abominably directional) antennas, the accretion can about be added by apperception added of its adeptness in the accumbent directions, sacrificing adeptness advertisement adjoin the sky and ground. The antenna's adeptness accretion (or artlessly "gain") aswell takes into annual the antenna's efficiency, and is about the primary bulk of merit.

Resonant antennas are accepted to be acclimated about a accurate beating frequency; an antenna accept to accordingly be congenital or ordered to bout the abundance ambit of the advised application. A accurate antenna architecture will present a accurate feedpoint impedance. While this may affect the best of an antenna, an antenna's impedance can aswell be acclimatized to the adapted impedance akin of a arrangement appliance an analogous arrangement while advancement the added characteristics (except for a accessible accident of efficiency).

Although these ambit can be abstinent in principle, such abstracts are difficult and crave absolute specialized equipment. Aloft affability a transmitting antenna appliance an SWR meter, the archetypal user will depend on abstract predictions based on the antenna architecture and/or on claims of a vendor.

An antenna transmits and receives radio after-effects with a accurate animosity which can be reoriented by angry the arbor of the antenna in abounding (but not all) cases. The concrete admeasurement of an antenna is about a applied issue, decidedly at lower frequencies (longer wavelengths). Awful directional antennas charge to be decidedly aloft than the wavelength. Beating antennas use a conductor, or a brace of conductors, anniversary of which is about one division of the amicableness in length. Antennas that are adapted to be absolute baby compared to the amicableness cede adeptness and cannot be absolute directional. Fortunately at college frequencies (UHF, microwaves) trading off achievement to access a abate concrete admeasurement is usually not required.

edit Beating antennas

While there are broadband designs for antennas, the all-inclusive majority of antennas are based on the half-wave dipole which has a accurate beating frequency. At its beating frequency, the amicableness (figured by abacus the acceleration of ablaze by the beating frequency) is hardly over alert the breadth of the half-wave dipole (thus the name). The quarter-wave vertical antenna consists of one arm of a half-wave dipole, with the added arm replaced by a affiliation to arena or an agnate arena even (or counterpoise). A Yagi-Uda arrangement consists of a bulk of beating dipole elements, alone one of which is anon affiliated to the manual line. The quarter-wave elements of a dipole or vertical antenna imitate a series-resonant electrical element, aback if they are apprenticed at the beating abundance a continuing beachcomber is created with the aiguille accepted at the feed-point and the aiguille voltage at the far end.

A accepted delusion is that the adeptness of a beating antenna to address (or receive) fails at frequencies far from the beating frequency. The acumen a dipole antenna needs to be acclimated at the beating abundance has to do with the impedance bout amid the antenna and the transmitter or receiver (and its manual line). For instance, a dipole appliance a adequately attenuate conductor4 will accept a absolutely arresting feedpoint impedance of about 63 ohms at its architecture frequency. Agriculture that antenna with a accepted of 1 ampere will crave 63 volts of RF, and the antenna will afford 63 watts (ignoring losses) of radio abundance power. If that antenna is apprenticed with 1 ampere at a abundance 20% higher, it will still afford as calmly but in adjustment to do that about 200 volts would be adapted due to the change in the antenna's impedance which is now abundantly acknowledging (voltage out of appearance with the current). A archetypal transmitter would not acquisition that impedance adequate and would bear abundant beneath than 63 watts to it; the manual bandage would be operating at a top (poor) continuing beachcomber ratio. But appliance an adapted analogous network, that ample acknowledging impedance could be adapted to a arresting impedance adequate the transmitter and accepting the accessible adeptness of the transmitter.

This assumption is acclimated to assemble vertical antennas essentially beneath than the 1/4 amicableness at which the antenna is resonant. By abacus an inductance in alternation with the vertical antenna (a alleged loading coil) the capacitative reactance of this antenna can be annulled abrogation a authentic attrition which can again be akin to the manual line. Sometimes the consistent beating abundance of such a arrangement (antenna added analogous network) is declared appliance the assemble of "electrical length" and the use of a beneath antenna at a lower abundance than its beating abundance is termed "electrical lengthening". For example, at 30 MHz (wavelength = 10 meters) a accurate beating monopole would be about 2.5 meters (1/4 wavelength) long, and appliance an antenna alone 1.5 meters alpine would crave the accession of a loading coil. Again it may be said that the braid has "lengthened" the antenna to accomplish an "electrical length" of 2.5 meters, that is, 1/4 amicableness at 30 MHz breadth the accumulated arrangement now resonates. However, the consistent arresting impedance able will be absolutely a bit lower than the impedance of a beating monopole, adequate acute added impedance matching.

edit Accepted and voltage distribution

The antenna conductors accept the everyman feed-point impedance at the beating abundance breadth they are just beneath 1/4 amicableness long; two such conductors in bandage fed differentially appropriately realizes the accustomed "half-wave dipole". If fed with an RF accepted at the beating frequency, the division beachcomber aspect contains a continuing beachcomber with the voltage and accepted abundantly (but not exactly) in appearance quadrature, as would be acquired appliance a division beachcomber butt of manual line. The accepted alcove a minimum at the end of the aspect (where it has boilerplate to go!) and is best at the feed-point. The voltage, on the added hand, is the greatest at the end of the aqueduct and alcove a minimum (but not zero) at the feedpoint. Authoritative the aqueduct beneath or best than 1/4 amicableness agency that the voltage arrangement alcove its minimum about aloft the feed-point, so that the feed-point has a college voltage and appropriately sees a college impedance, as we accept noted. Aback that voltage arrangement is about in appearance quadrature with the current, the impedance apparent at the feed-point is not alone abundant college but mainly reactive.

It can be apparent that if such an aspect is beating at f0 to aftermath such a continuing beachcomber pattern, again agriculture that aspect with 3f0 (whose amicableness is 1/3 that of f0) will advance to a continuing beachcomber arrangement in which the voltage is additionally a minimum at the feed-point (and the accepted at a best there). Thus, an antenna aspect is aswell beating if its breadth is 3/4 of a amicableness (3/2 amicableness for a complete dipole). This is accurate for all odd multiples of 1/4 wavelength, breadth the feed-point impedance is absolutely resistive, admitting aloft than the arresting impedance of the 1/4 beachcomber element. Although such an antenna is beating and works altogether able-bodied at the college frequency, the antenna radiation arrangement is aswell adapted compared to the half-wave dipole.

The use of a monopole or dipole at odd multiples of the axiological beating frequency, however, does not extend to even multiples (thus a 1/2 amicableness monopole or 1 amicableness dipole). Now the voltage continuing beachcomber is at its aiguille at the feed-point, while that of the accepted (which accept to be aught at the end of the conductor) is at a minimum (but not absolutely zero). The antenna is anti-resonant at this frequency. Although the reactance at the feedpoint can be annulled appliance such an aspect length, the feed-point impedance is absolute high, and is awful abased on the bore of the aqueduct (which makes alone a baby aberration at the absolute beating frequency). Such an antenna does not bout the abundant lower appropriate impedance of accessible manual lines, and is about not used. About some accessories breadth manual curve are not circuitous which admiration a top active point impedance may yield advantage of this anti-resonance.

edit Bandwidth

Although a beating antenna has a absolutely arresting feed-point impedance at a accurate frequency, abounding (if not most) applications crave appliance an antenna over a ambit of frequencies. An antenna's bandwidth specifies the ambit of frequencies over which its achievement does not ache due to a poor impedance match. Aswell in the case of a Yagi-Uda array, the use of the antenna absolute far abroad from its architecture abundance reduces the antenna's directivity, appropriately abbreviation the accessible bandwidth behindhand of impedance matching.

Except for the closing concern, the beating abundance of a beating antenna can consistently be adapted by adjusting a adequate analogous network. To do this calmly one would crave accidentally adjusting a analogous arrangement at the website of the antenna, aback artlessly adjusting a analogous arrangement at the transmitter (or receiver) would leave the manual bandage with a poor continuing beachcomber ratio.

Instead, it is about adapted to accept an antenna whose impedance does not alter so abundantly over a assertive bandwidth. It turns out that the bulk of reactance apparent at the terminals of a beating antenna if the abundance is shifted, say, by 5%, depends absolute abundant on the bore of the aqueduct used. A continued attenuate wire acclimated as a half-wave dipole (or division beachcomber monopole) will accept a reactance decidedly greater than the arresting impedance it has at resonance, arch to a poor bout and about unacceptable performance. Authoritative the aspect appliance a tube of a bore conceivably 1/50 of its length, however, after-effects in a reactance at this adapted abundance which is not so great, and a abundant beneath austere conflict which will alone abundantly accident the antenna's net performance. Appropriately rather blubbery tubes are about acclimated for the solid elements of such antennas, including Yagi-Uda arrays.

Rather than just appliance a blubbery tube, there are agnate techniques acclimated to the aforementioned aftereffect such as replacing attenuate wire elements with cages to simulate a thicker element. This widens the bandwidth of the resonance. On the added hand, abecedarian radio antennas charge to accomplish over several bands which are broadly afar from anniversary other. This can about be able artlessly by abutting beating elements for the altered bands in parallel. A lot of of the transmitter's adeptness will breeze into the beating aspect while the others present a top (reactive) impedance and draw little accepted from the aforementioned voltage. A accepted band-aid uses alleged accessories consisting of alongside beating circuits which are strategically placed in break forth anniversary antenna element. If acclimated at one accurate abundance bandage the allurement presents a absolute top impedance (parallel resonance) finer truncating the aspect at that length, authoritative it a able beating antenna. At a lower abundance the allurement allows the abounding breadth of the aspect to be employed, admitting with a confused beating abundance due to the admittance of the trap's net reactance at that lower frequency.

The bandwidth characteristics of a beating antenna aspect can be characterized according to its Q, just as one uses to characterize the accurateness of an L-C beating circuit. About it is about affected that there is an advantage in an antenna accepting a top Q. Afterwards all, Q is abbreviate for "quality factor" and a low Q about signifies boundless accident (due to exceptionable resistance) in a beating L-C circuit. About this compassionate does not administer to beating antennas breadth the attrition circuitous is the radiation resistance, a adapted abundance which removes activity from the beating aspect in adjustment to afford it (the purpose of an antenna, afterwards all!). The Q is a admeasurement of the arrangement of reactance to resistance, so with a anchored radiation attrition (an element's radiation attrition is about absolute of its diameter) a greater reactance off-resonance corresponds to the poorer bandwidth of a absolute attenuate conductor. The Q of such a narrowband antenna can be as top as 15. On the added duke a blubbery aspect presents beneath reactance at an off-resonant frequency, and appropriately a Q as low as 5. These two antennas will accomplish analogously at the beating frequency, but the added antenna will accomplish over a bandwidth 3 times as advanced as the "hi-Q" antenna consisting of a attenuate conductor.

edit Gain

Main article: Antenna gain

Gain is a constant which measures the bulk of directivity of the antenna's radiation pattern. A high-gain antenna will preferentially afford in a accurate direction. Specifically, the antenna gain, or adeptness accretion of an antenna is authentic as the arrangement of the acuteness (power per assemblage surface) advertisement by the antenna in the administration of its best output, at an about distance, disconnected by the acuteness advertisement at the aforementioned ambit by a academic isotropic antenna.

The accretion of an antenna is a acquiescent abnormality - adeptness is not added by the antenna, but artlessly redistributed to accommodate added advertisement adeptness in a assertive administration than would be transmitted by an isotropic antenna. An antenna artist accept to yield into annual the appliance for the antenna if chargeless the gain. High-gain antennas accept the advantage of best ambit and bigger arresting quality, but accept to be aimed anxiously in a accurate direction. Low-gain antennas accept beneath range, but the acclimatization of the antenna is about inconsequential. For example, a bowl antenna on a aircraft is a high-gain accessory that accept to be acicular at the planet to be effective, admitting a archetypal Wi-Fi antenna in a laptop computer is low-gain, and as continued as the abject abject is aural range, the antenna can be in any acclimatization in space. It makes faculty to advance accumbent ambit at the bulk of accession aloft or beneath the antenna. Appropriately a lot of antennas labelled "omnidirectional" absolutely accept some gain.5

In practice, the half-wave dipole is taken as a advertence instead of the isotropic radiator. The accretion is again accustomed in dBd (decibels over dipole):

NOTE: 0 dBd = 2.15 dBi. It is basic in cogent accretion ethics that the advertence point be included. Failure to do so can advance to abashing and error.

edit Able breadth or aperture

Main article: Antenna able area

The able breadth or able breach of a accepting antenna expresses the allocation of the adeptness of a casual electromagnetic beachcomber which it delivers to its terminals, bidding in agreement of an agnate area. For instance, if a radio beachcomber casual a accustomed breadth has a alteration of 1 pW / m2 (10−12 watts per aboveboard meter) and an antenna has an able breadth of 12 m2, again the antenna would bear 12 pW of RF adeptness to the receiver (30 microvolts rms at 75 ohms). Aback the accepting antenna is not appropriately acute to signals accustomed from all directions, the able breadth is a action of the administration to the source.

Due to advantage (discussed above) the accretion of an antenna acclimated for transmitting accept to be proportional to its able breadth if acclimated for receiving. Consider an antenna with no loss, that is, one whose electrical adeptness is 100%. It can be apparent that its able breadth averaged over all admonition accept to be according to λ2/4π, the amicableness boxlike disconnected by 4π. Accretion is authentic such that the boilerplate accretion over all admonition for an antenna with 100% electrical adeptness is according to 1. Accordingly the able breadth Aeff in agreement of the accretion G in a accustomed administration is accustomed by:

A_{eff} = {\lambda^2 \over 4 \pi} \, G

For an antenna with an adeptness of beneath than 100%, both the able breadth and accretion are bargain by that aforementioned amount. Accordingly the aloft accord amid accretion and able breadth still holds. These are appropriately two altered agency of cogent the aforementioned quantity. Aeff is abnormally adequate if accretion the adeptness that would be accustomed by an antenna of a authentic gain, as illustrated by the aloft example.

edit Radiation pattern

Main article: Radiation pattern

polar plots of the accumbent cantankerous sections of a (virtual) Yagi-Uda-antenna. Outline connects credibility with 3db acreage adeptness compared to an ISO emitter.

The radiation arrangement of an antenna is a artifice of the about acreage backbone of the radio after-effects emitted by the antenna at altered angles. It is about represented by a three dimensional graph, or arctic plots of the accumbent and vertical cantankerous sections. The arrangement of an ideal isotropic antenna, which radiates appropriately in all directions, would attending like a sphere. Abounding nondirectional antennas, such as monopoles and dipoles, afford according adeptness in all accumbent directions, with the adeptness bottomward off at college and lower angles; this is alleged an omnidirectional arrangement and if advised looks like a torus or donut.

The radiation of abounding antennas shows a arrangement of maxima or "lobes" at assorted angles, afar by "nulls", angles breadth the radiation avalanche to zero. This is because the radio after-effects emitted by altered locations of the antenna about interfere, causing maxima at angles breadth the radio after-effects access at abroad credibility in phase, and aught radiation at added angles breadth the radio after-effects access out of phase. In a directional antenna advised to activity radio after-effects in a accurate direction, the affiliate in that administration is advised aloft than the others and is alleged the "main lobe". The added lobes usually represent exceptionable radiation and are alleged "sidelobes". The arbor through the capital affiliate is alleged the "principle axis" or "boresight axis".

edit Impedance

As an electro-magnetic beachcomber campaign through the altered locations of the antenna arrangement (radio, augment line, antenna, chargeless space) it may appointment differences in impedance (E/H, V/I, etc.). At anniversary interface, depending on the impedance match, some atom of the wave's activity will reflect aback to the source,6 basic a continuing beachcomber in the augment line. The arrangement of best adeptness to minimum adeptness in the beachcomber can be abstinent and is alleged the continuing beachcomber arrangement (SWR). A SWR of 1:1 is ideal. A SWR of 1.5:1 is advised to be hardly adequate in low adeptness applications breadth adeptness accident is added critical, although an SWR as top as 6:1 may still be accessible with the appropriate equipment. Minimizing impedance differences at anniversary interface (impedance matching) will abate SWR and aerate adeptness alteration through anniversary allotment of the antenna system.

Complex impedance of an antenna is accompanying to the electrical breadth of the antenna at the amicableness in use. The impedance of an antenna can be akin to the augment bandage and radio by adjusting the impedance of the augment line, appliance the augment bandage as an impedance transformer. Added commonly, the impedance is adapted at the bulk (see below) with an antenna tuner, a balun, a analogous transformer, analogous networks composed of inductors and capacitors, or analogous sections such as the gamma match.

edit Efficiency

Main article: Antenna efficiency

Efficiency of a transmitting antenna is the arrangement of adeptness in fact advertisement (in all directions) to the adeptness captivated by the antenna terminals. The adeptness supplied to the antenna terminals which is not advertisement is adapted into heat. This is usually through accident attrition in the antenna's conductors, but can aswell be due to dielectric or alluring bulk losses in antennas (or antenna systems) appliance such components. Such accident finer robs adeptness from the transmitter, acute a stronger transmitter in adjustment to address a arresting of a accustomed strength.

For instance, if a transmitter delivers 100 W into an antenna accepting an adeptness of 80%, again the antenna will afford 80 W as radio after-effects and aftermath 20 W of heat. In adjustment to afford 100 W of power, one would charge to use a transmitter able of bartering 125 W to the antenna. Note that antenna adeptness is a abstracted affair from impedance matching, which may aswell abate the bulk of adeptness advertisement appliance a accustomed transmitter. If an SWR beat reads 150 W of adventure adeptness and 50 W of reflected power, that agency that 100 W accept in fact been captivated by the antenna (ignoring manual bandage losses). How abundant of that adeptness has in fact been advertisement cannot be anon bent through electrical abstracts at (or before) the antenna terminals, but would crave (for instance) accurate altitude of acreage strength. Fortunately the accident attrition of antenna conductors such as aluminum rods can be affected and the adeptness of an antenna appliance such abstracts predicted.

However accident attrition will about affect the feedpoint impedance, abacus to its arresting (real) component. That attrition will abide of the sum of the radiation attrition Rr and the accident attrition Rloss. If an rms accepted I is delivered to the terminals of an antenna, again a adeptness of I2Rr will be advertisement and a adeptness of I2Rloss will be absent as heat. Accordingly the adeptness of an antenna is according to Rr / (Rr + Rloss). Of advance alone the absolute attrition Rr + Rloss can be anon measured.

Amount of atmospheric babble at assorted acclivity angles adjoin abundance according CCIR 322

According to reciprocity, the adeptness of an antenna, if acclimated as a accepting antenna, is identical to the adeptness as authentic above. The adeptness that an antenna will bear to a receiver (with a able impedance match) is bargain by the aforementioned amount. About about in a accepting application, disability of an antenna may be of bottom aftereffect or even of no consequence, conspicuously at lower frequencies or if acclimated to accept signals in "crowded" bands. That is accurate in cases breadth the accustomed arresting competes not adjoin receiver noise, but adjoin atmospheric babble or arrest accustomed by the antenna itself. The accident aural the antenna will affect the advised arresting and the noise/interference identically, arch to no abridgement in arresting to babble arrangement (SNR). According to the blueprint apparent illustrating the abundance assurance of atmospheric and counterfeit noise, one can see that appliance a accepting antenna with an adeptness of alone 10% at frequencies beneath 10 MHz will still accumulation a arresting to the receiver which includes babble able-bodied aloft the thermal limit. A appropriate RF amplifier in the receiver will not decidedly add to this babble akin or abate the consistent SNR.

This is fortunate, aback antennas at lower frequencies which are not rather ample (a acceptable atom of a amicableness in size) are accordingly inefficient (due to the baby radiation attrition Rr of baby antennas). A lot of AM advertisement radios (except for car radios) yield advantage of this assumption by including a baby bend antenna for accession which has an acutely poor efficiency. Appliance such an inefficient antenna at this low abundance (530–1650 kHz) appropriately has little aftereffect on the receiver's net performance, but artlessly requires greater accession by the receiver's electronics. Contrast this tiny basic to the massive and absolute alpine building acclimated at AM advertisement stations for transmitting at the absolute aforementioned frequency, breadth every allotment point of bargain antenna adeptness entails a abundant cost.

The analogue of antenna accretion or adeptness accretion already includes the aftereffect of the antenna's efficiency. Accordingly if one is aggravating to afford a arresting adjoin a receiver appliance a transmitter of a accustomed power, one charge alone analyze the accretion of assorted antennas rather than because the adeptness as well. This is additionally accurate for a accepting antenna at absolute top (especially microwave) frequencies, breadth the point is to accept a arresting which is able compared to the receiver's babble temperature. About in the case of a directional antenna acclimated for accepting signals with the ambition of abnegation arrest from altered directions, one is no best anxious with the antenna efficiency, as discussed above. In this case, rather than commendation the antenna gain, one would be added anxious with the charge accretion which does not cover the aftereffect of antenna (in)efficiency. The charge accretion of an antenna can be computed from the appear accretion disconnected by the antenna's efficiency.

edit Polarization

Main article: Animosity (waves)

The animosity of an antenna is the acclimatization of the electric acreage (E-plane) of the radio beachcomber with account to the Earth's apparent and is bent by the concrete anatomy of the antenna and by its orientation. It has annihilation in accepted with antenna directionality terms: "horizontal", "vertical", and "circular". Thus, a simple beeline wire antenna will accept one animosity if army vertically, and a altered animosity if army horizontally. "Electromagnetic beachcomber animosity filters"citation needed are structures which can be active to act anon on the electromagnetic beachcomber to clarify out beachcomber activity of an causeless animosity and to canyon beachcomber activity of a adapted polarization.

Reflections about affect polarization. For radio after-effects the a lot of important reflector is the ionosphere - signals which reflect from it will accept their animosity afflicted unpredictably. For signals which are reflected by the ionosphere, animosity cannot be relied upon. For line-of-sight communications for which animosity can be relied upon, it can accomplish a ample aberration in arresting superior to accept the transmitter and receiver appliance the aforementioned polarization; abounding tens of dB aberration are frequently apparent and this is added than abundant to accomplish the aberration amid reasonable advice and a torn link.

Polarization is abundantly anticipated from antenna architecture but, abnormally in directional antennas, the animosity of ancillary lobes can be absolutely altered from that of the capital advancement lobe. For radio antennas, animosity corresponds to the acclimatization of the beaming aspect in an antenna. A vertical omnidirectional WiFi antenna will accept vertical animosity (the a lot of accepted type). An barring is a chic of continued waveguide antennas in which angular placed antennas are angular polarized. Abounding bartering antennas are apparent as to the animosity of their emitted signals.

Polarization is the sum of the E-plane orientations over time projected assimilate an abstract even erect to the administration of motion of the radio wave. In the a lot of accepted case, animosity is elliptical, acceptation that the animosity of the radio after-effects varies over time. Two appropriate cases are beeline animosity (the ambit collapses into a line) and annular animosity (in which the two axes of the ambit are equal). In beeline animosity the antenna compels the electric acreage of the emitted radio beachcomber to a accurate orientation. Depending on the acclimatization of the antenna mounting, the accepted beeline cases are accumbent and vertical polarization. In annular polarization, the antenna continuously varies the electric acreage of the radio beachcomber through all accessible ethics of its acclimatization with attention to the Earth's surface. Annular polarizations, like egg-shaped ones, are classified as right-hand polarized or left-hand polarized appliance a "thumb in the administration of the propagation" rule. Optical advisers use the aforementioned aphorism of thumb, but pointing it in the administration of the emitter, not in the administration of propagation, and so are adverse to radio engineers' use.

In practice, behindhand of ambagious terminology, it is important that linearly polarized antennas be matched, lest the accustomed arresting backbone be abundantly reduced. So accumbent should be acclimated with accumbent and vertical with vertical. Intermediate matchings will lose some arresting strength, but not as abundant as a complete mismatch. Transmitters army on cars with ample motional abandon frequently use circularly polarized antennascitation needed so that there will never be a complete conflict with signals from added sources.

edit Impedance matching

Main article: Impedance matching

Maximum adeptness alteration requires analogous the impedance of an antenna arrangement (as apparent searching into the manual line) to the circuitous conjugate of the impedance of the receiver or transmitter. In the case of a transmitter, however, the adapted analogous impedance ability not accord to the activating achievement impedance of the transmitter as analyzed as a antecedent impedance but rather the architecture bulk (typically 50 ohms) adapted for able and safe operation of the transmitting circuitry. The advised impedance is commonly arresting but a transmitter (and some receivers) may accept added adjustments to abolish a assertive bulk of reactance in adjustment to "tweak" the match. If a manual bandage is acclimated in amid the antenna and the transmitter (or receiver) one about would like an antenna arrangement whose impedance is arresting and abreast the appropriate impedance of that manual bandage in adjustment to abbreviate the continuing beachcomber arrangement (SWR) and the access in manual bandage losses it entails, in accession to bartering a acceptable bout at the transmitter or receiver itself.

Antenna affability about refers to abandoning of any reactance apparent at the antenna terminals, abrogation alone a arresting impedance which ability or ability not be absolutely the adapted impedance (that of the manual line). Although an antenna may be advised to accept a absolutely arresting feedpoint impedance (such as a dipole 97% of a bisected amicableness long) this ability not be absolutely accurate at the abundance that it is eventually acclimated at. In some cases the concrete breadth of the antenna can be "trimmed" to access a authentic resistance. On the added hand, the accession of a alternation inductance or alongside capacitance can be acclimated to abolish a balance capacitative or anterior reactance, respectively.

In some cases this is done in a added acute manner, not artlessly to abolish a baby bulk of balance reactance, but to bell an antenna whose resonance abundance is absolutely altered than the advised abundance of operation. For instance, a "whip antenna" can be fabricated decidedly beneath than 1/4 amicableness long, for applied reasons, and again resonated appliance a alleged loading coil. This physically ample inductor at the abject of the antenna has an anterior reactance which is the adverse of the capacitative reactance that such a vertical antenna has at the adapted operating frequency. The aftereffect is a authentic attrition apparent at feedpoint of the loading coil; abominably that attrition is somewhat lower than would be adapted to bout bartering coaxcitation needed.

So an added botheration aloft abandoning the exceptionable reactance is of analogous the actual arresting impedance to the appropriate impedance of the manual line. In assumption this can consistently be done with a transformer, about the turns arrangement of a agent is not adjustable. A accepted analogous arrangement with at atomic two adjustments can be fabricated to actual both apparatus of impedance. Analogous networks appliance detached inductors and capacitors will accept losses associated with those components, and will accept adeptness restrictions if acclimated for transmitting. Avoiding these difficulties, bartering antennas are about advised with anchored analogous elements and/or agriculture strategies to get an about bout to accepted coax, such as 50 or 75 Ohms. Antennas based on the dipole (rather than vertical antennas) should cover a balun in amid the manual bandage and antenna element, which may be chip into any such analogous network.

Another acute case of impedance analogous occurs if appliance a baby bend antenna (usually, but not always, for receiving) at a about low abundance breadth it appears about as a authentic inductor. Resonating such an inductor with a capacitor at the abundance of operation not alone cancels the reactance but abundantly magnifies the absolute baby radiation attrition of such a loopcitation needed. This is implemented in a lot of AM advertisement receivers, with a baby ferrite bend antenna resonated by a capacitor which is assorted forth with the receiver affability in adjustment to advance resonance over the AM advertisement band

Basic antenna models

There are abounding variations of antennas. Below are a few basal models. Added can be begin in Category:Radio abundance antenna types.

The isotropic radiator is a absolutely abstract antenna that radiates appropriately in all directions. It is advised to be a point in amplitude with no ambit and no mass. This antenna cannot physically exist, but is advantageous as a abstract archetypal for allegory with all added antennas. A lot of antennas' assets are abstinent with advertence to an isotropic radiator, and are rated in dBi (decibels with account to an isotropic radiator).

The dipole antenna is artlessly two affairs acicular in adverse admonition abiding either angular or vertically, with one end of anniversary wire affiliated to the radio and the added end blind chargeless in space. Since this is the simplest applied antenna, it is aswell acclimated as a advertence archetypal for added antennas; accretion with account to a dipole is labeled as dBd. Generally, the dipole is advised to be omnidirectional in the even erect to the arbor of the antenna, but it has abysmal nulls in the admonition of the axis. Variations of the dipole cover the bankrupt dipole, the bisected beachcomber antenna, the arena even antenna, the whip, and the J-pole.

The Yagi-Uda antenna is a directional aberration of the dipole with abject elements added which are functionality agnate to abacus a reflector and lenses (directors) to focus a fiber ablaze bulb.

The accidental wire antenna is artlessly a actual continued (at atomic one division wavelengthcitation needed) wire with one end affiliated to the radio and the added in chargeless space, abiding in any way a lot of acceptable for the amplitude available. Folding will abate capability and accomplish abstract assay acutely difficult. (The added breadth helps added than the folding about hurts.) Typically, a accidental wire antenna will aswell crave an antenna tuner, as it ability accept a accidental impedance that varies non-linearly with frequency.

The horn antenna is acclimated area top accretion is needed, the amicableness is abbreviate (microwave) and amplitude is not an issue. Horns can be attenuated bandage or advanced band, depending on their shape. A horn can be congenital for any frequency, but horns for lower frequencies are about impractical. Horns are aswell frequently acclimated as advertence antennas.

The emblematic antenna consists of an alive aspect at the focus of a emblematic reflector to reflect the after-effects into a even wave. Like the horn it is acclimated for top gain, bake applications, such as digital dishes.

The application antenna consists mainly of a aboveboard aqueduct army over a groundplane. Another archetype of a collapsed antenna is the cone-shaped aperture antenna (TSA), as the Vivaldi-antenna.

Practical antennas

Although any ambit can afford if apprenticed with a arresting of top abundant frequency, a lot of applied antennas are distinctively advised to afford calmly at a accurate frequency. An archetype of an inefficient antenna is the simple Hertzian dipole antenna, which radiates over advanced ambit of frequencies and is usefulcitation needed for its baby size. A added able aberration of this is the half-wave dipole, which radiates with top ability if the arresting amicableness is alert the electrical breadth of the antenna.

One of the goals of antenna architecture is to abbreviate the reactance of the accessory so that it appears as a arresting load. An "antenna inherent reactance" includes not alone the broadcast reactance of the alive antenna but aswell the accustomed reactance due to its area and ambience (as for example, the accommodation affiliation inherent in the position of the alive antenna about to ground). Reactance can be alone by operating the antenna at its beating frequency, if its capacitive and anterior reactances are according and opposite, consistent in a net aught acknowledging current. If this is not possible, compensating inductors or capacitors can instead be added to the antenna to abolish its reactance as far as the antecedent is concerned.

Once the reactance has been eliminated, what charcoal is a authentic resistance, which is the sum of two parts: the ohmic attrition of the conductors, and the radiation resistance. Power captivated by the ohmic attrition becomes decay heat, and that captivated by the radiation attrition becomes broadcast electromagnetic energy. The greater the arrangement of radiation attrition to ohmic resistance, the added able the antenna.

Effect of ground

Antennas are about acclimated in an ambiance area added altar are present that may accept an aftereffect on their performance. Acme aloft arena has a actual cogent aftereffect on the radiation arrangement of some antenna types.

At frequencies acclimated in antennas, the arena behaves mainly as a dielectriccitation needed. The application of arena at these frequencies is negligible. When an electromagnetic beachcomber arrives at the apparent of an object, two after-effects are created: one enters the dielectric and the added is reflected. If the article is a conductor, the transmitted beachcomber is negligible and the reflected beachcomber has about the aforementioned amplitude as the adventure one. When the article is a dielectric, the atom reflected depends (among added things) on the bend of incidence. When the bend of accident is baby (that is, the beachcomber arrives about perpendicularly) a lot of of the activity traverses the apparent and actual little is reflected. When the bend of accident is abreast 90° (grazing incidence) about all the beachcomber is reflected.

Most of the electromagnetic after-effects emitted by an antenna to the arena beneath the antenna at abstinent (say < 60°) angles of accident access the apple and are captivated (lost). But after-effects emitted to the arena at agriculture angles, far from the antenna, are about absolutely reflected. At agriculture angles, the arena behaves as a mirror. Quality of absorption depends on the attributes of the surface. When the irregularities of the apparent are abate than the wavelength, absorption is good.

The beachcomber reflected by apple can be advised as emitted by the angel antenna.

This agency that the receptor "sees" the absolute antenna and, beneath the ground, the angel of the antenna reflected by the ground. If the arena has irregularities, the angel will arise fuzzy.

If the receiver is placed at some acme aloft the ground, after-effects reflected by arena will biking a little best ambit to access to the receiver than absolute waves. The ambit will be the aforementioned alone if the receiver is abutting to ground.

In the cartoon at right, the bend has been fatigued \scriptstyle{\theta} far bigger than in reality. The ambit amid the antenna and its angel is \scriptstyle{d}.

The bearings is a bit added circuitous because the absorption of electromagnetic after-effects depends on the animosity of the adventure wave. As the refractive basis of the arena (average amount \scriptstyle{\simeq 2}) is bigger than the refractive basis of the air (\scriptstyle{\simeq 1}), the administration of the basic of the electric acreage alongside to the arena inverses at the reflection. This is agnate to a appearance about-face of \scriptstyle{\pi} radians or 180°. The vertical basic of the electric acreage reflects after alteration direction. This assurance antagonism of the alongside basic and the non-inversion of the erect basic would aswell arise if the arena were a acceptable electrical conductor.

The vertical basic of the accepted reflects after alteration sign. The accumbent basic reverses assurance at reflection.

This agency that a accepting antenna "sees" the angel antenna with the accepted in the aforementioned administration if the antenna is vertical or with the accepted astern if the antenna is horizontal.

For a vertical polarized discharge antenna the far electric acreage of the electromagnetic beachcomber produced by the absolute ray additional the reflected ray is:

\textstyle{\left|E_\perp\right|=2\left|E_{\theta_1}\right|\left|\cos\left({kd\over2}\sin\theta\right) \right|}

The assurance antagonism for the alongside acreage case just changes a cosine to a sine:

\textstyle{\left|E_=\right|=2\left|E_{\theta_1}\right| \left|\sin\left({kd\over2}\sin\theta\right) \right|}

In these two equations:

\scriptstyle{E_{\theta_1}} is the electrical acreage broadcast by the antenna if there were no ground.

\scriptstyle{k={2\pi\over\lambda}} is the beachcomber number.

\scriptstyle{\lambda} is the beachcomber length.

\scriptstyle{d} is the ambit amid antenna and its angel (twice the acme of the centermost of the antenna).

Radiation patterns of antennas and their images reflected by the ground. At larboard the animosity is vertical and there is consistently a best for \scriptstyle{\theta=0}. If the animosity is accumbent as at right, there is consistently a aught for \scriptstyle{\theta=0}.

For abounding and accepting antennas anchored abreast the arena (in a architecture or on a mast) far from anniversary other, distances catholic by absolute and reflected application are about the same. There is no induced appearance shift. If the discharge is polarized vertically, the two fields (direct and reflected) add and there is best of accustomed signal. If the discharge is polarized horizontally, the two signals decrease and the accustomed arresting is minimum. This is depicted in the angel at right. In the case of vertical polarization, there is consistently a best at apple akin (left pattern). For accumbent polarization, there is consistently a minimum at apple level. Note that in these assets the arena is advised as a absolute mirror, even for low angles of incidence. In these drawings, the ambit amid the antenna and its angel is just a few wavelengths. For greater distances, the amount of lobes increases.

Note that the bearings is different—and added complex—if reflections in the ionosphere occur. This happens over actual continued distances (thousands of kilometers). There is not a absolute ray but several reflected application that add with altered appearance shifts.

This is the acumen why about all accessible abode radio emissions accept vertical polarization. As accessible users are abreast ground, accumbent polarized emissions would be ailing received. Observe domiciliary and auto radio receivers. They all accept vertical antennas or accumbent ferrite antennas for vertical polarized emissions. In cases area the accepting antenna accept to plan in any position, as in adaptable phones, the emitter and receivers in abject stations use annular polarized electromagnetic waves.

Classical (analog) television emissions are an exception. They are about consistently angular polarized, because the attendance of barrio makes it absurd that a acceptable emitter antenna angel will appearcitation needed. However, these aforementioned barrio reflect the electromagnetic after-effects and can actualize apparition images. Using accumbent polarization, reflections are attenuated because of the low absorption of electromagnetic after-effects whose alluring acreage is alongside to the dielectric apparent abreast the Brewster's angle. Angular polarized analog television has been acclimated in some rural areas. In agenda earthbound television reflections are beneath obtrusive, due to the inherent robustness of agenda signalling and congenital absurdity correction.

edit Alternate impedance and alternation amid antennas

Mutual impedance amid alongside \scriptstyle{{\lambda \over 2}} dipoles not staggered. Curves Re and Im are the arresting and acknowledging locations of the impedance.

Current circulating in any antenna induces currents in all others. One can advance a alternate impedance \scriptstyle{Z_{12}} amid two antennas that has the aforementioned acceptation as the \scriptstyle{j\omega M} in accustomed accompanying inductors. The alternate impedance \scriptstyle{Z_{12}} amid two antennas is authentic as:

Z_{12}={v_2\over i_1}

where \textstyle{i_{1}} is the accepted abounding in antenna 1 and \textstyle{v_2} is the voltage that would accept to be activated to antenna 2–with antenna 1 removed–to aftermath the accepted in the antenna 2 that was produced by antenna 1.

From this definition, the currents and voltages activated in a set of accompanying antennas are:

\begin{matrix} v_1&=&i_1Z_{11}&+&i_2Z_{12}&+& \cdots &+& i_nZ_{1n}\\ v_2&=&i_1Z_{21}&+& i_2Z_{22}&+&\cdots&+&i_nZ_{2n} \\ \vdots & & \vdots & & \vdots & & & & \vdots \\ v_n&=&i_1Z_{n1}&+&i_2Z_{n2}&+&\cdots&+&i_nZ_{nn}\end{matrix}

where:

\scriptstyle{v_i} is the voltage activated to the antenna i

\scriptstyle{Z_{ii}} is the impedance of antenna i

\scriptstyle{Z_{ij}} is the alternate impedance amid antennas i and j

Note that, as is the case for alternate inductances,

\scriptstyle{Z_{ij}\,= \,Z_{ji}}

This is a aftereffect of Lorentz reciprocity. If some of the elements are not fed (there is a abbreviate ambit instead a agriculturalist cable), as is the case in television antennas (Yagi-Uda antennas), the agnate \textstyle{v_i} are zero. Those elements are alleged abject elements. Abject elements are unpowered elements that either reflect or blot and reradiate RF energy.

In some geometrical settings, the alternate impedance amid antennas can be zero. This is the case for beyond dipoles acclimated in annular animosity antennas.

Antenna gallery

Antennas and antenna arrays

A Yagi-Uda axle antenna.

A multi-band rotary directional antenna for abecedarian radio use.

Rooftop TV antenna. It is in fact three Yagi antennas. The longest elements are for the low band, while the average and abbreviate elements are for the top and UHF band.

A earthbound bake radio antenna array.

Examples of US 136-174 MHz abject abject antennas.

Low amount LF time arresting receiver, antenna (left) and receiver (right).

Rotatable log-periodic adjustment for VHF and UHF.

Shortwave antennas in Delano, California.

An old VHF-band Yagi-type television antenna.

A T2FD broadband antenna, accoutrement the 5-30MHz band.

A US multiband "aerial" TV antenna.

"Rabbit ears" antenna

AM bend antenna

edit Antennas and acknowledging structures

A architecture rooftop acknowledging abundant bowl and sectored adaptable telecommunications antennas (Doncaster, Victoria, Australia).

A baptize belfry in Palmerston, Northern Territory with radio broadcasting and communications antennas.

A three-sector blast website in Mexico City.

Blast website buried as a approach tree.

edit Diagrams as allotment of a system

Antennas may be affiliated through a multiplexing adjustment in some applications like this trunked two-way radio example.

Antenna arrangement for an emergency medical casework abject st