Antenna (radio)

For other uses, see Antenna.

An antenna (plural antennae or antennas), or aerial,is an electrical device which converts electric power intoradio waves, and vice versa.[1] It is usually used with aradio transmitter or radio receiver. In transmission, a ra-dio transmitter supplies an electric current oscillating atradio frequency (i.e. a high frequency alternating current(AC)) to the antenna’s terminals, and the antenna radiatesthe energy from the current as electromagnetic waves (ra-dio waves). In reception, an antenna intercepts some ofthe power of an electromagnetic wave in order to producea tiny voltage at its terminals, that is applied to a receiverto be amplified.Antennas are essential components of all equipmentthat uses radio. They are used in systems such asradio broadcasting, broadcast television, two-way ra-dio, communications receivers, radar, cell phones, andsatellite communications, as well as other devices such asgarage door openers, wireless microphones, Bluetooth-enabled devices, wireless computer networks, baby mon-itors, and RFID tags on merchandise.Typically an antenna consists of an arrangement of metal-lic conductors (elements), electrically connected (oftenthrough a transmission line) to the receiver or transmitter.An oscillating current of electrons forced through the an-tenna by a transmitter will create an oscillating magneticfield around the antenna elements, while the charge of theelectrons also creates an oscillating electric field along theelements. These time-varying fields radiate away fromthe antenna into space as a moving transverse electromag-netic field wave. Conversely, during reception, the oscil-lating electric and magnetic fields of an incoming radiowave exert force on the electrons in the antenna elements,causing them to move back and forth, creating oscillatingcurrents in the antenna.Antennas can be designed to transmit and receiveradio waves in all horizontal directions equally(omnidirectional antennas), or preferentially in aparticular direction (directional or high gain antennas).In the latter case, an antenna may also include additionalelements or surfaces with no electrical connection tothe transmitter or receiver, such as parasitic elements,parabolic reflectors or horns, which serve to direct theradio waves into a beam or other desired radiationpattern.The first antennas were built in 1888 by German physicistHeinrich Hertz in his pioneering experiments to prove the

existence of electromagnetic waves predicted by the the-ory of James Clerk Maxwell. Hertz placed dipole anten-nas at the focal point of parabolic reflectors for both trans-mitting and receiving. He published his work in Annalender Physik und Chemie (vol. 36, 1889).

Animation of a half-wave dipole antenna transmitting radiowaves, showing the electric field lines. The antenna in the cen-ter is two vertical metal rods, with an alternating current appliedat its center from a radio transmitter (not shown). The voltagecharges the two sides of the antenna alternately positive (+) andnegative (−). Loops of electric field (black lines) leave the an-tenna and travel away at the speed of light; these are the radiowaves.

Animated diagram of a half-wave dipole antenna receiving en-ergy from a radio wave. The antenna consists of two metal rodsconnected to a receiver R. The electric field (E, green arrows)of the incoming wave pushes the electrons in the rods back andforth, charging the ends alternately positive (+) and negative (−).Since the length of the antenna is one half the wavelength of thewave, the oscillating field induces standing waves of voltage (V,represented by red band) and current in the rods. The oscillat-ing currents (black arrows) flow down the transmission line andthrough the receiver (represented by the resistance R).

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2 2 OVERVIEW

1 Terminology

The words antenna (plural: antennas[2] in US English, al-though both “antennas” and “antennae” are used in In-ternational English[3]) and aerial are used interchange-ably. Occasionally the term “aerial” is used to mean awire antenna. However, note the important internationaltechnical journal, the IEEE Transactions on Antennas andPropagation.[4] In the United Kingdom and other areaswhere British English is used, the term aerial is some-times used although 'antenna' has been universal in pro-fessional use for many years.The origin of the word antenna relative to wireless ap-paratus is attributed to Italian radio pioneer GuglielmoMarconi. In the summer of 1895, Marconi began test-ing his wireless system outdoors on his father’s estatenear Bologna and soon began to experiment with longwire “aerials”. Marconi discovered that by raising the“aerial” wire above the ground and connecting the otherside of his transmitter to ground, the transmission rangewas increased.[5] Soon he was able to transmit signals overa hill, a distance of approximately 2.4 kilometres (1.5mi).[6] In Italian a tent pole is known as l'antenna centrale,and the pole with the wire was simply called l'antenna.Until then wireless radiating transmitting and receivingelements were known simply as aerials or terminals.Because of his prominence, Marconi’s use of the wordantenna (Italian for pole) spread among wireless re-searchers, and later to the general public.[7][8][9]

In common usage, the word antennamay refer broadly toan entire assembly including support structure, enclosure(if any), etc. in addition to the actual functional compo-nents. Especially at microwave frequencies, a receivingantenna may include not only the actual electrical antennabut an integrated preamplifier or mixer.An antenna, in converting radio waves to electrical signalsor vice versa, is a form of transducer.[10]

2 Overview

Antennas are required by any radio receiver or trans-mitter to couple its electrical connection to the electro-magnetic field. Radio waves are electromagnetic waveswhich carry signals through the air (or through space) atthe speed of light with almost no transmission loss. Ra-dio transmitters and receivers are used to convey signals(information) in systems including broadcast (audio) ra-dio, television, mobile telephones, Wi-Fi (WLAN) datanetworks, trunk lines and point-to-point communicationslinks (telephone, data networks), satellite links, manyremote controlled devices such as garage door openers,and wireless remote sensors, among many others. Radiowaves are also used directly for measurements in tech-nologies including radar, GPS, and radio astronomy. Ineach and every case, the transmitters and receivers in-

volved require antennas, although these are sometimeshidden (such as the antenna inside an AM radio or insidea laptop computer equipped with Wi-Fi).According to their applications and technology available,antennas generally fall in one of two categories:

1. Omnidirectional or only weakly directional anten-nas which receive or radiate more or less in all di-rections. These are employed when the relative po-sition of the other station is unknown or arbitrary.They are also used at lower frequencies where a di-rectional antenna would be too large, or simply tocut costs in applications where a directional antennaisn't required.

2. Directional or beam antennas which are intended topreferentially radiate or receive in a particular direc-tion or directional pattern.

In common usage “omnidirectional” usually refers to allhorizontal directions, typically with reduced performancein the direction of the sky or the ground (a truly isotropicradiator is not even possible). A “directional” antennausually is intended to maximize its coupling to the elec-tromagnetic field in the direction of the other station, orsometimes to cover a particular sector such as a 120° hor-izontal fan pattern in the case of a panel antenna at a cellsite.One example of omnidirectional antennas is the verycommon vertical antenna or whip antenna consisting of ametal rod (often, but not always, a quarter of a wavelengthlong). A dipole antenna is similar but consists of two suchconductors extending in opposite directions, with a totallength that is often, but not always, a half of a wavelengthlong. Dipoles are typically oriented horizontally in whichcase they are weakly directional: signals are reasonablywell radiated toward or received from all directions withthe exception of the direction along the conductor itself;this region is called the antenna blind cone or null.Both the vertical and dipole antennas are simple in con-struction and relatively inexpensive. The dipole antenna,which is the basis for most antenna designs, is a balancedcomponent, with equal but opposite voltages and currentsapplied at its two terminals through a balanced transmis-sion line (or to a coaxial transmission line through a so-called balun). The vertical antenna, on the other hand, isa monopole antenna. It is typically connected to the innerconductor of a coaxial transmission line (or a matchingnetwork); the shield of the transmission line is connectedto ground. In this way, the ground (or any large con-ductive surface) plays the role of the second conductorof a dipole, thereby forming a complete circuit. Sincemonopole antennas rely on a conductive ground, a so-called grounding structure may be employed to providea better ground contact to the earth or which itself acts asa ground plane to perform that function regardless of (orin absence of) an actual contact with the earth.

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Antennas more complex than the dipole or vertical de-signs are usually intended to increase the directivity andconsequently the gain of the antenna. This can be ac-complished in many different ways leading to a plethoraof antenna designs. The vast majority of designs are fedwith a balanced line (unlike a monopole antenna) and arebased on the dipole antenna with additional components(or elements) which increase its directionality. Antenna“gain” in this instance describes the concentration of ra-diated power into a particular solid angle of space, as op-posed to the spherically uniform radiation of the ideal ra-diator. The increased power in the desired direction is atthe expense of that in the undesired directions. Power isconserved, and there is no net power increase over thatdelivered from the power source (the transmitter.)For instance, a phased array consists of two or more sim-ple antennas which are connected together through anelectrical network. This often involves a number of par-allel dipole antennas with a certain spacing. Dependingon the relative phase introduced by the network, the samecombination of dipole antennas can operate as a “broad-side array” (directional normal to a line connecting theelements) or as an “end-fire array” (directional along theline connecting the elements). Antenna arrays may em-ploy any basic (omnidirectional or weakly directional) an-tenna type, such as dipole, loop or slot antennas. Theseelements are often identical.However a log-periodic dipole array consists of a numberof dipole elements of different lengths in order to obtaina somewhat directional antenna having an extremely widebandwidth: these are frequently used for television recep-tion in fringe areas. The dipole antennas composing it areall considered “active elements” since they are all electri-cally connected together (and to the transmission line).On the other hand, a superficially similar dipole array,the Yagi-Uda Antenna (or simply “Yagi”), has only onedipole element with an electrical connection; the otherso-called parasitic elements interact with the electromag-netic field in order to realize a fairly directional antennabut one which is limited to a rather narrow bandwidth.The Yagi antenna has similar looking parasitic dipole el-ements but which act differently due to their somewhatdifferent lengths. There may be a number of so-called“directors” in front of the active element in the directionof propagation, and usually a single (but possibly more)“reflector” on the opposite side of the active element.Greater directionality can be obtained using beam-forming techniques such as a parabolic reflector or a horn.Since high directivity in an antenna depends on it beinglarge compared to the wavelength, narrow beams of thistype are more easily achieved at UHF and microwave fre-quencies.At low frequencies (such as AM broadcast), arrays of ver-tical towers are used to achieve directionality [12] and theywill occupy large areas of land. For reception, a longBeverage antenna can have significant directivity. For

non directional portable use, a short vertical antenna orsmall loop antenna works well, with the main design chal-lenge being that of impedance matching. With a ver-tical antenna a loading coil at the base of the antennamay be employed to cancel the reactive component ofimpedance; small loop antennas are tuned with parallelcapacitors for this purpose.An antenna lead-in is the transmission line (or feed line)which connects the antenna to a transmitter or receiver.The antenna feed may refer to all components connect-ing the antenna to the transmitter or receiver, such as animpedance matching network in addition to the transmis-sion line. In a so-called aperture antenna, such as a hornor parabolic dish, the “feed” may also refer to a basic an-tenna inside the entire system (normally at the focus ofthe parabolic dish or at the throat of a horn) which couldbe considered the one active element in that antenna sys-tem. A microwave antenna may also be fed directly froma waveguide in place of a (conductive) transmission line.An antenna counterpoise or ground plane is a structureof conductive material which improves or substitutes forthe ground. It may be connected to or insulated from thenatural ground. In a monopole antenna, this aids in thefunction of the natural ground, particularly where varia-tions (or limitations) of the characteristics of the naturalground interfere with its proper function. Such a structureis normally connected to the return connection of an un-balanced transmission line such as the shield of a coaxialcable.An electromagnetic wave refractor in some aperture an-tennas is a component which due to its shape and posi-tion functions to selectively delay or advance portions ofthe electromagnetic wavefront passing through it. The re-fractor alters the spatial characteristics of the wave on oneside relative to the other side. It can, for instance, bringthe wave to a focus or alter the wave front in other ways,generally in order to maximize the directivity of the an-tenna system. This is the radio equivalent of an opticallens.An antenna coupling network is a passive network (gen-erally a combination of inductive and capacitive circuitelements) used for impedance matching in between theantenna and the transmitter or receiver. This may be usedto improve the standing wave ratio in order to minimizelosses in the transmission line and to present the transmit-ter or receiver with a standard resistive impedance that itexpects to see for optimum operation.

3 Reciprocity

It is a fundamental property of antennas that the elec-trical characteristics of an antenna described in thenext section, such as gain, radiation pattern, impedance,bandwidth, resonant frequency and polarization, arethe same whether the antenna is transmitting or

4 4 CHARACTERISTICS

receiving.[13][14] For example, the "receiving pattern"(sensitivity as a function of direction) of an antenna whenused for reception is identical to the radiation pattern ofthe antenna when it is driven and functions as a radia-tor. This is a consequence of the reciprocity theorem ofelectromagnetics.[14] Therefore, in discussions of antennaproperties no distinction is usually made between receiv-ing and transmitting terminology, and the antenna can beviewed as either transmitting or receiving, whichever ismore convenient.A necessary condition for the aforementioned reciprocityproperty is that the materials in the antenna and trans-mission medium are linear and reciprocal. Reciprocal (orbilateral) means that the material has the same responseto an electric current or magnetic field in one direction,as it has to the field or current in the opposite direction.Most materials used in antennas meet these conditions,but some microwave antennas use high-tech componentssuch as isolators and circulators, made of nonreciprocalmaterials such as ferrite.[13][14] These can be used to givethe antenna a different behavior on receiving than it hason transmitting,[13] which can be useful in applicationslike radar.

4 Characteristics

See also: Antenna measurement § Antenna parameters

Antennas are characterized by a number of performancemeasures which a user would be concerned with in select-ing or designing an antenna for a particular application.Chief among these relate to the directional characteristics(as depicted in the antenna’s radiation pattern) and theresulting gain. Even in omnidirectional (or weakly direc-tional) antennas, the gain can often be increased by con-centrating more of its power in the horizontal directions,sacrificing power radiated toward the sky and ground.The antenna’s power gain (or simply “gain”) also takesinto account the antenna’s efficiency, and is often the pri-mary figure of merit.Resonant antennas are expected to be used around a par-ticular resonant frequency; an antenna must therefore bebuilt or ordered to match the frequency range of theintended application. A particular antenna design willpresent a particular feedpoint impedance. While this mayaffect the choice of an antenna, an antenna’s impedancecan also be adapted to the desired impedance level of asystem using a matching network while maintaining theother characteristics (except for a possible loss of effi-ciency).Although these parameters can be measured in principle,such measurements are difficult and require very special-ized equipment. Beyond tuning a transmitting antennausing an SWRmeter, the typical user will depend on the-oretical predictions based on the antenna design or on

claims of a vendor.An antenna transmits and receives radio waves with a par-ticular polarization which can be reoriented by tilting theaxis of the antenna in many (but not all) cases. The phys-ical size of an antenna is often a practical issue, partic-ularly at lower frequencies (longer wavelengths). Highlydirectional antennas need to be significantly larger thanthe wavelength. Resonant antennas usually use a linearconductor (or element), or pair of such elements, eachof which is about a quarter of the wavelength in length(an odd multiple of quarter wavelengths will also be res-onant). Antennas that are required to be small comparedto the wavelength sacrifice efficiency and cannot be verydirectional. Fortunately at higher frequencies (UHF, mi-crowaves) trading off performance to obtain a smallerphysical size is usually not required.

4.1 Resonant antennas

The majority of antenna designs are based on the reso-nance principle. This relies on the behaviour of movingelectrons, which reflect off surfaces where the dielectricconstant changes, in a fashion similar to the way light re-flects when optical properties change. In these designs,the reflective surface is created by the end of a conduc-tor, normally a thin metal wire or rod, which in the sim-plest case has a feed point at one end where it is con-nected to a transmission line. The conductor, or element,is aligned with the electrical field of the desired signal,normally meaning it is perpendicular to the line from theantenna to the source (or receiver in the case of a broad-cast antenna).[15]

The radio signal’s electrical component induces a voltagein the conductor. This causes an electrical current to be-gin flowing in the direction of the signal’s instantaneousfield. When the resulting current reaches the end of theconductor, it reflects, which is equivalent to a 180 degreechange in phase. If the conductor is 1⁄4 of a wavelengthlong, current from the feed point will undergo 90 degreephase change by the time it reaches the end of the con-ductor, reflect through 180 degrees, and then another 90degrees as it travels back. That means it has undergone atotal 360 degree phase change, returning it to the originalsignal. The current in the element thus adds to the currentbeing created from the source at that instant. This pro-cess creates a standing wave in the conductor, with themaximum current at the feed.[16]

The half-wave dipole is probably the most widely usedantenna design. This consists of two 1⁄4-wavelength el-ements arranged end-to-end (or collinear), each feedingone side of a two-conductor transmission wire. The twoelements physical arrangement places them 180 degreesout of phase, which means that at any given instant one ofthe elements is driving current into the transmission linewhile the other is pulling it out. The monopole antennais essentially one half of the half-wave dipole, a single

4.1 Resonant antennas 5

1⁄4-wavelength element with the other side connected toground or an equivalent ground plane (or counterpoise).Monopoles, which are one-half the size of a dipole, arecommon for long-wavelength radio signals where a dipolewould be impractically large. Another common design isthe folded dipole, which is essentially two dipoles placedside-by-side and connected at their ends to make a singleone-wavelength antenna.The standing wave forms with this desired pattern at thedesign frequency, f0, and antennas are normally designedto be this size. However, feeding that element with 3f0(whose wavelength is 1⁄3 that of f0) will also lead to astanding wave pattern. Thus, an antenna element is alsoresonant when its length is 3⁄4 of a wavelength. This istrue for all odd multiples of 1⁄4 wavelength. This allowssome flexibility of design in terms of antenna lengths andfeed points. Antennas used in such a fashion are knownto be harmonically operated.[17]

4.1.1 Current and voltage distribution

The quarter-wave elements imitate a series-resonant elec-trical element due to the standing wave present along theconductor. At the resonant frequency, the standing wavehas a current peak and voltage node (minimum) at thefeed. In electrical terms, this means the element hasminimum reactance, generating the maximum current forminimum voltage. This is the ideal situation, because itproduces the maximum output for the minimum input,producing the highest possible efficiency. Contrary to anideal (lossless) series-resonant circuit, a finite resistanceremains (corresponding to the relatively small voltage atthe feed-point) due to the antenna’s radiation resistanceas well as any actual electrical losses.Recall that a current will reflect when there are changes inthe electrical properties of the material. In order to effi-ciently send the signal into the transmission line, it is im-portant that the transmission line has the same impedanceas the elements, otherwise some of the signal will be re-flected back into the antenna. This leads to the conceptof impedance matching, the design of the overall sys-tem of antenna and transmission line so the impedanceis as close as possible, thereby reducing these losses.Impedance matching between antennas and transmissionlines is commonly handled through the use of a balun, al-though other solutions are also used in certain roles. Animportant measure of this basic concept is the standingwave ratio, whichmeasures themagnitude of the reflectedsignal.Consider a half-wave dipole designed to work with sig-nals 1 m wavelength, meaning the antenna would be ap-proximately 50 cm across. If the element has a length-to-diameter ratio of 1000, it will have an inherent resistanceof about 63 ohms. Using the appropriate transmissionwire or balun, we match that resistance to ensure mini-mum signal loss. Feeding that antenna with a current of

1 ampere will require 63 volts of RF, and the antennawill radiate 63 watts (ignoring losses) of radio frequencypower. Now consider the case when the antenna is fed asignal with a wavelength of 1.25 m; in this case the re-flected current would arrive at the feed out-of-phase withthe signal, causing the net current to drop while the volt-age remains the same. Electrically this appears to be avery high impedance. The antenna and transmission lineno longer have the same impedance, and the signal willbe reflected back into the antenna, reducing output. Thiscould be addressed by changing the matching system be-tween the antenna and transmission line, but that solutiononly works well at the new design frequency.The end result is that the resonant antenna will efficientlyfeed a signal into the transmission line only when thesource signal’s frequency is close to that of the designfrequency of the antenna, or one of the resonant multi-ples. This makes resonant antenna designs inherently nar-rowband, and they are most commonly used with a sin-gle target signal. They are particularly common on radarsystems, where the same antenna is used for both broad-cast and reception, or for radio and television broadcasts,where the antenna is working with a single frequency.They are less commonly used for reception where mul-tiple channels are present, in which case additional mod-ifications are used to increase the bandwidth, or entirelydifferent antenna designs are used.

4.1.2 Modified resonant designs

It is possible to use the impedance matching conceptsto construct vertical antennas substantially shorter thanthe 1⁄4 wavelength at which the antenna is resonant. Byadding an inductance in series with the antenna, a so-called loading coil, the capacitive reactance of this an-tenna can be cancelled leaving a pure resistance which canthen be matched to the transmission line. Sometimes theresulting resonant frequency of such a system (antennaplus matching network) is described using the constructof electrical length and the use of a shorter antenna ata lower frequency than its resonant frequency is termedelectrical lengthening.For example, at 300 MHz (1 m wavelength) a true reso-nant 1⁄4-wavelength monopole would be almost 2.5 me-ters long, and using an antenna only 1.5 meters tall wouldrequire the addition of a loading coil. Then it may besaid that the coil has lengthened the antenna to achievean electrical length of 2.5 meters. However, the result-ing resistive impedance achieved will be quite a bit lowerthan the impedance of a resonant monopole, likely requir-ing further impedance matching. In addition to a lowerradiation resistance, the reactance becomes higher as theantenna size is reduced, and the resonant circuit formedby the antenna and the tuning coil has a Q factor that risesand eventually causes the bandwidth of the antenna to beinadequate for the signal being transmitted. This is themajor factor that sets the size of antennas at 1 MHz and

6 4 CHARACTERISTICS

lower frequencies.

4.1.3 Arrays and reflectors

The amount of signal received from a distant transmissionsource is essentially geometric in nature due to the inversesquare law, and this leads to the concept of effective area.This measures the performance of an antenna by com-paring the amount of power it generates to the amountof power in the original signal, measured in terms of thesignal’s power density in Watts per square metre. A half-wave dipole has an effective area of 0.13 λ 2. If moreperformance is needed, one cannot simply make the an-tenna larger. Although this would intercept more energyfrom the signal, due to the considerations above, it woulddecrease the output significantly. In roles where higherperformance is needed, designers often use multiple ele-ments combined together.Returning to the basic concept of current flows in a con-ductor, consider what happens if a half-wave dipole is notconnected to a feed point, but instead shorted out. Elec-trically this forms a single 1⁄2-wavelength element. Butthe overall current pattern is the same; the current will bezero at the two ends, and reach a maximum in the cen-ter. Thus signals near the design frequency will continueto create a standing wave pattern. Any varying electricalcurrent, like the standing wave in the element, will radiatea signal. In this case, aside from losses in the element, itwill be significantly similar to the original signal in bothmagnitude and shape. If this element is placed so its sig-nal reaches the main dipole in-phase, it will reinforce theoriginal signal, and increase the current in the dipole. El-ements used in this way are known as passive elements.A Yagi-Uda array uses passive elements to greatly in-crease gain. It consists of a support boom that is pointedtoward the signal, and thus sees no induced signal anddoes not contribute to the antenna’s operation. The endcloser to the source is referred to as the front. Nearthe rear is a single active element, typically a half-wavedipole or folded dipole Passive elements are arranged infront (directors) and behind (reflectors) the active elementalong the boom. The Yagi has the inherent quality thatit becomes increasingly directional, and thus has highergain, as the number of elements increases. However,this also makes it increasingly sensitive to changes in fre-quency; in addition to the changes in impedance notedabove, the radiated signals will no longer reach the activeelement in phase, and thus it becomes inherently morenarrowband.It is also possible to usemultiple active elements and com-bine them together with transmission lines to produce asimilar system where the phases add up to reinforce theoutput. The antenna array and very similar reflective ar-ray antenna consist of multiple elements, often half-wavedipoles, spaced out on a plane and wired together withtransmission lines with specific phase lengths to produce

a single in-phase signal at the output. The log-periodicantenna is a more complex design that uses multiple in-line elements similar in appearance to the Yagi-Uda butusing transmission lines between the elements to producethe output.Reflection of the original signal also occurs when it hits anextended conductive surface, in a fashion similar to a mir-ror. This effect can also be used to increase signal throughthe use of a reflector, normally placed behind the activeelement and spaced so the reflected signal reaches the ele-ment in-phase. Generally the reflector will remain highlyreflective even if it is not solid; gaps less than 1⁄10 gen-erally have little effect on the outcome. For this reason,reflectors often take the form of wire meshes or rows ofpassive elements. The parabolic reflector is perhaps thebest known example of a reflector-based antenna, whichhas an effective area far greater than the active elementalone.

4.2 Bandwidth

Main article: Antenna bandwidth

Although a resonant antenna has a purely resistive feed-point impedance at a particular frequency, many (if notmost) applications require using an antenna over a rangeof frequencies. An antenna’s bandwidth specifies therange of frequencies over which its performance does notsuffer due to a poor impedance match. Also in the caseof a Yagi-Uda array, the use of the antenna very far awayfrom its design frequency reduces the antenna’s direc-tivity, thus reducing the usable bandwidth regardless ofimpedance matching.Except for the latter concern, the resonant frequency ofa resonant antenna can always be altered by adjustinga suitable matching network. To do this efficiently onewould require remotely adjusting a matching network atthe site of the antenna, since simply adjusting a matchingnetwork at the transmitter (or receiver) would leave thetransmission line with a poor standing wave ratio.Instead, it is often desired to have an antenna whoseimpedance does not vary so greatly over a certain band-width. It turns out that the amount of reactance seen atthe terminals of a resonant antenna when the frequency isshifted, say, by 5%, depends very much on the diameterof the conductor used. A long thin wire used as a half-wave dipole (or quarter wave monopole) will have a reac-tance significantly greater than the resistive impedance ithas at resonance, leading to a poor match and generallyunacceptable performance. Making the element using atube of a diameter perhaps 1/50 of its length, however,results in a reactance at this altered frequency which isnot so great, and a much less serious mismatch whichwill onlymodestly damage the antenna’s net performance.Thus rather thick tubes are typically used for the solid el-ements of such antennas, including Yagi-Uda arrays.

4.4 Effective area or aperture 7

Rather than just using a thick tube, there are similar tech-niques used to the same effect such as replacing thin wireelements with cages to simulate a thicker element. Thiswidens the bandwidth of the resonance. On the otherhand, amateur radio antennas need to operate over sev-eral bands which are widely separated from each other.This can often be accomplished simply by connecting res-onant elements for the different bands in parallel. Mostof the transmitter’s power will flow into the resonant ele-ment while the others present a high (reactive) impedanceand draw little current from the same voltage. A pop-ular solution uses so-called traps consisting of parallelresonant circuits which are strategically placed in breaksalong each antenna element. When used at one particularfrequency band the trap presents a very high impedance(parallel resonance) effectively truncating the element atthat length, making it a proper resonant antenna. At alower frequency the trap allows the full length of the el-ement to be employed, albeit with a shifted resonant fre-quency due to the inclusion of the trap’s net reactance atthat lower frequency.The bandwidth characteristics of a resonant antenna el-ement can be characterized according to its Q, just asone uses to characterize the sharpness of an L-C reso-nant circuit. However it is often assumed that there is anadvantage in an antenna having a high Q. After all, Q isshort for “quality factor” and a low Q typically signifiesexcessive loss (due to unwanted resistance) in a resonantL-C circuit. However this understanding does not applyto resonant antennas where the resistance involved is theradiation resistance, a desired quantity which removes en-ergy from the resonant element in order to radiate it (thepurpose of an antenna, after all!). The Q is a measureof the ratio of reactance to resistance, so with a fixedradiation resistance (an element’s radiation resistance isalmost independent of its diameter) a greater reactanceoff-resonance corresponds to the poorer bandwidth of avery thin conductor. The Q of such a narrowband antennacan be as high as 15. On the other hand, a thick elementpresents less reactance at an off-resonant frequency, andconsequently a Q as low as 5. These two antennas willperform equivalently at the resonant frequency, but thesecond antenna will perform over a bandwidth 3 times aswide as the “hi-Q” antenna consisting of a thin conductor.

4.3 Gain

Main article: Antenna gain

Gain is a parameter which measures the degree ofdirectivity of the antenna’s radiation pattern. A high-gainantenna will radiate most of its power in a particular di-rection, while a low-gain antenna will radiate over a widerangle. The antenna gain, or power gain of an antenna isdefined as the ratio of the intensity (power per unit sur-face area) I radiated by the antenna in the direction ofits maximum output, at an arbitrary distance, divided by

the intensity Iiso radiated at the same distance by a hy-pothetical isotropic antenna which radiates equal powerin all directions. This dimensionless ratio is usually ex-pressed logarithmically in decibels, these units are called“decibels-isotropic” (dBi)

GdBi = 10 log I

Iiso

A second unit used to measure gain is the ratio of thepower radiated by the antenna to the power radiated bya half-wave dipole antenna Idipole ; these units are called“decibels-dipole” (dBd)

GdBd = 10 log I

Idipole

Since the gain of a half-wave dipole is 2.15 dBi and thelogarithm of a product is additive, the gain in dBi is just2.15 decibels greater than the gain in dBd

GdBi = GdBd + 2.15

High-gain antennas have the advantage of longer rangeand better signal quality, but must be aimed carefully atthe other antenna. An example of a high-gain antennais a parabolic dish such as a satellite television antenna.Low-gain antennas have shorter range, but the orientationof the antenna is relatively unimportant. An example ofa low-gain antenna is the whip antenna found on portableradios and cordless phones. Antenna gain should not beconfused with amplifier gain, a separate parameter mea-suring the increase in signal power due to an amplifyingdevice.

4.4 Effective area or aperture

Main article: Antenna effective area

The effective area or effective aperture of a receiving an-tenna expresses the portion of the power of a passing elec-tromagnetic wave which it delivers to its terminals, ex-pressed in terms of an equivalent area. For instance, if aradio wave passing a given location has a flux of 1 pW /m2 (10−12 watts per square meter) and an antenna has aneffective area of 12 m2, then the antenna would deliver12 pW of RF power to the receiver (30 microvolts rms at75 ohms). Since the receiving antenna is not equally sen-sitive to signals received from all directions, the effectivearea is a function of the direction to the source.Due to reciprocity (discussed above) the gain of an an-tenna used for transmitting must be proportional to itseffective area when used for receiving. Consider an an-tenna with no loss, that is, one whose electrical efficiencyis 100%. It can be shown that its effective area averaged

8 4 CHARACTERISTICS

over all directions must be equal to λ2/4π, the wavelengthsquared divided by 4π. Gain is defined such that the av-erage gain over all directions for an antenna with 100%electrical efficiency is equal to 1. Therefore, the effec-tive area Aₑff in terms of the gain G in a given directionis given by:

Aeff =λ2

4πG

For an antenna with an efficiency of less than 100%,both the effective area and gain are reduced by that sameamount. Therefore, the above relationship between gainand effective area still holds. These are thus two differentways of expressing the same quantity. Aₑff is especiallyconvenient when computing the power that would be re-ceived by an antenna of a specified gain, as illustrated bythe above example.

4.5 Radiation pattern

Main article: Radiation patternThe radiation pattern of an antenna is a plot of the relativefield strength of the radio waves emitted by the antennaat different angles. It is typically represented by a three-dimensional graph, or polar plots of the horizontal andvertical cross sections. The pattern of an ideal isotropicantenna, which radiates equally in all directions, wouldlook like a sphere. Many nondirectional antennas, suchas monopoles and dipoles, emit equal power in all hori-zontal directions, with the power dropping off at higherand lower angles; this is called an omnidirectional patternand when plotted looks like a torus or donut.The radiation of many antennas shows a pattern of max-ima or "lobes" at various angles, separated by "nulls", an-gles where the radiation falls to zero. This is because theradio waves emitted by different parts of the antenna typ-ically interfere, causing maxima at angles where the radiowaves arrive at distant points in phase, and zero radiationat other angles where the radio waves arrive out of phase.In a directional antenna designed to project radio wavesin a particular direction, the lobe in that direction is de-signed larger than the others and is called the "main lobe".The other lobes usually represent unwanted radiation andare called "sidelobes". The axis through the main lobe iscalled the "principal axis" or "boresight axis".

4.6 Field regions

Main article: Near and far field

The space surrounding an antenna can be divided intothree concentric regions: the reactive near-field, the radi-ating near-field (Fresnell region) and the far-field (Fraun-hofer) regions. These regions are useful to identify the

field structure in each, although there are no preciseboundaries.In the far-field region, we are far enough from the an-tenna to neglect its size and shape. We can assume thatthe electromagnetic wave is purely a radiating plane wave(electric and magnetic fields are in phase and perpendic-ular to each other and to the direction of propagation).This simplifies the mathematical analysis of the radiatedfield.

4.7 Impedance

As an electro-magnetic wave travels through the differ-ent parts of the antenna system (radio, feed line, antenna,free space) it may encounter differences in impedance(E/H, V/I, etc.). At each interface, depending on theimpedancematch, some fraction of the wave’s energy willreflect back to the source,[18] forming a standing wavein the feed line. The ratio of maximum power to min-imum power in the wave can be measured and is calledthe standing wave ratio (SWR). A SWR of 1:1 is ideal. ASWR of 1.5:1 is considered to be marginally acceptablein low power applications where power loss is more crit-ical, although an SWR as high as 6:1 may still be usablewith the right equipment. Minimizing impedance differ-ences at each interface (impedance matching) will reduceSWR and maximize power transfer through each part ofthe antenna system.Complex impedance of an antenna is related to theelectrical length of the antenna at the wavelength in use.The impedance of an antenna can be matched to the feedline and radio by adjusting the impedance of the feedline, using the feed line as an impedance transformer.More commonly, the impedance is adjusted at the load(see below) with an antenna tuner, a balun, a matchingtransformer, matching networks composed of inductorsand capacitors, or matching sections such as the gammamatch.

4.8 Efficiency

Main article: Antenna efficiency

Efficiency of a transmitting antenna is the ratio of poweractually radiated (in all directions) to the power absorbedby the antenna terminals. The power supplied to the an-tenna terminals which is not radiated is converted intoheat. This is usually through loss resistance in the an-tenna’s conductors, but can also be due to dielectric ormagnetic core losses in antennas (or antenna systems) us-ing such components. Such loss effectively robs powerfrom the transmitter, requiring a stronger transmitter inorder to transmit a signal of a given strength.For instance, if a transmitter delivers 100 W into an an-tenna having an efficiency of 80%, then the antenna will

4.9 Polarization 9

radiate 80 W as radio waves and produce 20 W of heat.In order to radiate 100 W of power, one would need touse a transmitter capable of supplying 125 W to the an-tenna. Note that antenna efficiency is a separate issuefrom impedance matching, which may also reduce theamount of power radiated using a given transmitter. Ifan SWR meter reads 150 W of incident power and 50W of reflected power, that means that 100 W have actu-ally been absorbed by the antenna (ignoring transmissionline losses). How much of that power has actually beenradiated cannot be directly determined through electricalmeasurements at (or before) the antenna terminals, butwould require (for instance) careful measurement of fieldstrength. Fortunately the loss resistance of antenna con-ductors such as aluminum rods can be calculated and theefficiency of an antenna using such materials predicted.However loss resistance will generally affect the feed-point impedance, adding to its resistive (real) component.That resistance will consist of the sum of the radiation re-sistance Rᵣ and the loss resistance R ₒ . If an rms currentI is delivered to the terminals of an antenna, then a powerof I2Rᵣ will be radiated and a power of I2R ₒ will be lostas heat. Therefore, the efficiency of an antenna is equalto Rᵣ / (Rᵣ + R ₒ ). Of course only the total resistance Rᵣ+ R ₒ can be directly measured.According to reciprocity, the efficiency of an antennaused as a receiving antenna is identical to the efficiencyas defined above. The power that an antenna will deliverto a receiver (with a proper impedance match) is reducedby the same amount. In some receiving applications, thevery inefficient antennas may have little impact on per-formance. At low frequencies, for example, atmosphericor man-made noise can mask antenna inefficiency. Forexample, CCIR Rep. 258-3 indicates man-made noise ina residential setting at 40 MHz is about 28 dB above thethermal noise floor. Consequently, an antenna with a 20dB loss (due to inefficiency) would have little impact onsystem noise performance. The loss within the antennawill affect the intended signal and the noise/interferenceidentically, leading to no reduction in signal to noise ratio(SNR).This is fortunate, since antennas at lower frequencieswhich are not rather large (a good fraction of a wavelengthin size) are inevitably inefficient (due to the small radia-tion resistance Rᵣ of small antennas). Most AM broad-cast radios (except for car radios) take advantage of thisprinciple by including a small loop antenna for receptionwhich has an extremely poor efficiency. Using such an in-efficient antenna at this low frequency (530–1650 kHz)thus has little effect on the receiver’s net performance,but simply requires greater amplification by the receiver’selectronics. Contrast this tiny component to the massiveand very tall towers used at AM broadcast stations fortransmitting at the very same frequency, where every per-centage point of reduced antenna efficiency entails a sub-stantial cost.

The definition of antenna gain or power gain already in-cludes the effect of the antenna’s efficiency. Therefore,if one is trying to radiate a signal toward a receiver usinga transmitter of a given power, one need only comparethe gain of various antennas rather than considering theefficiency as well. This is likewise true for a receivingantenna at very high (especially microwave) frequencies,where the point is to receive a signal which is strong com-pared to the receiver’s noise temperature. However, in thecase of a directional antenna used for receiving signalswith the intention of rejecting interference from differentdirections, one is no longer concerned with the antennaefficiency, as discussed above. In this case, rather thanquoting the antenna gain, one would be more concernedwith the directive gainwhich does not include the effect ofantenna (in)efficiency. The directive gain of an antennacan be computed from the published gain divided by theantenna’s efficiency.

4.9 Polarization

See also: Polarization (waves) § Antennas

The polarization of an antenna refers to the orientationof the electric field (E-plane) of the radio wave with re-spect to the Earth’s surface and is determined by the phys-ical structure of the antenna and by its orientation; notethat this designation is totally distinct from the antenna’sdirectionality. Thus, a simple straight wire antenna willhave one polarization when mounted vertically, and adifferent polarization when mounted horizontally. As atransverse wave, the magnetic field of a radio wave is atright angles to that of the electric field, but by convention,talk of an antenna’s “polarization” is understood to referto the direction of the electric field.Reflections generally affect polarization. For radiowaves, one important reflector is the ionosphere whichcan change the wave’s polarization. Thus for signals re-ceived following reflection by the ionosphere (a skywave),a consistent polarization cannot be expected. For line-of-sight communications or ground wave propagation, hori-zontally or vertically polarized transmissions generally re-main in about the same polarization state at the receivinglocation. Matching the receiving antenna’s polarizationto that of the transmitter can make a very substantial dif-ference in received signal strength.Polarization is predictable from an antenna’s geometry,although in some cases it is not at all obvious (such asfor the quad antenna). An antenna’s linear polarizationis generally along the direction (as viewed from the re-ceiving location) of the antenna’s currents when such adirection can be defined. For instance, a vertical whip an-tenna or Wi-Fi antenna vertically oriented will transmitand receive in the vertical polarization. Antennas withhorizontal elements, such as most rooftop TV antennasin the United States, are horizontally polarized (broad-

10 4 CHARACTERISTICS

cast TV in the U.S. usually uses horizontal polarization).Even when the antenna system has a vertical orientation,such as an array of horizontal dipole antennas, the polar-ization is in the horizontal direction corresponding to thecurrent flow. The polarization of a commercial antennais an essential specification.Polarization is the sum of the E-plane orientations overtime projected onto an imaginary plane perpendicular tothe direction of motion of the radio wave. In the mostgeneral case, polarization is elliptical, meaning that thepolarization of the radio waves varies over time. Twospecial cases are linear polarization (the ellipse collapsesinto a line) as we have discussed above, and circular po-larization (in which the two axes of the ellipse are equal).In linear polarization the electric field of the radio waveoscillates back and forth along one direction; this can beaffected by the mounting of the antenna but usually thedesired direction is either horizontal or vertical polariza-tion. In circular polarization, the electric field (and mag-netic field) of the radio wave rotates at the radio frequencycircularly around the axis of propagation. Circular or el-liptically polarized radio waves are designated as right-handed or left-handed using the “thumb in the directionof the propagation” rule. Note that for circular polariza-tion, optical researchers use the opposite right hand rulefrom the one used by radio engineers.It is best for the receiving antenna to match the polariza-tion of the transmitted wave for optimum reception. In-termediate matchings will lose some signal strength, butnot as much as a complete mismatch. A circularly polar-ized antenna can be used to equally well match vertical orhorizontal linear polarizations. Transmission from a cir-cularly polarized antenna received by a linearly polarizedantenna (or vice versa) entails a 3 dB reduction in signal-to-noise ratio as the received power has thereby been cutin half.

4.10 Impedance matching

Main article: Impedance matching

Maximum power transfer requires matching theimpedance of an antenna system (as seen looking intothe transmission line) to the complex conjugate of theimpedance of the receiver or transmitter. In the case ofa transmitter, however, the desired matching impedancemight not correspond to the dynamic output impedanceof the transmitter as analyzed as a source impedance butrather the design value (typically 50 ohms) required forefficient and safe operation of the transmitting circuitry.The intended impedance is normally resistive but atransmitter (and some receivers) may have additionaladjustments to cancel a certain amount of reactance inorder to “tweak” the match. When a transmission lineis used in between the antenna and the transmitter (orreceiver) one generally would like an antenna system

whose impedance is resistive and near the characteristicimpedance of that transmission line in order to minimizethe standing wave ratio (SWR) and the increase intransmission line losses it entails, in addition to supplyinga good match at the transmitter or receiver itself.Antenna tuning generally refers to cancellation of any re-actance seen at the antenna terminals, leaving only a resis-tive impedance which might or might not be exactly thedesired impedance (that of the transmission line). Al-though an antenna may be designed to have a purely re-sistive feedpoint impedance (such as a dipole 97% of ahalf wavelength long) this might not be exactly true at thefrequency that it is eventually used at. In some cases thephysical length of the antenna can be “trimmed” to ob-tain a pure resistance. On the other hand, the additionof a series inductance or parallel capacitance can be usedto cancel a residual capacitative or inductive reactance,respectively.In some cases this is done in a more extreme manner, notsimply to cancel a small amount of residual reactance,but to resonate an antenna whose resonance frequency isquite different from the intended frequency of operation.For instance, a “whip antenna” can be made significantlyshorter than 1/4 wavelength long, for practical reasons,and then resonated using a so-called loading coil. Thisphysically large inductor at the base of the antenna hasan inductive reactance which is the opposite of the ca-pacitative reactance that such a vertical antenna has at thedesired operating frequency. The result is a pure resis-tance seen at feedpoint of the loading coil; unfortunatelythat resistance is somewhat lower than would be desiredto match commercial coax.So an additional problem beyond canceling the un-wanted reactance is of matching the remaining resistiveimpedance to the characteristic impedance of the trans-mission line. In principle this can always be done witha transformer, however the turns ratio of a transformeris not adjustable. A general matching network with atleast two adjustments can be made to correct both com-ponents of impedance. Matching networks using discreteinductors and capacitors will have losses associated withthose components, and will have power restrictions whenused for transmitting. Avoiding these difficulties, com-mercial antennas are generally designed with fixedmatch-ing elements or feeding strategies to get an approximatematch to standard coax, such as 50 or 75 Ohms. An-tennas based on the dipole (rather than vertical antennas)should include a balun in between the transmission lineand antenna element, which may be integrated into anysuch matching network.Another extreme case of impedance matching occurswhen using a small loop antenna (usually, but not always,for receiving) at a relatively low frequency where it ap-pears almost as a pure inductor. Resonating such an in-ductor with a capacitor at the frequency of operation notonly cancels the reactance but greatly magnifies the very

5.1 Monopole 11

small radiation resistance of such a loop. This is imple-mented in most AM broadcast receivers, with a small fer-rite loop antenna resonated by a capacitor which is variedalong with the receiver tuning in order to maintain reso-nance over the AM broadcast band

5 Antenna types

Antennas can be classified in various ways. The list belowgroups together antennas under common operating prin-ciples, following the way antennas are classified in manyengineering textbooks.[19][20][21]

Isotropic: An isotropic antenna (isotropic radiator) is ahypothetical antenna that radiates equal signal power inall directions. It is a mathematical model that is used asthe base of comparison to calculate the gain of real an-tennas. No real antenna can have an isotropic radiationpattern. However approximately isotropic antennas, con-structed with multiple elements, are used in antenna test-ing.The first four groups below are resonant antennas; whendriven at their resonant frequency their elements act asresonators. Waves of voltage and current bounce backand forth between the ends, creating standing waves inthe metal elements, increasing output power

5.1 Monopole

• Quarter-wave whip antenna on an FM radio for 88-108 MHz

• Rubber Ducky antenna on UHF 446 MHz walkietalkie with rubber cover removed.

• VHF ground plane antenna

• Mast radiator antenna of medium wave AM radiostation, Germany

• T antenna of amateur radio station, 80 ft high, usedat 1.5 MHz.

Monopole antennas consist of a single radiating elementsuch as a metal rod, often mounted over a conductingsurface, a ground plane.[20][22] One side of the feedlinefrom the receiver or transmitter is connected to the rod,and the other side to the ground plane, which may bethe Earth. The most common form is the quarter-wavemonopole which is one-quarter of a wavelength long andhas a gain of 5.12 dBi whenmounted over a ground plane.Monopoles have an omnidirectional radiation pattern, sothey are used for broad coverage of an area, and havevertical polarization. The ground waves used for broad-casting at low frequencies must be vertically polarized, solarge vertical monopole antennas are used for broadcast-ing in the MF, LF, and VLF bands. Small monopoles are

used as nondirectional antennas on portable radios in theHF, VHF, and UHF bands.

• Whip - Type of antenna used onmobile and portableradios in the VHF and UHF bands such as boomboxes, consists of a flexible rod, often made of tele-scoping segments.

• Rubber Ducky - Most common antenna usedon portable two way radios and cordlessphones due to its compactness, consists of anelectrically short wire helix. The helix addsinductance to cancel the capacitive reactanceof the short radiator, making it resonant. Verylow gain.

• Ground plane - a whip antenna with severalrods extending horizontally from base of whipattached to the ground side of the feedline.Since whips are mounted above ground, thehorizontal rods form an artificial ground planeunder the antenna to increase its gain. Usedas base station antennas for land mobile ra-dio systems such as police, ambulance and taxidispatchers.

• Mast radiator - A radio tower in which the towerstructure itself serves as the antenna. Common formof transmitting antenna for AM radio stations andother MF and LF transmitters. At its base the toweris mounted on a ceramic insulator to isolate it fromthe ground.

• T and inverted L - Consist of a long horizontal wiresuspended between two towers with insulators, witha vertical wire hanging down from it, attached to afeedline to the receiver or transmitter. Used on LFand VLF bands. The vertical wire serves as the ra-diator. Since at these frequencies the vertical wire iselectrically short, much shorter than a quarter wave-length, the horizontal wire(s) serve as a capacitive“hat” to increase the current in the vertical radia-tor, increasing the gain. Very narrow bandwidth,requires loading coil to tune out the capacitive reac-tance and make it resonant. Requires low resistanceground (electricity)

• Inverted F - Combines the advantages of theinverted-L antenna and the F-type antenna of, re-spectively, compactness and good matching. Theantenna is grounded at the base and fed at some in-termediate point. The position of the feed point de-termines the antenna impedance. Thus, matchingcan be achieved without the need for an extraneousmatching network.

• Umbrella - Very large wire transmitting antennasused on VLF bands. Consists of a central mast radi-ator tower attached at the top to multiple wires ex-tending out radially from the mast to ground, like atent or umbrella, insulated at the ends. Extremely

12 5 ANTENNA TYPES

narrow bandwidth, requires large loading coil andlow resistance counterpoise ground. Used for longrange military communications.

5.2 Dipole

• Rabbit ears half-wave dipole television antenna forVHF channels 54-217 MHz

• Yagi-Uda television antenna for analog channels 2-4, 47-68 MHz

• Log-periodic antenna covering 140-470 MHz

• Corner reflector UHF TV antenna with “bowtie”dipole driven element

• Two-element turnstile antenna for reception ofweather satellite data, 137 MHz. Has circular po-larization.

The most widely used class of antenna, a dipole antennaconsists of two symmetrical radiators such as metal rodsor wires, with one side of the balanced feedline from thetransmitter or receiver attached to each.[20][23] A horizon-tal dipole radiates in two lobes perpendicular to the an-tenna’s axis. A half-wave dipole the most common type,has two collinear elements each a quarter wavelength longand a gain of 2.15 dBi. Used individually as low gain an-tennas, dipoles are also used as driven elements in manymore complicated higher gain types of antennas.

• Yagi-Uda - One of the most common directional an-tennas at HF, VHF, and UHF frequencies. Con-sists of multiple half wave dipole elements in a line,with a single driven element and multiple parasiticelements which serve to focus the radio waves in abeam. It radiates a unidirectional beam with gainup to about 17 dBi depending on the number ofelements used, and has a narrow bandwidth of afew percent. Used for rooftop television anten-nas, point-to-point communication links, and longdistance shortwave communication using skywave(“skip”) reflection from the ionosphere.

• Log periodic - Unique in that it is a high gain dipoleantenna with broad bandwidth. It consists of manydipole driven elements in a line, with alternating po-larization and gradually increasing width. Used forrooftop television antennas.

• Turnstile - Two dipole antennas mounted at right an-gles, fed with a phase difference of 90°. This canradiate horizontally polarized radio waves broadsidein an omnidirectional pattern, and circularly polar-ized radio waves off the end. Used for receiving sig-nals from satellites, as circular polarization is usedfor satellite communication since it is not sensitiveto the orientation of the satellite’s antenna.

• Corner reflector - A directive antenna with moderategain of 15 - 20 dBi used at UHF frequencies. Con-sists of a dipole mounted in front of two reflectivemetal screens joined at an angle, usually 90°. Usedas a rooftop UHF television antenna and for point-to-point data links.

• Patch (microstrip) - A type of antenna with elementsconsisting of metal sheets mounted over a groundplane. Similar to dipole with gain of 6 - 9 dBi. In-tegrated into surfaces such as aircraft bodies. Theireasy fabrication using PCB techniques have madethem popular in modern wireless devices. Oftenused in arrays.

5.3 Array

• VHF collinear array of folded dipoles

• Sector antennas (white bars) on cell phone tower.Collinear arrays of dipoles, these radiate a flat, fan-shaped beam.

• 108 MHz reflective array antenna of AN-270 radarused during WW2.

• Reflective array UHF TV antenna, with bowtiedipoles to cover the UHF 470-890 MHz band

• US Air Force PAVE PAWS phased array radar an-tenna for ballistic missile detection, Alaska. Thetwo circular arrays are composed of thousands ofcrossed dipole antennas.

• Batwing VHF television broadcasting antenna

• Crossed-dipole FM radio broadcast antenna

• Curtain array shortwave transmitting antenna, Aus-tria. Wire dipoles suspended between towers

• Turnstile antenna array used for satellite communi-cation

• Flat microstrip array antenna for satellite TV recep-tion.

Array antennas consist of multiple antennas working as asingle antenna. Typically they consist of arrays of iden-tical driven elements, usually dipoles fed in phase, givingincreased gain over that of a single dipole.[20][24][25]

• Collinear - Consist of a number of dipoles in a ver-tical line. It is a high gain omnidirectional antenna,meaning more of the power is radiated in horizon-tal directions and less into the sky or ground andwasted. Gain of 8 to 10 dBi. Used as base sta-tion antennas for land mobile radio systems such aspolice, fire, ambulance, and taxi dispatchers, andsector antennas for cellular base stations.

5.5 Aperture 13

• Reflective array - multiple dipoles in a two-dimensional array mounted in front of a flat reflect-ing screen. Used for radar and UHF television trans-mitting and receiving antennas.

• Phased array - A high gain antenna used at UHFand microwave frequencies which is electronicallysteerable. It consists of multiple dipoles in a two-dimensional array, each fed through an electronicphase shifter, with the phase shifters controlled by acomputer control system. The beam can be instantlypointed in any direction over a wide angle in front ofthe antenna. Used for military radar and jammingsystems.

• Curtain array - Large directional wire transmittingantenna used at HF by shortwave broadcasting sta-tions. It consists of a vertical rectangular array ofwire dipoles suspended in front of a flat reflectorscreen consisting of a vertical “curtain” of parallelwires, all supported between twometal towers. It ra-diates a horizontal beam of radio waves into the skyabove the horizon, which is reflected by the iono-sphere to Earth beyond the horizon

• Batwing or superturnstile - A specialized antennaused in television broadcasting consisting of perpen-dicular pairs of dipoles with radiators resemblingbat wings. Multiple batwing antennas are stackedvertically on a mast to make VHF television broad-cast antennas. Omnidirectional radiation patternwith high gain in horizontal directions. The batwingshape gives them wide bandwidth.

• microstrip - an array of patch antennas on a substratefed by microstrip feedlines. Microwave antenna thatcan achieve large gains in compact space. Ease offabrication by PCB techniques havemade them pop-ular in modern wireless devices. Beamwidth and po-larization can be actively reconfigurable.

5.4 Loop

Ferrite rod receiving antenna from AM radio, 550 -1600 KHz. The antenna also serves as the inductor inthe tuned circuit for the receiver.

Loop direction finding antenna covers 1.75 - 30 MHz, 6ft diameter

Loop UHF television antenna

Loop antennas consist of a loop or coil of wire.[20][26][27]Loops with circumference of a wavelength or larger actsimilarly to dipole antennas. However loops small incomparison to a wavelength act differently. They interactwith the magnetic field of the radio wave instead of theelectric field as other antennas do, and so are relativelyinsensitive to nearby electrical noise. However they havelow radiation resistance, and so are inefficient for trans-mitting. They are used as receiving antennas at low fre-quencies, and also as direction finding antennas.

• Ferrite (loopstick) - Used as the receiving antennain virtually all AM radios at MF, LF, and VLF fre-quencies. A coil of wire wrapped around a ferritemagnetic core. The magnetic core concentrates themagnetic field, enabling this small compact antennato receive radio waves many times its size. Has twomain lobes off the ends of the ferrite stick.

5.5 Aperture

• NASA Cassegrain parabolic spacecraft communi-cation antenna, Australia. Uses X band, 8 – 12 GHz.Extremely high gain ~70 dBi.

• Microwave horn antenna bandwidth 0.8–18 GHz

• X band marine radar slot antenna on ship, 8 – 12GHz.

• Dielectric lens antenna used in millimeter wave ra-dio telescope

Aperture antennas are the main type of directional an-tennas used at microwave frequencies and above.[20][28]They consist of a small dipole or loop feed antenna insidea three-dimensional guiding structure large compared toa wavelength, with an aperture to emit the radio waves.Since the antenna structure itself is nonresonant they canbe used over a wide frequency range by replacing or tun-ing the feed antenna.

• Parabolic - The most widely used high gain antennaat microwave frequencies and above. Consists of adish-shapedmetal parabolic reflector with a feed an-tenna at the focus. It can have some of the high-est gains of any antenna type, up to 60 dBi, but thedish must be large compared to a wavelength. Used

14 6 EFFECT OF GROUND

for radar antennas, point-to-point data links, satel-lite communication, and radio telescopes

• Horn - Simple antenna with moderate gains of 15to 25 dBi consists of a flaring metal horn attachedto a waveguide. Used for applications such as radarguns, radiometers and as feed antennas for parabolicdishes.

• Slot - Consist of a waveguide with one or more slotscut in it to emit the microwaves. Linear slot anten-nas emit narrow fan-shaped beams. Used as UHFbroadcast antennas and marine radar antennas.

• Dielectric resonator - consists of small ball or puck-shaped piece of dielectric material excited by aper-ture in waveguide Used at millimeter wave frequen-cies

5.6 Traveling wave

• A typical random wire antenna for shortwave recep-tion, strung between two buildings.

• Quadrant antenna, similar to rhombic, at an Aus-trian shortwave broadcast station. Radiates horizon-tal beam at 5-9 MHz, 100 kW

• Array of four axial-mode helical antennas used forsatellite tracking, France

Unlike the above antennas, traveling wave anten-nas are nonresonant so they have inherently broadbandwidth.[20][29] They are typically wire antennas multi-ple wavelengths long, through which the voltage and cur-rent waves travel in one direction, instead of bouncingback and forth to form standing waves as in resonant an-tennas. They have linear polarization (except for the heli-cal antenna). Unidirectional traveling wave antennas areterminated by a resistor at one end equal to the antenna’scharacteristic resistance, to absorb the waves from one di-rection. This makes them inefficient as transmitting an-tennas.

• Random wire - This describes the typical antennaused to receive shortwave radio, consisting of a ran-dom length of wire either strung outdoors betweensupports or indoors in a zigzag pattern along walls,connected to the receiver at one end. Can have com-plex radiation patterns with several lobes at angles tothe wire.

• Beverage - Simplest unidirectional traveling waveantenna. Consists of a straight wire one to severalwavelengths long, suspended near the ground, con-nected to the receiver at one end and terminated bya resistor equal to its characteristic impedance, 400to 800Ω at the other end. It’s radiation pattern hasa main lobe at a shallow angle in the sky off the ter-minated end. It is used for reception of skywaves

reflected off the ionosphere in long distance “skip”shortwave communication.

• Rhombic - Consists of four equal wire sectionsshaped like a rhombus. It is fed by a balanced feed-line at one of the acute corners, and the two sidesare connected to a resistor equal to the characteristicresistance of the antenna at the other. It has a mainlobe in a horizontal direction off the terminated endof the rhombus. Used for skywave communicationon shortwave bands.

• Helical (axial mode) - Consists of a wire in the shapeof a helix mounted above a reflecting screen. It radi-ates circularly polarized waves in a beam off the end,with a typical gain of 15 dBi. It is used at VHF andUHF frequencies for communication with satellitesand animal tracking transmitters, which use circularpolarization because it is insensitive to the relativeorientation of the antennas.

• Leaky wave - Microwave antennas consisting of awaveguide or coaxial cable with a slot or aperturescut in it so it radiates continuously along its length.

6 Effect of ground

Main article: Multipath propagation

Ground reflections is one of the common types ofmultipath.[30][31][32]

The radiation pattern and even the driving pointimpedance of an antenna can be influenced by the di-electric constant and especially conductivity of nearbyobjects. For a terrestrial antenna, the ground is usuallyone such object of importance. The antenna’s heightabove the ground, as well as the electrical properties(permittivity and conductivity) of the ground, can then beimportant. Also, in the particular case of a monopole an-tenna, the ground (or an artificial ground plane) serves asthe return connection for the antenna current thus havingan additional effect, particularly on the impedance seenby the feed line.When an electromagnetic wave strikes a plane surfacesuch as the ground, part of the wave is transmitted into theground and part of it is reflected, according to the Fresnelcoefficients. If the ground is a very good conductor thenalmost all of the wave is reflected (180° out of phase),whereas a ground modeled as a (lossy) dielectric can ab-sorb a large amount of the wave’s power. The power re-maining in the reflected wave, and the phase shift uponreflection, strongly depend on the wave’s angle of inci-dence and polarization. The dielectric constant and con-ductivity (or simply the complex dielectric constant) isdependent on the soil type and is a function of frequency.For very low frequencies to high frequencies (<30 MHz),

15

the ground behaves as a lossy dielectric,[33] Thus theground is characterized both by a conductivity [34] andpermittivity (dielectric constant) which can be measuredfor a given soil (but is influenced by fluctuating moisturelevels) or can be estimated from certain maps. At lowerfrequencies the ground acts mainly as a good conductor,which AM middle wave broadcast (.5 - 1.6 MHz) anten-nas depend on.At frequencies between 3 and 30 MHz, a large portionof the energy from a horizontally polarized antenna re-flects off the ground, with almost total reflection at thegrazing angles important for ground wave propagation.That reflected wave, with its phase reversed, can eithercancel or reinforce the direct wave, depending on the an-tenna height in wavelengths and elevation angle (for a skywave).On the other hand, vertically polarized radiation is notwell reflected by the ground except at grazing incidenceor over very highly conducting surfaces such as seawater.[35] However the grazing angle reflection importantfor ground wave propagation, using vertical polarization,is in phase with the direct wave, providing a boost of upto 6 db, as is detailed below.At VHF and above (>30 MHz) the ground becomes apoorer reflector. However it remains a good reflectorespecially for horizontal polarization and grazing anglesof incidence. That is important as these higher frequen-cies usually depend on horizontal line-of-sight propaga-tion (except for satellite communications), the groundthen behaving almost as a mirror.The net quality of a ground reflection depends on the to-pography of the surface. When the irregularities of thesurface are much smaller than the wavelength, we are inthe regime of specular reflection, and the receiver seesboth the real antenna and an image of the antenna underthe ground due to reflection. But if the ground has irregu-larities not small compared to the wavelength, reflectionswill not be coherent but shifted by random phases. Withshorter wavelengths (higher frequencies), this is generallythe case.Whenever both the receiving or transmitting antenna areplaced at significant heights above the ground (relative tothe wavelength), waves specularly reflected by the groundwill travel a longer distance than direct waves, inducing aphase shift which can sometimes be significant. When asky wave is launched by such an antenna, that phase shiftis always significant unless the antenna is very close to theground (compared to the wavelength).The phase of reflection of electromagnetic waves dependson the polarization of the incident wave. Given the largerrefractive index of the ground (typically n=2) comparedto air (n=1), the phase of horizontally polarized radiationis reversed upon reflection (a phase shift of π radians or180°). On the other hand, the vertical component of thewave’s electric field is reflected at grazing angles of inci-dence approximately in phase. These phase shifts apply

as well to a ground modelled as a good electrical conduc-tor.This means that a receiving antenna “sees” an image ofthe antenna but with reversed currents. That current is inthe same absolute direction as the actual antenna if theantenna is vertically oriented (and thus vertically polar-ized) but opposite the actual antenna if the antenna cur-rent is horizontal.The actual antenna which is transmitting the original wavethen also may receive a strong signal from its own im-age from the ground. This will induce an additional cur-rent in the antenna element, changing the current at thefeedpoint for a given feedpoint voltage. Thus the an-tenna’s impedance, given by the ratio of feedpoint volt-age to current, is altered due to the antenna’s proximityto the ground. This can be quite a significant effect whenthe antenna is within a wavelength or two of the ground.But as the antenna height is increased, the reduced powerof the reflected wave (due to the inverse square law) al-lows the antenna to approach its asymptotic feedpointimpedance given by theory. At lower heights, the effecton the antenna’s impedance is very sensitive to the ex-act distance from the ground, as this affects the phase ofthe reflected wave relative to the currents in the antenna.Changing the antenna’s height by a quarter wavelength,then changes the phase of the reflection by 180°, with acompletely different effect on the antenna’s impedance.The ground reflection has an important effect on the netfar field radiation pattern in the vertical plane, that is,as a function of elevation angle, which is thus differentbetween a vertically and horizontally polarized antenna.Consider an antenna at a height h above the ground, trans-mitting a wave considered at the elevation angle θ. Fora vertically polarized transmission the magnitude of theelectric field of the electromagnetic wave produced by thedirect ray plus the reflected ray is:

|EV | = 2 |E0|∣∣cos ( 2πh

λ sin θ)∣∣

Thus the power received can be as high as 4 times that dueto the direct wave alone (such as when θ=0), following thesquare of the cosine. The sign inversion for the reflectionof horizontally polarized emission instead results in:

|EH | = 2 |E0|∣∣sin ( 2πh

λ sin θ)∣∣

where:

• E0 is the electrical field that would be received bythe direct wave if there were no ground.

• θ is the elevation angle of the wave being considered.

• λ is the wavelength.

16 7 MUTUAL IMPEDANCE AND INTERACTION BETWEEN ANTENNAS

• h is the height of the antenna (half the distance be-tween the antenna and its image).

For horizontal propagation between transmitting and re-ceiving antennas situated near the ground reasonably farfrom each other, the distances traveled by tne direct andreflected rays are nearly the same. There is almost no rel-ative phase shift. If the emission is polarized vertically,the two fields (direct and reflected) add and there is max-imum of received signal. If the signal is polarized hori-zontally, the two signals subtract and the received signalis largely cancelled. The vertical plane radiation patternsare shown in the image at right. With vertical polarizationthere is always a maximum for θ=0, horizontal propaga-tion (left pattern). For horizontal polarization, there iscancellation at that angle. Note that the above formulaeand these plots assume the ground as a perfect conductor.These plots of the radiation pattern correspond to a dis-tance between the antenna and its image of 2.5λ. As theantenna height is increased, the number of lobes increasesas well.The difference in the above factors for the case of θ=0 isthe reason that most broadcasting (transmissions intendedfor the public) uses vertical polarization. For receiversnear the ground, horizontally polarized transmissions suf-fer cancellation. For best reception the receiving antennasfor these signals are likewise vertically polarized. In someapplications where the receiving antenna must work inany position, as in mobile phones, the base station anten-nas use mixed polarization, such as linear polarization atan angle (with both vertical and horizontal components)or circular polarization.On the other hand, classical (analog) television transmis-sions are usually horizontally polarized, because in urbanareas buildings can reflect the electromagnetic waves andcreate ghost images due to multipath propagation. Us-ing horizontal polarization, ghosting is reduced becausethe amount of reflection of electromagnetic waves in thep polarization (horizontal polarization off the side of abuilding) is generally less than s (vertical, in this case) po-larization. Vertically polarized analog television has nev-ertheless been used in some rural areas. In digital terres-trial television such reflections are less problematic, dueto robustness of binary transmissions and error correc-tion.

7 Mutual impedance and interac-tion between antennas

Current circulating in one antenna generally induces avoltage across the feedpoint of nearby antennas or an-tenna elements. The mathematics presented below areuseful in analyzing the electrical behaviour of antennaarrays, where the properties of the individual array el-ements (such as half wave dipoles) are already known.

If those elements were widely separated and driven in acertain amplitude and phase, then each would act inde-pendently as that element is known to. However, becauseof the mutual interaction between their electric and mag-netic fields due to proximity, the currents in each elementare not simply a function of the applied voltage (accord-ing to its driving point impedance), but depend on thecurrents in the other nearby elements. Note that this nowis a near field phenomenon which could not be properlyaccounted for using the Friis transmission equation forinstance.The elements’ feedpoint currents and voltages can berelated to each other using the concept of mutualimpedance Zji between every pair of antennas just as themutual impedance jωM describes the voltage induced inone inductor by a current through a nearby coil coupled toit through a mutual inductanceM. The mutual impedanceZ21 between two antennas is defined[36] as:

Zji =vjii

where ii is the current flowing in antenna i and vj is thevoltage induced at the open-circuited feedpoint of an-tenna j due to i1 when all other currents ik are zero. Themutual impendances can be viewed as the elements of asymmetric square impedance matrix Z. Note that the di-agonal elements, Zii =

viii, are simply the driving point

impedances of each element.Using this definition, the voltages present at the feed-points of a set of coupled antennas can be expressed as themultiplication of the impedance matrix times the vectorof currents. Written out as discrete equations, that means:

v1 = i1Z11 + i2Z12 + · · · + inZ1n

v2 = i1Z21 + i2Z22 + · · · + inZ2n

......

......

vn = i1Zn1 + i2Zn2 + · · · + inZnn

where:

• vi is the voltage at the terminals of antenna i

• ii is the current flowing between the terminals of an-tenna i

• Zii is the driving point impedance of antenna i

• Zij is the mutual impedance between antennas i andj .

As is the case for mutual inductances,

Zij =Zji.

17

This is a consequence of Lorentz reciprocity. For anantenna element i not connected to anything (open cir-cuited) one can write ii = 0 . But for an element i whichis short circuited, a current is generated across that shortbut no voltage is allowed, so the corresponding vi = 0 .This is the case, for instance, with the so-called parasiticelements of a Yagi-Uda antenna where the solid rod canbe viewed as a dipole antenna shorted across its feedpoint.Parasitic elements are unpowered elements that absorband reradiate RF energy according to the induced currentcalculated using such a system of equations.With a particular geometry, it is possible for the mutualimpedance between nearby antennas to be zero. This isthe case, for instance, between the crossed dipoles usedin the turnstile antenna.

8 Gallery

8.1 Antennas and supporting structures

• A building rooftop supporting numerous dishand sectored mobile telecommunications antennas(Doncaster, Victoria, Australia).

• A water tower in Palmerston, Northern Territorywith radio broadcasting and communications anten-nas.

• A three-sector telephone site in Mexico City.

• Telephone site concealed as a palm tree.

8.2 Diagrams as part of a system

• Antennas may be connected through a multiplexingarrangement in some applications like this trunkedtwo-way radio example.

• Antenna network for an emergency medical servicesbase station.

9 See also• Amateur radio

• Antenna measurement

• AWX antenna

• Category:Radio frequency antenna types

• Category:Radio frequency propagation

• Cellular repeater

• DXing

• Electromagnetism

• Fractal antenna

• Mast radiator

• Mobile broadband modem

• Numerical Electromagnetics Code

• Radio masts and towers

• Radio telescope

• RF connector

• Satellite television

• Smart antenna

• Television antenna

• TETRA

• Whip antenna

10 Notes[1] Graf, Rudolf F. (1999). ModernDictionary of Electronics.

Newnes. p. 29. ISBN 0750698667.

[2] In the context of electrical engineering and physics, theplural of antenna is antennas, and it has been this waysince about 1950 (or earlier), when a cornerstone text-book in this field, Antennas, was published by the physi-cist and electrical engineer John D. Kraus of The OhioState University. Besides in the title, Dr. Kraus noted thisin a footnote on the first page of his book. Insects mayhave "antennae", but this form is not used in the contextof electronics or physics.

[3] For example http://www.telegraph.co.uk/science/science-news/7810454/British-scientists-launch-major-radio-telescope.html;http://www.ic.gc.ca/eic/site/smt-gst.nsf/eng/sf09377.html; http://www.ska.ac.za/media/meerkat_cad.php

[4] “IEEE Transactions on Antennas and Propagation”.

[5] Marconi, "Wireless Telegraphic Communication: NobelLecture, 11 December 1909." Nobel Lectures. Physics1901–1921. Amsterdam: Elsevier Publishing Company,1967: 196–222. p. 206.

[6] “The Nobel Prize in Physics 1909”.

[7] Slyusar, Vadym (20–23 September 2011). “To historyof radio engineering’s term “antenna"" (PDF). VIII Inter-national Conference on Antenna Theory and Techniques(ICATT’11). Kyiv, Ukraine. pp. 83–85.

[8] Slyusar, Vadym (21–24 February 2012). “An Italian pe-riod on the history of radio engineering’s term “antenna""(PDF). 11th International Conference Modern Problemsof Radio Engineering, Telecommunications and ComputerScience (TCSET’2012). Lviv-Slavske, Ukraine. p. 174.

18 11 REFERENCES

[9] Slyusar, Vadym (June 2011). "Антенна: историярадиотехнического термина" [The Antenna: A Historyof Radio Engineering’s Term] (PDF). ПЕРВАЯ МИЛЯLast mile: Electronics: Science, Technology, Business (inRussian) ? (6): 52–64.

[10] Schantz, Hans Gregory (2003), “Introduction to ultra-wideband antennas” (PDF), Proceedings of the 2003 IEEEUWBST Conference.

[11] “Media Advisory: Apply Now to Attend the ALMA Ob-servatory Inauguration”. ESO Announcement. Retrieved4 December 2012.

[12] Carl Smith (1969). Standard Broadcast Antenna Systems,p. 2-1212. Cleveland, Ohio: Smith Electronics, Inc.

[13] Lonngren, Karl Erik; Savov, Sava V.; Jost, Randy J.(2007). Fundamentals of Electomagnetics With Matlab,2nd Ed. SciTech Publishing. p. 451. ISBN 1891121588.

[14] Stutzman, Warren L.; Thiele, Gary A. (2012). AntennaTheory and Design, 3rd Ed. JohnWiley& Sons. pp. 560–564. ISBN 0470576642.

[15] Hall 1998, p. 24.

[16] Hall 1998, p. 25.

[17] Hall 1998, pp. 31-32.

[18] Impedance is caused by the same physics as refractiveindex in optics, although impedance effects are typi-cally one-dimensional, where effects of refractive indexis three-dimensional.

[19] Bevelaqua, Peter J. “Types of Antennas”. Antenna The-ory. Antenna-theory.com Peter Bevelaqua’s private web-site. Retrieved June 28, 2015.

[20] Aksoy, Serkan (2008). “Antennas” (PDF). Lecture Notes-v.1.3.4. Electrical Engineering Dept., Gebze TechnicalUniversity, Gebze, Turkey. Retrieved June 29, 2015.

[21] Balanis, Constantine A. (2005). Antenna Theory: Analy-sis and Design, 3rd Ed., Vol. 1. John Wiley and Sons. p.4. ISBN 047166782X.

[22] Bevelaqua, Monopole Antenna, Antenna-Theory.com

[23] Bevelaqua, Dipole Antenna, Antenna-Theory.com

[24] Bevelaqua, Antenna Arrays, Antenna-Theory.com

[25] Balanis, Antenna Theory, Vol. 1, p. 283-371

[26] Bevelaqua, Loop Antennas, Antenna-Theory.com

[27] Balanis, Antenna Theory, Vol. 1, p. 231-275

[28] Balanis, Antenna Theory, Vol. 1, p. 653-728

[29] Balanis, Antenna Theory, Vol. 1, p. 549-602

[30] Fixed Broadband Wireless System Design, p. 130, atGoogle Books

[31] Monopole Antennas, p. 340, at Google Books

[32] Wireless and Mobile Communication, p. 37, at GoogleBooks

[33] H. Ward Silver, ed. (2011). ARRL Antenna Book, p. 3-2.Newington, Connecticut: American Radio Relay League.ISBN 978-0-87259-694-8

[34] http://www.fcc.gov/encyclopedia/m3-map-effective-ground-conductivity-united-states-wall-sized-map-am-broadcast-stations

[35] H. Ward Silver, ed. (2011). ARRL Antenna Book, p.3-23. Newington, Connecticut: American Radio RelayLeague. ISBN 978-0-87259-694-8

[36] Kai Fong Lee (1984). Principles of Antenna Theory. JohnWiley and Sons Ltd. ISBN 0-471-90167-9.

11 References

11.1 General references

• Antenna Theory (3rd edition), by C. Balanis, Wiley,2005, ISBN 0-471-66782-X;

• Antenna Theory and Design (2nd edition), by W.Stutzman and G. Thiele, Wiley, 1997, ISBN 0-471-02590-9;

• Antennas (4th edition), by J. Kraus and R. Marhe-fka, McGraw-Hill, 2001, ISBN 0-07-232103-2;

• Antennenbuch, by Karl Rothammel, publ.Franck’sche Verlagshandlung Stuttgart, 1991,ISBN 3-440-05853-0; other editions (in German)

• Antennas for portable Devices, Zhi Ning Chen(edited), John Wiley & Sons in March 2007

• Broadband Planar Antennas: Design and Applica-tions, Zhi Ning Chen and M. Y. W. Chia, John Wi-ley & Sons in February 2006

• Template:Cite book first=Gerald

11.2 “Practical antenna” references

• Antenna Theory antenna-theory.com

• Antennas Antenna types

• Patch Antenna: From Simulation to Realization EMTalk

• Why Antennas Radiate, Stuart G. Downs, WY6EE(PDF)

• Understanding electromagnetic fields and antennaradiation takes (almost) no math, Ron Schmitt, EDNMagazine, March 2 2000 (PDF)

• Antennas: Generalities, Principle of operation, Aselectronic component, Hertz Marconi and Other typesAntennas etc etc

11.4 Patents and USPTO 19

11.3 Theory and simulations

• AN-SOF, "Antenna Simulation Software". Pro-gram system for the modeling of antennas and scat-terers.

• http://www.dipoleanimator.com

• EM Talk, "Microstrip Patch Antenna", (Theory andsimulation of microstrip patch antenna)

• "" Formulas for simulating and optimizing Antennaspecs and placement

• "Microwave Antenna Design Calculator" Providesquick estimation of antenna size required for a givengain and frequency. 3 dB and 10 dB beamwidths arealso derived; the calculator additionally gives the far-field range required for a given antenna.

• Sophocles J. Orfanidis, "Electromagnetic Wavesand Antennas", Rutgers University (20 PDF Chaps.Basic theory, definitions and reference)

• Hans Lohninger, “Learning by Simulations:Physics: Coupled Radiators". vias.org, 2005. (ed.Interactive simulation of two coupled antennas)

• NEC Lab - NEC Lab is a tool that uses NumericalElectromagnetics Code and Artificial Intelligence todesign and simulate antennas.

• Justin Smith "Aerials". A.T.V (Aerials and Televi-sion), 2009. (ed. Article on the (basic) theory anduse of FM, DAB & TV aerials)

• Antennas Research Group, "Virtual (Reality) An-tennas". Democritus University of Thrace, 2005.

• “Support > Knowledgebase > RF Basics > Anten-nas / Cables > dBi vs. dBd detail". MaxStream,Inc., 2005. (ed. How to measure antennagain) (New location: http://www.digi.com/support/kbase/kbaseresultdetl?id=2146 Note: to skip theregistration form click the link below it)

• Yagis and Log Periodics, Astrosurf article.

• Raines, J. K., “Virtual Outer Conductor for LinearAntennas,”Microwave Journal, Vol. 52, No. 1, Jan-uary, 2009, pp. 76–86

• Tests of FM/VHF receiving antennas.

Effect of ground references

• Electronic Radio and Engineering. F.E. Terman.McGraw-Hill

• Lectures on physics. Feynman, Leighton and Sands.Addison-Wesley

• Classical Electricity and Magnetism. W. Panofskyand M. Phillips. Addison-Wesley

11.4 Patents and USPTO

• CLASS 343, Communication: Radio Wave An-tenna

12 Further reading• Antennas for Base Stations inWireless Communica-tions, edited by Zhi Ning Chen and Kwai-Man Luk,McGraw-Hill Companies, Inc, USA in May 2009

The dictionary definition of antenna at Wiktionary

20 12 FURTHER READING

Electronic symbol for an antenna

Antennas of the Atacama Large Millimeter submillimeter Ar-ray.[11]

Whip antenna on car, common example of an omnidirectionalantenna

Half-wave dipole antenna

21

Diagram of the electric fields (blue) and magnetic fields (red)radiated by a dipole antenna (black rods) during transmission.

Cell phone base station antennas

Standing waves on a half wave dipole driven at its resonant fre-quency. The waves are shown graphically by bars of color (redfor voltage, V and blue for current, I) whose width is propor-tional to the amplitude of the quantity at that point on the an-tenna.

Rooftop television Yagi-Uda antennas like these are widely usedat VHF and UHF frequencies.

22 12 FURTHER READING

0°

270°

180°

90° antenna faces to 0°

main lobe side lobes

back lobe

Polar plots of the horizontal cross sections of a (virtual) Yagi-Uda-antenna. Outline connects points with 3db field power com-pared to an ISO emitter.

The wave reflected by earth can be considered as emitted by theimage antenna.

The currents in an antenna appear as an image in opposite phasewhen reflected at grazing angles. This causes a phase reversal forwaves emitted by a horizontally polarized antenna (left) but nota vertically polarized antenna (center).

Radiation patterns of antennas and their images reflected by theground. At left the polarization is vertical and there is always amaximum for θ=0 . If the polarization is horizontal as at right,there is always a zero for θ=0 .

Mutual impedance between parallel λ2dipoles not staggered.

Curves Re and Im are the resistive and reactive parts of theimpedance.

23

13 Text and image sources, contributors, and licenses

13.1 Text• Antenna (radio) Source: https://en.wikipedia.org/wiki/Antenna_(radio)?oldid=698911547 Contributors: Bryan Derksen, Timo

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• File:Dipole_receiving_antenna_animation_6_800x394x150ms.gif Source: https://upload.wikimedia.org/wikipedia/commons/d/dd/Dipole_receiving_antenna_animation_6_800x394x150ms.gif License: CC0 Contributors: Own work Original artist: Chetvorno

24 13 TEXT AND IMAGE SOURCES, CONTRIBUTORS, AND LICENSES

• File:Dipole_xmting_antenna_animation_4_408x318x150ms.gif Source: https://upload.wikimedia.org/wikipedia/commons/a/a6/Dipole_xmting_antenna_animation_4_408x318x150ms.gif License: CC0 Contributors: Own work Original artist: Chetvorno

• File:Felder_um_Dipol.jpg Source: https://upload.wikimedia.org/wikipedia/commons/d/da/Felder_um_Dipol.jpg License: CC-BY-SA-3.0 Contributors: http://de.wikipedia.org/wiki/Datei:Felder_um_Dipol.jpg Original artist: Averse

• File:Ferritantenne_2.jpg Source: https://upload.wikimedia.org/wikipedia/commons/c/ca/Ferritantenne_2.jpg License: CC-BY-SA-3.0Contributors: Transferred from da.wikipedia Original artist: Glenn at da.wikipedia

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• File:Loop_Antenna.jpg Source: https://upload.wikimedia.org/wikipedia/commons/8/8b/Loop_Antenna.jpg License: CC-BY-SA-3.0Contributors: Own work ( ) Original artist: Kuha455405

• File:Loop_antenna.jpg Source: https://upload.wikimedia.org/wikipedia/commons/0/01/Loop_antenna.jpg License: CCBY-SA 3.0 Con-tributors: Own work Original artist: Trixt

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• File:The_Atacama_Large_Millimeter_submillimeter_Array_(ALMA)_by_night_under_the_Magellanic_Clouds.jpg Source:https://upload.wikimedia.org/wikipedia/commons/b/bd/The_Atacama_Large_Millimeter_submillimeter_Array_%28ALMA%29_by_night_under_the_Magellanic_Clouds.jpg License: CC BY 4.0 Contributors: http://www.eso.org/public/images/ann12092a/ Original artist:ESO/C. Malin

• File:Zij-en.png Source: https://upload.wikimedia.org/wikipedia/commons/0/0d/Zij-en.png License: CC-BY-SA-3.0 Contributors: ?Original artist: ?

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