All About Antenna

7/29/2019 All About Antenna

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Antenna (radio)From Wikipedia, the free encyclopedia

Whip antennaon car

Diagram of theelectric fields(blue) andmagnetic fields(red) radiated by adipole antenna(black rods)during transmission.

Largeparabolic antennafor communicating with spacecraft
http://en.wikipedia.org/wiki/Whip_antennahttp://en.wikipedia.org/wiki/Whip_antennahttp://en.wikipedia.org/wiki/Electric_fieldhttp://en.wikipedia.org/wiki/Electric_fieldhttp://en.wikipedia.org/wiki/Electric_fieldhttp://en.wikipedia.org/wiki/Magnetic_fieldhttp://en.wikipedia.org/wiki/Magnetic_fieldhttp://en.wikipedia.org/wiki/Magnetic_fieldhttp://en.wikipedia.org/wiki/Dipole_antennahttp://en.wikipedia.org/wiki/Dipole_antennahttp://en.wikipedia.org/wiki/Dipole_antennahttp://en.wikipedia.org/wiki/Parabolic_antennahttp://en.wikipedia.org/wiki/Parabolic_antennahttp://en.wikipedia.org/wiki/Parabolic_antennahttp://en.wikipedia.org/wiki/File:Canberra_Deep_Dish_Communications_Complex_-_GPN-2000-000502.jpghttp://en.wikipedia.org/wiki/File:Canberra_Deep_Dish_Communications_Complex_-_GPN-2000-000502.jpghttp://en.wikipedia.org/wiki/File:Felder_um_Dipol.jpghttp://en.wikipedia.org/wiki/File:Felder_um_Dipol.jpghttp://en.wikipedia.org/wiki/File:Car_radio_antenna_extended_portrait.jpeghttp://en.wikipedia.org/wiki/File:Car_radio_antenna_extended_portrait.jpeghttp://en.wikipedia.org/wiki/File:Canberra_Deep_Dish_Communications_Complex_-_GPN-2000-000502.jpghttp://en.wikipedia.org/wiki/File:Canberra_Deep_Dish_Communications_Complex_-_GPN-2000-000502.jpghttp://en.wikipedia.org/wiki/File:Felder_um_Dipol.jpghttp://en.wikipedia.org/wiki/File:Felder_um_Dipol.jpghttp://en.wikipedia.org/wiki/File:Car_radio_antenna_extended_portrait.jpeghttp://en.wikipedia.org/wiki/File:Car_radio_antenna_extended_portrait.jpeghttp://en.wikipedia.org/wiki/File:Canberra_Deep_Dish_Communications_Complex_-_GPN-2000-000502.jpghttp://en.wikipedia.org/wiki/File:Canberra_Deep_Dish_Communications_Complex_-_GPN-2000-000502.jpghttp://en.wikipedia.org/wiki/File:Felder_um_Dipol.jpghttp://en.wikipedia.org/wiki/File:Felder_um_Dipol.jpghttp://en.wikipedia.org/wiki/File:Car_radio_antenna_extended_portrait.jpeghttp://en.wikipedia.org/wiki/File:Car_radio_antenna_extended_portrait.jpeghttp://en.wikipedia.org/wiki/File:Canberra_Deep_Dish_Communications_Complex_-_GPN-2000-000502.jpghttp://en.wikipedia.org/wiki/File:Canberra_Deep_Dish_Communications_Complex_-_GPN-2000-000502.jpghttp://en.wikipedia.org/wiki/File:Felder_um_Dipol.jpghttp://en.wikipedia.org/wiki/File:Felder_um_Dipol.jpghttp://en.wikipedia.org/wiki/File:Car_radio_antenna_extended_portrait.jpeghttp://en.wikipedia.org/wiki/File:Car_radio_antenna_extended_portrait.jpeghttp://en.wikipedia.org/wiki/File:Canberra_Deep_Dish_Communications_Complex_-_GPN-2000-000502.jpghttp://en.wikipedia.org/wiki/File:Canberra_Deep_Dish_Communications_Complex_-_GPN-2000-000502.jpghttp://en.wikipedia.org/wiki/File:Felder_um_Dipol.jpghttp://en.wikipedia.org/wiki/File:Felder_um_Dipol.jpghttp://en.wikipedia.org/wiki/File:Car_radio_antenna_extended_portrait.jpeghttp://en.wikipedia.org/wiki/File:Car_radio_antenna_extended_portrait.jpeghttp://en.wikipedia.org/wiki/File:Canberra_Deep_Dish_Communications_Complex_-_GPN-2000-000502.jpghttp://en.wikipedia.org/wiki/File:Canberra_Deep_Dish_Communications_Complex_-_GPN-2000-000502.jpghttp://en.wikipedia.org/wiki/File:Felder_um_Dipol.jpghttp://en.wikipedia.org/wiki/File:Felder_um_Dipol.jpghttp://en.wikipedia.org/wiki/File:Car_radio_antenna_extended_portrait.jpeghttp://en.wikipedia.org/wiki/File:Car_radio_antenna_extended_portrait.jpeghttp://en.wikipedia.org/wiki/Parabolic_antennahttp://en.wikipedia.org/wiki/Dipole_antennahttp://en.wikipedia.org/wiki/Magnetic_fieldhttp://en.wikipedia.org/wiki/Electric_fieldhttp://en.wikipedia.org/wiki/Whip_antenna


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Rooftoptelevision antennasin Israel.Yagi-Uda antennaslike these six are widely used atVHFandUHFfrequencies.

An antenna (oraerial) is an electrical device which convertselectric powerintoradio waves, and vice versa. It

is usually used with aradio transmitterorradio receiver. Intransmission, a radio transmitter supplies an

oscillatingradio frequencyelectric current to the antenna's terminals, and the antenna radiates the energy from

the current aselectromagnetic waves(radio waves). In reception, an antenna intercepts some of the power of

an electromagnetic wave in order to produce a tiny voltage at its terminals, that is applied to a receiver to

beamplified.

Antennas are essential components of all equipment that usesradio. They are used in systems such asradio

broadcasting,broadcast television,two-way radio,communications receivers,radar,cell phones, andsatellite

communications, as well as other devices such asgarage door openers,wireless

microphones,bluetoothenabled devices,wireless computer networks,baby monitors, andRFID tagson

merchandise.

Typically an antenna consists of an arrangement of metallicconductors("elements"), electrically connected

(often through atransmission line) to the receiver or transmitter. An oscillating current ofelectronsforced

through the antenna by a transmitter will create an oscillatingmagnetic fieldaround the antenna elements,

while thechargeof the electrons also creates an oscillatingelectric fieldalong the elements. These time-

varying fields, when created in the proper proportions, radiate away from the antenna into space as a moving

transverse electromagnetic field wave. Conversely, during reception, the oscillating electric and magnetic fields

of an incoming radio wave exert force on the electrons in the antenna elements, causing them to move back

and forth, creating oscillating currents in the antenna.

Antennas may also include reflective or directive elements or surfaces not connected to the transmitter or

receiver, such asparasitic elements,parabolic reflectorsorhorns, which serve to direct the radio waves into a

beam or other desiredradiation pattern. Antennas can be designed to transmit or receive radio waves in all

directions equally (omnidirectional antennas), or transmit them in a beam in a particular direction, and receive

from that one direction only (directionalorhigh gainantennas).
http://en.wikipedia.org/wiki/Television_antennahttp://en.wikipedia.org/wiki/Television_antennahttp://en.wikipedia.org/wiki/Television_antennahttp://en.wikipedia.org/wiki/Yagi-Uda_antennahttp://en.wikipedia.org/wiki/Yagi-Uda_antennahttp://en.wikipedia.org/wiki/Yagi-Uda_antennahttp://en.wikipedia.org/wiki/Very_High_Frequencyhttp://en.wikipedia.org/wiki/Very_High_Frequencyhttp://en.wikipedia.org/wiki/Very_High_Frequencyhttp://en.wikipedia.org/wiki/Ultrahigh_frequencyhttp://en.wikipedia.org/wiki/Ultrahigh_frequencyhttp://en.wikipedia.org/wiki/Ultrahigh_frequencyhttp://en.wikipedia.org/wiki/Electric_powerhttp://en.wikipedia.org/wiki/Electric_powerhttp://en.wikipedia.org/wiki/Electric_powerhttp://en.wikipedia.org/wiki/Radio_wavehttp://en.wikipedia.org/wiki/Radio_wavehttp://en.wikipedia.org/wiki/Radio_wavehttp://en.wikipedia.org/wiki/Transmitterhttp://en.wikipedia.org/wiki/Transmitterhttp://en.wikipedia.org/wiki/Transmitterhttp://en.wikipedia.org/wiki/Receiver_(radio)http://en.wikipedia.org/wiki/Receiver_(radio)http://en.wikipedia.org/wiki/Receiver_(radio)http://en.wikipedia.org/wiki/Transmission_(telecommunications)http://en.wikipedia.org/wiki/Transmission_(telecommunications)http://en.wikipedia.org/wiki/Transmission_(telecommunications)http://en.wikipedia.org/wiki/Radio_frequencyhttp://en.wikipedia.org/wiki/Radio_frequencyhttp://en.wikipedia.org/wiki/Radio_frequencyhttp://en.wikipedia.org/wiki/Electromagnetic_radiationhttp://en.wikipedia.org/wiki/Electromagnetic_radiationhttp://en.wikipedia.org/wiki/Electromagnetic_radiationhttp://en.wikipedia.org/wiki/Amplifierhttp://en.wikipedia.org/wiki/Amplifierhttp://en.wikipedia.org/wiki/Amplifierhttp://en.wikipedia.org/wiki/Radiohttp://en.wikipedia.org/wiki/Radiohttp://en.wikipedia.org/wiki/Radiohttp://en.wikipedia.org/wiki/Radio_broadcastinghttp://en.wikipedia.org/wiki/Radio_broadcastinghttp://en.wikipedia.org/wiki/Radio_broadcastinghttp://en.wikipedia.org/wiki/Radio_broadcastinghttp://en.wikipedia.org/wiki/Broadcast_televisionhttp://en.wikipedia.org/wiki/Broadcast_televisionhttp://en.wikipedia.org/wiki/Broadcast_televisionhttp://en.wikipedia.org/wiki/Two-way_radiohttp://en.wikipedia.org/wiki/Two-way_radiohttp://en.wikipedia.org/wiki/Communications_receiverhttp://en.wikipedia.org/wiki/Communications_receiverhttp://en.wikipedia.org/wiki/Communications_receiverhttp://en.wikipedia.org/wiki/Radarhttp://en.wikipedia.org/wiki/Radarhttp://en.wikipedia.org/wiki/Radarhttp://en.wikipedia.org/wiki/Cell_phonehttp://en.wikipedia.org/wiki/Cell_phonehttp://en.wikipedia.org/wiki/Cell_phonehttp://en.wikipedia.org/wiki/Satellite_communicationshttp://en.wikipedia.org/wiki/Satellite_communicationshttp://en.wikipedia.org/wiki/Satellite_communicationshttp://en.wikipedia.org/wiki/Satellite_communicationshttp://en.wikipedia.org/wiki/Garage_door_openerhttp://en.wikipedia.org/wiki/Garage_door_openerhttp://en.wikipedia.org/wiki/Garage_door_openerhttp://en.wikipedia.org/wiki/Wireless_microphonehttp://en.wikipedia.org/wiki/Wireless_microphonehttp://en.wikipedia.org/wiki/Wireless_microphonehttp://en.wikipedia.org/wiki/Bluetoothhttp://en.wikipedia.org/wiki/Bluetoothhttp://en.wikipedia.org/wiki/Bluetoothhttp://en.wikipedia.org/wiki/Wireless_LANhttp://en.wikipedia.org/wiki/Wireless_LANhttp://en.wikipedia.org/wiki/Wireless_LANhttp://en.wikipedia.org/wiki/Baby_monitorhttp://en.wikipedia.org/wiki/Baby_monitorhttp://en.wikipedia.org/wiki/Baby_monitorhttp://en.wikipedia.org/wiki/RFID_taghttp://en.wikipedia.org/wiki/RFID_taghttp://en.wikipedia.org/wiki/RFID_taghttp://en.wikipedia.org/wiki/Conductor_(material)http://en.wikipedia.org/wiki/Conductor_(material)http://en.wikipedia.org/wiki/Conductor_(material)http://en.wikipedia.org/wiki/Driven_elementhttp://en.wikipedia.org/wiki/Driven_elementhttp://en.wikipedia.org/wiki/Driven_elementhttp://en.wikipedia.org/wiki/Transmission_linehttp://en.wikipedia.org/wiki/Transmission_linehttp://en.wikipedia.org/wiki/Transmission_linehttp://en.wikipedia.org/wiki/Electronhttp://en.wikipedia.org/wiki/Electronhttp://en.wikipedia.org/wiki/Electronhttp://en.wikipedia.org/wiki/Magnetic_fieldhttp://en.wikipedia.org/wiki/Magnetic_fieldhttp://en.wikipedia.org/wiki/Magnetic_fieldhttp://en.wikipedia.org/wiki/Electric_chargehttp://en.wikipedia.org/wiki/Electric_chargehttp://en.wikipedia.org/wiki/Electric_chargehttp://en.wikipedia.org/wiki/Electric_fieldhttp://en.wikipedia.org/wiki/Electric_fieldhttp://en.wikipedia.org/wiki/Electric_fieldhttp://en.wikipedia.org/wiki/Parasitic_elementhttp://en.wikipedia.org/wiki/Parasitic_elementhttp://en.wikipedia.org/wiki/Parasitic_elementhttp://en.wikipedia.org/wiki/Parabolic_antennahttp://en.wikipedia.org/wiki/Parabolic_antennahttp://en.wikipedia.org/wiki/Horn_antennahttp://en.wikipedia.org/wiki/Horn_antennahttp://en.wikipedia.org/wiki/Horn_antennahttp://en.wikipedia.org/wiki/Radiation_patternhttp://en.wikipedia.org/wiki/Radiation_patternhttp://en.wikipedia.org/wiki/Radiation_patternhttp://en.wikipedia.org/wiki/Omnidirectional_antennahttp://en.wikipedia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The first antennas were built in 1888 by German physicistHeinrich Hertzin his pioneering experiments to prove

the existence of electromagnetic waves predicted by the theory ofJames Clerk Maxwell. Hertz placeddipole

antennasat the focal point ofparabolic reflectorsfor both transmitting and receiving. He published his work

inAnnalen der Physik und Chemie(vol. 36, 1889).

Contents

[hide]

1 Terminology

2 Overview

3 Reciprocity

4 Parameters

o 4.1 Resonant antennas

4.1.1 Current and voltage distribution

4.1.2 Bandwidth

o 4.2 Gain

o 4.3 Effective area or aperture

o 4.4 Radiation pattern

o 4.5 Field regions

o 4.6 Impedance

o 4.7 Efficiency

o 4.8 Polarization

o 4.9 Impedance matching

5 Basic antenna models

6 Practical antennas

7 Effect of ground

8 Mutual impedance and interaction between antennas

9 Antenna gallery

o 9.1 Antennas and antenna arrays

o 9.2 Antennas and supporting structures

o 9.3 Diagrams as part of a system

10 See also

11 Notes

12 References
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fficiencyhttp://en.wikipedia.org/wiki/Antenna_(radio)#Impedancehttp://en.wikipedia.org/wiki/Antenna_(radio)#Field_regionshttp://en.wikipedia.org/wiki/Antenna_(radio)#Radiation_patternhttp://en.wikipedia.org/wiki/Antenna_(radio)#Effective_area_or_aperturehttp://en.wikipedia.org/wiki/Antenna_(radio)#Gainhttp://en.wikipedia.org/wiki/Antenna_(radio)#Bandwidthhttp://en.wikipedia.org/wiki/Antenna_(radio)#Current_and_voltage_distributionhttp://en.wikipedia.org/wiki/Antenna_(radio)#Resonant_antennashttp://en.wikipedia.org/wiki/Antenna_(radio)#Parametershttp://en.wikipedia.org/wiki/Antenna_(radio)#Reciprocityhttp://en.wikipedia.org/wiki/Antenna_(radio)#Overviewhttp://en.wikipedia.org/wiki/Antenna_(radio)#Terminologyhttp://en.wikipedia.org/wiki/Antenna_(radio)http://en.wikipedia.org/wiki/Annalen_der_Physik_und_Chemiehttp://en.wikipedia.org/wiki/Parabolic_reflectorhttp://en.wikipedia.org/wiki/Dipole_antennahttp://en.wikipedia.org/wiki/Dipole_antennahttp://en.wikipedia.org/wiki/James_Clerk_Maxwellhttp://en.wikipedia.org/wiki/Heinrich_Hertz


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o 12.1 General references

o 12.2 "Practical antenna" references

o 12.3 Theory and simulations

o 12.4 Patents and USPTO

13 Further reading

Terminology[edit source|editbeta]

The words antenna (plural: antennas[1]

) and aerialare used interchangeably. Occasionally a rigid metallic

structure is called an "antenna" while the wire form is called an "aerial". However, note the important

internationaltechnical journal, theIEEE Transactions on Antennas and Propagation.[2]

In theUnited

Kingdomand other areas whereBritish Englishis used, the term aerial is sometimes used although 'antenna'

has been universal in professional use for many years.

The origin of the word antenna relative to wireless apparatus is attributed to Italian radio pioneerGuglielmo

Marconi. In 1895, while testing early radio apparatus in theSwiss AlpsatSalvan, Switzerlandin theMont

Blancregion, Marconi experimented with long wire "aerials". He used a 2.5 meter vertical pole, with a wire

attached to the top running down to the transmitter, as a radiating and receiving aerial element. 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 receiving elements were known simply as aerials or terminals. Because of his

prominence, Marconi's use of the word antenna(Italianforpole) spread among wireless researchers, and later

to the general public.[3]

In common usage, the word antenna may refer broadly to an entire assembly including support structure,

enclosure (if any), etc. in addition to the actual functional components. Especially at microwave frequencies, a

receiving antenna may include not only the actual electrical antenna but an integrated preamplifier ormixer.

One of the 7-metre-diameter antennas of theAtacama Large Millimeter Array.[4]
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"Rabbit ears"dipole antennafor television reception

Cell phonebase stationantennas

Wi-FiWestNet Wi-Fi base station antennas inCalgary, Alberta

Parabolic antennaby Himalaya TelevisionNepal
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Yagi antennaused for mobile military communications station, Dresden, Germany, 1955

Turnstiletype transmitting antenna for VHF low band television broadcasting station, Germany.

Folded dipoleantenna

Large Yagi antenna used byamateur radiohobbyists
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Amast radiatorantenna for anAM radiostation inChapel Hill, North Carolina

Overview[edit source|editbeta]

Antennas of theAtacama Large Millimeter submillimeter Array.[5]

Antennas are required by any radio receiver or transmitter to couple its electrical connection to the

electromagnetic field.Radiowaves areelectromagnetic waveswhich carry signals through the air (or throughspace) at thespeed of lightwith almost notransmission loss. Radio transmitters and receivers are used to

convey signals (information) in systems including broadcast (audio) radio,television,mobile telephones,wi-

fi(WLAN) data networks,trunk linesand point-to-point communications links (telephone, data networks),

satellite links, manyremote controlleddevices such asgarage door openers, and wireless remote sensors,

among many others. Radio waves are also used directly for measurements in technologies

includingRADAR,GPS, andradio astronomy. In each and every case, the transmitters and receivers involved

require antennas, although these are sometimes hidden (such as the antenna inside an AM radio or inside a

laptop computer equipped with wi-fi).

According to their applications and technology available, antennas generally fall in one of two categories:

1. Omnidirectionalor only weakly directional antennas which receive or radiate more or less in all

directions. These are employed when the relative position of the other station is unknown or arbitrary.

They are also used at lower frequencies where a directional antenna would be too large, or simply to

cut costs in applications where a directional antenna isn't required.

2. Directionalorbeam antennas which are intended to preferentially radiate or receive in a particular

direction or directional pattern.

In common usage "omnidirectional" usually refers to all horizontal directions, typically with reduced

performance in the direction of the sky or the ground (a trulyisotropicradiator is not even possible). A

"directional" antenna usually is intended to maximize its coupling to the electromagnetic field in the direction of

the other station, or sometimes to cover a particular sector such as a 120 horizontal fan pattern in the case of

a panel antenna at acell site.
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ia.org/wiki/AM_radiohttp://en.wikipedia.org/wiki/Mast_radiator


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One example of omnidirectional antennas is the very common vertical antenna orwhip antennaconsisting of a

metal rod (often, but not always, a quarter of a wavelength long). Adipole antennais similar but consists of two

such conductors extending in opposite directions, with a total length that is often, but not always, a half of a

wavelength long. Dipoles are typically oriented horizontally in which case they are weakly directional: signals

are reasonably well radiated toward or received from all directions with the exception of the direction along the

conductor itself; this region is called the antenna blind cone or null.

Half-wavedipole antenna

Both the vertical and dipole antennas are simple in construction and relatively inexpensive. The dipole antenna,

which is the basis for most antenna designs, is abalancedcomponent, with equal but opposite voltages and

currents applied at its two terminals through abalanced transmission line(or to a coaxial transmission line

through a so-calledbalun). The vertical antenna, on the other hand, is a monopole antenna. It is typically

connected to the inner conductor of acoaxial transmission line(or a matching network); the shield of the

transmission line is connected toground. In this way, the ground (or any large conductive surface) plays the

role of the second conductor of a dipole, thereby forming acomplete circuit.[6]

Since monopole antennas rely on

a conductive ground, a so-calledgroundingstructure may be employed to provide a better ground contact to the

earth or which itself acts as aground planeto perform that function regardless of (or in absence of) an actual

contact with the earth.

Antennas more complex than the dipole or vertical designs are usually intended to increase the directivity and

consequently the gain of the antenna. This can be accomplished in many different ways leading to a plethora of

antenna designs. The vast majority of designs are fed with a balanced line (unlike a monopole antenna) and

are based on the dipole antenna with additional components (orelements) which increase its directionality.

Antenna "gain" in this instance describes the concentration of radiated power into a particular solid angle of

space, as opposed to the spherically uniform radiation of the ideal radiator. The increased power in the desired

direction is at the expense of that in the undesired directions. Power is conserved, and there is no net power

increase over that delivered from the power source (the transmitter.)

For instance, aphased arrayconsists of two or more simple antennas which are connected together through

an electrical network. This often involves a number of parallel dipole antennas with a certain spacing.

Depending on the relativephaseintroduced by the network, the same combination of dipole antennas can
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operate as a "broadside array" (directional normal to a line connecting the elements) or as an "end-fire array"

(directional along the line connecting the elements). Antenna arrays may employ any basic (omnidirectional or

weakly directional) antenna type, such as dipole, loop or slot antennas. These elements are often identical.

However alog-periodic dipole arrayconsists of a number of dipole elements ofdifferentlengths in order to

obtain a somewhat directional antenna having an extremely wide bandwidth: these are frequently used for

television reception in fringe areas. The dipole antennas composing it are all considered "active elements"

since they are all electrically connected together (and to the transmission line). On the other hand, a

superficially similar dipole array, theYagi-Uda Antenna(or simply "Yagi"), has only one dipole element with an

electrical connection; the other so-calledparasitic elementsinteract with the electromagnetic field in order to

realize a fairly directional antenna but one which is limited to a rather narrow bandwidth. The Yagi antenna has

similar looking parasitic dipole elements but which act differently due to their somewhat different lengths. There

may be a number of so-called "directors" in front of the active element in the direction of 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 aparabolic reflectoror a horn.

Since the size of a directional antenna depends on it being large compared to the wavelength, very directional

antennas of this sort are mainly feasible at UHF and microwave frequencies. On the other hand, at low

frequencies (such as AM broadcast) where a practical antenna must be much smaller than a wavelength,

significant directionality isn't even possible. A vertical antenna orloop antennasmall compared to the

wavelength is typically used, with the main design challenge being that ofimpedance matching. With a vertical

antenna a loading coilat the base of the antenna may be employed to cancel thereactive component of

impedance;small loop antennasare tuned with parallel capacitors for this purpose.

An antenna lead-in is thetransmission line(orfeed line) which connects the antenna to a transmitter or

receiver. Theantenna feedmay refer to all components connecting the antenna to the transmitter or receiver,

such as animpedance matchingnetwork in addition to the transmission line. In a so-called aperture antenna,

such as a horn or parabolic dish, the "feed" may also refer to a basic antenna inside the entire system

(normally at the focus of the parabolic dish or at the throat of a horn) which could be considered the one active

element in that antenna system. A microwave antenna may also be fed directly from awaveguidein lieu of a

(conductive)transmission line.

An antennacounterpoiseorground planeis a structure of conductive material which improves or substitutes for

the ground. It may be connected to or insulated from the natural ground. In a monopole antenna, this aids in

the function of the natural ground, particularly where variations (or limitations) of the characteristics of the

natural ground interfere with its proper function. Such a structure is normally connected to the return connection

of an unbalanced transmission line such as the shield of acoaxial cable.
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An electromagnetic wave refractorin some aperture antennas is a component which due to its shape and

position functions to selectively delay or advance portions of the electromagnetic wavefront passing through it.

The refractor alters the spatial characteristics of the wave on one side relative to the other side. It can, for

instance, bring the wave to a focus or alter the wave front in other ways, generally in order to maximize the

directivity of the antenna system. This is the radio equivalent of anoptical lens.

An antenna coupling network is a passive network (generally a combination of inductive and capacitive circuit

elements) used forimpedance matchingin between the antenna and the transmitter or receiver. This may be

used to improve thestanding wave ratioin order to minimize losses in the transmission line and to present the

transmitter or receiver with a standard resistive impedance that it expects to see for optimum operation.

Reciprocity[edit source|editbeta]

It is a fundamental property of antennas that the electrical characteristics of an antenna described in the next

section, such asgain,radiation pattern,impedance,bandwidth,resonant frequencyandpolarization, are the

same whether the antenna istransmittingorreceiving.[7][8]

For example, the "receiving pattern" (sensitivity as a

function of direction) of an antenna when used for reception is identical to theradiation patternof the antenna

when it is driven and functions as a radiator. This is a consequence of thereciprocity theoremof

electromagnetics.[8]

Therefore in discussions of antenna properties no distinction is usually made between

receiving and transmitting terminology, and the antenna can be viewed as either transmitting or receiving,

whichever is more convenient.

A necessary condition for the aforementioned reciprocity property is that the materials in the antenna and

transmission medium arelinearand reciprocal. Reciprocal(orbilateral) means that the material has the same

response to 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

components such asisolatorsandcirculators, made of nonreciprocal materials such asferrite.[7][8]

These can

be used to give the antenna a different behavior on receiving than it has on transmitting,[7]

which can be useful

in applications likeradar.

Parameters[edit source|editbeta]

Main article:Antenna measurement

Antennas are characterized by a number of performance measures which a user would be concerned with in

selecting or designing an antenna for a particular application. Chief among these relate to the directional

characteristics (as depicted in the antenna'sradiation pattern) and the resultinggain. Even in omnidirectional

(or weakly directional) antennas, the gain can often be increased by concentrating more of its power in the
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/Antenna_(radio)#cite_note-Lonngren-7http://en.wikipedia.org/wiki/Antenna_(radio)#cite_note-Lonngren-7http://en.wikipedia.org/wiki/Antenna_(radio)#cite_note-Lonngren-7http://en.wikipedia.org/wiki/Ferrite_(iron)http://en.wikipedia.org/wiki/Circulatorhttp://en.wikipedia.org/wiki/Isolator_(microwave)http://en.wikipedia.org/wiki/Linear_functionhttp://en.wikipedia.org/wiki/Antenna_(radio)#cite_note-Stutzman-8http://en.wikipedia.org/wiki/Reciprocity_(electromagnetism)http://en.wikipedia.org/wiki/Radiation_patternhttp://en.wikipedia.org/wiki/Antenna_(radio)#cite_note-Lonngren-7http://en.wikipedia.org/wiki/Antenna_(radio)#cite_note-Lonngren-7http://en.wikipedia.org/wiki/Radio_receiverhttp://en.wikipedia.org/wiki/Transmitterhttp://en.wikipedia.org/wiki/Polarization_(waves)http://en.wikipedia.org/wiki/Resonant_frequencyhttp://en.wikipedia.org/wiki/Bandwidth_(signal_processing)http://en.wikipedia.org/wiki/Electrical_impedancehttp://en.wikipedia.org/wiki/Radiation_patternhttp://en.wikipedia.org/wiki/Antenna_gainhttp://en.wikipedia.org/w/index.php?title=Antenna_(radio)&veaction=edit&section=3http://en.wikipedia.org/w/index.php?title=Antenna_(radio)&action=edit&section=3http://en.wikipedia.org/wiki/Standing_wave_ratiohttp://en.wikipedia.org/wiki/Impedance_matchinghttp://en.wikipedia.org/wiki/Optical_lens


11/28

horizontal directions, sacrificing power radiated toward the sky and ground. The antenna'spower gain(or

simply "gain") also takes into account the antenna's efficiency, and is often the primary figure of merit.

Resonant antennas are expected to be used around a particularresonant frequency; an antenna must

therefore be built or ordered to match the frequency range of the intended application. A particular antenna

design will present a particular feedpointimpedance. While this may affect the choice of an antenna, an

antenna's impedance can also be adapted to the desired impedance level of a system using amatching

networkwhile maintaining the other characteristics (except for a possible loss of efficiency).

Although these parameters can bemeasuredin principle, such measurements are difficult and require very

specialized equipment. Beyond tuning a transmitting antenna using anSWRmeter, the typical user will depend

on theoretical predictions based on the antenna design or on claims of a vendor.

An antenna transmits and receives radio waves with a particularpolarizationwhich can be reoriented by tilting

the axis of the antenna in many (but not all) cases. The physical size of an antenna is often a practical issue,particularly at lower frequencies (longer wavelengths). Highly directional antennas need to be significantly

larger than the wavelength. Resonant antennas use a conductor, or a pair of conductors, each of which is

about one quarter of the wavelength in length. Antennas that are required to be very small compared to the

wavelength sacrifice efficiency and cannot be very directional. Fortunately at higher frequencies (UHF,

microwaves) trading off performance to obtain a smaller physical size is usually not required.

Resonant antennas[edit source|editbeta]

While there arebroadband designsfor antennas, the vast majority of antennas are based on the half-

wavedipolewhich has a particularresonant frequency. At its resonant frequency, thewavelength(figured by

dividing thespeed of lightby the resonant frequency) is slightly over twice the length of the half-wave dipole

(thus the name). The quarter-wave vertical antenna consists of one arm of a half-wave dipole, with the other

arm replaced by a connection togroundor an equivalentground plane(orcounterpoise). AYagi-Udaarray

consists of a number of resonant dipole elements, only one of which is directly connected to the transmission

line. The quarter-wave elements of a dipole or vertical antenna imitate a series-resonant electrical element,

since if they are driven at the resonant frequency astanding waveis created with the peak current at the feed-

point and the peak voltage at the far end.

A common misconception is that the ability of a resonant antenna to transmit (or receive) fails at frequencies far

from the resonant frequency. The reason a dipole antenna needs to be used at the resonant frequency has to

do with theimpedance matchbetween the antenna and the transmitter or receiver (and its transmission line).

For instance, a dipole using a fairly thin conductor[9]

will have a purely resistive feedpoint impedance of about

63 ohms at its design frequency. Feeding that antenna with a current of 1 ampere will require 63 volts of RF,

and the antenna will radiate 63 watts (ignoring losses) of radio frequency power. If that antenna is driven with 1

ampere at a frequency 20% higher, it will still radiate as efficiently but in order to do that about 200 volts would
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12/28

be required due to the change in the antenna's impedance which is now largely reactive (voltage out of phase

with the current). A typical transmitter would not find that impedance acceptable and would deliver much less

than 63 watts to it; the transmission line would be operating at a high (poor)standing wave ratio. But using an

appropriate matching network, that large reactive impedance could be converted to a resistive impedance

satisfying the transmitter and accepting the available power of the transmitter.

This principle is used to construct vertical antennas substantially shorter than the 1/4 wavelength at which the

antenna is resonant. By adding an inductance in series with the vertical antenna (a so-calledloading coil) the

capacitive reactance of this antenna can be cancelled leaving a pure resistance which can then be matched to

the transmission line. Sometimes the resulting resonant frequency of such a system (antenna plus matching

network) is described using the construct of "electrical length" and the use of a shorter antenna at a lower

frequency than its resonant frequency is termed "electrical lengthening". For example, at 30 MHz (wavelength

= 10 meters) a true resonant monopole would be almost 2.5 meters (1/4 wavelength) long, and using an

antenna only 1.5 meters tall would require the addition of a loading coil. Then it may be said that the coil has

"lengthened" the antenna to achieve an "electrical length" of 2.5 meters, that is, 1/4 wavelength at 30 MHz

where the combined system now resonates. However, the resulting resistive impedance achieved will be quite

a bit lower than the impedance of a resonant monopole, likely requiring further impedance matching. In addition

to a lower radiation resistance, the reactance becomes higher as the antenna size is reduced, and the resonant

circuit formed by the antenna and the tuning coil has aQ factorthat rises and eventually causes the bandwidth

of the antenna to be inadequate for the signal being transmitted. This is the major factor that sets the size of

antennas at 1 MHz and lower frequencies.

Current and voltage distribution[edit source|editbeta]

The antenna conductors have the lowest feed-point impedance at the resonant frequency where they are just

under 1/4 wavelength long; two such conductors in line fed differentially thus realizes the familiar "half-wave

dipole". When fed with an RF current at the resonant frequency, the quarter wave element contains a standing

wavewith the voltage and current largely (but not exactly) in phase quadrature, as would be obtained using a

quarter wave stub of transmission line. The current reaches a minimum at the end of the element (where it has

nowhere to go!) and is maximum at the feed-point. The voltage, on the other hand, is the greatest at the end of

the conductor and reaches a minimum (but not zero) at the feedpoint. Making the conductor shorter or longer

than 1/4 wavelength means that the voltage pattern reaches its minimum somewhere beyond the feed-point, so

that the feed-point has a higher voltage and thus sees a higher impedance, as we have noted. Since that

voltage pattern is almost in phase quadrature with the current, the impedance seen at the feed-point is not only

much higher but mainly reactive.

It can be seen that if such an element is resonant at f0 to produce such a standing wave pattern, then feeding

that element with 3f0(whose wavelength is 1/3 that off0) will lead to a standing wave pattern in which the
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voltage is likewise a minimum at the feed-point (and the current at a maximum there). Thus, an antenna

element is also resonant when its length is 3/4 of a wavelength (3/2 wavelength for a complete dipole). This is

true for all odd multiples of 1/4 wavelength, where the feed-point impedance is purely resistive, though larger

than the resistive impedance of the 1/4 wave element. Although such an antenna is resonant and works

perfectly well at the higher frequency, the antenna radiation pattern is also altered compared to the half-wave

dipole.

The use of a monopole or dipole at odd multiples of the fundamental resonant frequency, however,

does notextend to even multiples (thus a 1/2 wavelength monopole or 1 wavelength dipole). Now the voltage

standing wave is at itspeakat the feed-point, while that of the current (which must be zero at the end of the

conductor) is at a minimum (but not exactly zero). The antenna is anti-resonantat this frequency. Although the

reactance at the feedpoint can be cancelled using such an element length, the feed-point impedance is very

high, and is highly dependent on the diameter of the conductor (which makes only a small difference at the

actual resonant frequency). Such an antenna does not match the much lower characteristic impedance of

available transmission lines, and is generally not used. However some equipment where transmission lines are

not involved which desire a high driving point impedance may take advantage of this anti-resonance.

Bandwidth[edit source|editbeta]

Although a resonant antenna has a purely resistive feed-point impedance at a particular frequency, many (if not

most) applications require using an antenna over a range of frequencies. An antenna'sbandwidthspecifies the

range of frequencies over which its performance does not suffer due to a poor impedance match. Also in the

case of aYagi-Udaarray, the use of the antenna very far away from its design frequency reduces the

antenna's directivity, thus reducing the usable bandwidth regardless of impedance matching.

Except for the latter concern, the resonant frequency of a resonant antenna can always be altered by adjusting

a suitable matching network. To do this efficiently one would require remotely adjusting a matching network at

the site of the antenna, since simply adjusting a matching network at the transmitter (or receiver) would leave

the transmission line with a poorstanding wave ratio.

Instead, it is often desired to have an antenna whose impedance does not vary so greatly over a certain

bandwidth. It turns out that the amount of reactance seen at the terminals of a resonant antenna when the

frequency is shifted, say, by 5%, depends very much on the diameter of the conductor used. A long thin wire

used as a half-wave dipole (or quarter wave monopole) will have a reactance significantly greater than the

resistive impedance it has at resonance, leading to a poor match and generally unacceptable performance.

Making the element using a tube of a diameter perhaps 1/50 of its length, however, results in a reactance at

this altered frequency which is not so great, and a much less serious mismatch which will only modestly

damage the antenna's net performance. Thus rather thick tubes are typically used for the solid elements of

such antennas, including Yagi-Uda arrays.
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Rather than just using a thick tube, there are similar techniques used to the same effect such as replacing thin

wire elements with cages to simulate a thicker element. This widens the bandwidth of the resonance. On the

other hand, amateur radio antennas need to operate over several bands which are widely separated from each

other. This can often be accomplished simply by connecting resonant elements for the different bands in

parallel. Most of the transmitter's power will flow into the resonant element while the others present a high

(reactive) impedance and draw little current from the same voltage. A popular solution uses so-

called traps consisting of parallel resonant circuits which are strategically placed in breaks along each antenna

element. When used at one particular frequency band the trap presents a very high impedance (parallel

resonance) effectively truncating the element at that length, making it a proper resonant antenna. At a lower

frequency the trap allows the full length of the element to be employed, albeit with a shifted resonant frequency

due to the inclusion of the trap's net reactance at that lower frequency.

The bandwidth characteristics of a resonant antenna element can be characterized according to itsQ, just as

one uses to characterize the sharpness of anL-C resonant circuit. However it is often assumed that there is an

advantage in an antenna having a highQ. After all, Q is short for "quality factor" and a low Q typically signifies

excessive loss (due to unwanted resistance) in a resonantL-C circuit. However this understanding does not

apply to resonant antennas where the resistance involved is theradiation resistance, a desired quantity which

removes energy from the resonant element in order to radiate it (the purpose of an antenna, after all!). The Q is

a measure of the ratio of reactance to resistance, so with a fixed radiation resistance(an element's radiation

resistance is almost independent of its diameter) a greater reactance off-resonance corresponds to the poorer

bandwidth of a very thin conductor. The Q of such a narrowband antenna can be as high as 15. On the other

hand a thick element presents less reactance at an off-resonant frequency, and consequently a Q as low as 5.

These two antennas will perform equivalently at the resonant frequency, but the second antenna will perform

over a bandwidth 3 times as wide as the "hi-Q" antenna consisting of a thin conductor.

Gain[edit source|editbeta]

Main article:Antenna gain

Gainis a parameter which measures the degree of directivity of the antenna's radiation pattern. A high-gain

antenna will preferentially radiate in a particular direction. Specifically, the antenna gain, orpower gain of an

antenna is defined as the ratio of theintensity(power per unit surface) radiated by the antenna in the direction

of its maximum output, at an arbitrary distance, divided by the intensity radiated at the same distance by a

hypotheticalisotropic antenna.

The gain of an antenna is a passive phenomenon - power is not added by the antenna, but simply redistributed

to provide more radiated power in a certain direction than would be transmitted by an isotropic antenna. An

antenna designer must take into account the application for the antenna when determining the gain. High-gain

antennas have the advantage of longer range and better signal quality, but must be aimed carefully in a
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particular direction. Low-gain antennas have shorter range, but the orientation of the antenna is relatively

inconsequential. For example, a dish antenna on a spacecraft is a high-gain device that must be pointed at the

planet to be effective, whereas a typicalWi-Fiantenna in a laptop computer is low-gain, and as long as the

base station is within range, the antenna can be in any orientation in space. It makes sense to improve

horizontal range at the expense of reception above or below the antenna.[10]

In practice, the half-wave dipole is taken as a reference instead of the isotropic radiator. The gain is then given

in dBd (decibels overdipole):

NOTE: 0 dBd = 2.15 dBi. It is vital in expressing gain values that the reference point be included.

Failure to do so can lead to confusion and error.

Effective area or aperture[edit source|editbeta]

Main article:Antenna effective area

Theeffective areaor effective aperture of a receiving antenna expresses the portion of the power of a

passing electromagnetic wave which it delivers to its terminals, expressed in terms of an equivalent area.

For instance, if a radio wave passing a given location has a flux of 1 pW / m2

(1012

watts per square

meter) and an antenna has an effective area of 12 m2, then the antenna would deliver 12 pW ofRFpower

to the receiver (30 microvoltsrmsat 75 ohms). Since the receiving antenna is not equally sensitive to

signals received from all directions, the effective area is a function of the direction to the source.

Due toreciprocity(discussed above) the gain of an antenna used for transmitting must be proportional to

its effective area when used for receiving. Consider an antenna with noloss, that is, one whoseelectrical

efficiencyis 100%. It can be shown that its effective area averaged over all directions must be equal to

2/4, the wavelength squared divided by 4. Gain is defined such that the average gain over all directions

for an antenna with 100%electrical efficiencyis equal to 1. Therefore the effective area Aeff in terms of the

gain G in a given direction is given by:

For an antenna with anefficiencyof less than 100%, both the effective area and gain are reduced by

that same amount. Therefore the above relationship between gain and effective area still holds. These

are thus two different ways of expressing the same quantity. Aeff is especially convenient whencomputing the power that would be received by an antenna of a specified gain, as illustrated by the

above example.

Radiation pattern[edit source|editbeta]

Main article:Radiation pattern
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