Antenna (Radio) - Wikipedia, The Free Encyclopedia

7/30/2019 Antenna (Radio) - Wikipedia, The Free Encyclopedia

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Whip antenna on car

Diagram of the electric

fields (blue) and

magnetic fields (red)

radiated by a dipole

antenna (black rods)

during transmission.

Large parabolic antenna

for communicating with

spacecraft

Antenna (radio)From Wikipedia, the free encyclopedia

An antenna (oraerial) is an electrical device which converts electric power into

radio waves, and vice versa. It is usually used with a radio transmitter or radio

receiver. In transmission, a radio transmitter supplies an oscillating radio frequency

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

the current as electromagnetic waves (radio waves). In reception, an antenna

intercepts some of thepower of an electromagnetic wave in order to produce a tiny

voltage at its terminals, that is applied to a receiver to be amplified.

Antennas are essential components of all equipment that uses radio. They are used in

systems such as radio broadcasting, broadcast television, two-way radio,

communications receivers, radar, cell phones, and satellite communications, as well

as other devices such as garage door openers, wireless microphones, bluetooth

enabled devices, wireless computer networks, baby monitors, and RFID tags on

merchandise.

Typically an antenna consists of an arrangement of metallic conductors ("elements"),

electrically connected (often through a transmission line) to the receiver ortransmitter. An oscillating current of electrons forced through the antenna by a

transmitter will create an oscillating magnetic field around the antenna elements, while

the charge ofthe electrons also creates an oscillating electric field along 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 connectedto the transmitteror receiver, such as parasitic elements, parabolic reflectors or

horns, which serve to direct the radio waves into a beam or other desired radiation

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 (directional or high gain antennas).

The first antennas werebuilt in 1888 by German physicist Heinrich Hertz in his

pioneering experiments to prove the existence of electromagnetic waves predicted by

the theory of James Clerk Maxwell. Hertz placed dipole antennas at the focal point

of parabolic reflectors for both transmitting and receiving. He published his work innnalen der Physik und Chemie (vol. 36, 1889).

Contents

1 Terminology

2 Overview

3 Reciprocity
http://en.wikipedia.org/wiki/File:Felder_um_Dipol.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/Antenna_(radio)#Reciprocityhttp://en.wikipedia.org/wiki/Antenna_(radio)#Terminologyhttp://en.wikipedia.org/wiki/Antenna_(radio)#Terminologyhttp://en.wikipedia.org/wiki/Antenna_(radio)#Terminologyhttp://en.wikipedia.org/wiki/Annalen_der_Physik_und_Chemiehttp://en.wikipedia.org/wiki/Parabolic_reflectorhttp://en.wikipedia.org/wiki/James_Clerk_Maxwellhttp://en.wikipedia.org/wiki/Dipole_antennahttp://en.wikipedia.org/wiki/Heinrich_Hertzhttp://en.wikipedia.org/wiki/File:Canberra_Deep_Dish_Communications_Complex_-_GPN-2000-000502.jpghttp://en.wikipedia.org/wiki/Directional_antennahttp://en.wikipedia.org/wiki/High_gain_antennahttp://en.wikipedia.org/wiki/Omnidirectional_antennahttp://en.wikipedia.org/wiki/File:Canberra_Deep_Dish_Communications_Complex_-_GPN-2000-000502.jpghttp://en.wikipedia.org/wiki/Radiation_patternhttp://en.wikipedia.org/wiki/File:Canberra_Deep_Dish_Communications_Complex_-_GPN-2000-000502.jpghttp://en.wikipedia.org/wiki/Radiation_patternhttp://en.wikipedia.org/wiki/Dipole_antennahttp://en.wikipedia.org/wiki/Electric_fieldhttp://en.wikipedia.org/wiki/File:Felder_um_Dipol.jpghttp://en.wikipedia.org/wiki/Electric_chargehttp://en.wikipedia.org/wiki/Electric_fieldhttp://en.wikipedia.org/wiki/File:Felder_um_Dipol.jpghttp://en.wikipedia.org/wiki/Magnetic_fieldhttp://en.wikipedia.org/wiki/File:Felder_um_Dipol.jpghttp://en.wikipedia.org/wiki/Transmission_linehttp://en.wikipedia.org/wiki/Conductor_(material)http://en.wikipedia.org/wiki/Driven_elementhttp://en.wikipedia.org/wiki/File:Felder_um_Dipol.jpghttp://en.wikipedia.org/wiki/Wireless_LANhttp://en.wikipedia.org/wiki/Baby_monitorhttp://en.wikipedia.org/wiki/RFID_taghttp://en.wikipedia.org/wiki/Communications_receiverhttp://en.wikipedia.org/wiki/Radarhttp://en.wikipedia.org/wiki/Cell_phonehttp://en.wikipedia.org/wiki/Satellite_communicationshttp://en.wikipedia.org/wiki/File:Car_radio_antenna_extended_portrait.jpeghttp://en.wikipedia.org/wiki/Radio_broadcastinghttp://en.wikipedia.org/wiki/Broadcast_televisionhttp://en.wikipedia.org/wiki/Two-way_radiohttp://en.wikipedia.org/wiki/File:Car_radio_antenna_extended_portrait.jpeghttp://en.wikipedia.org/wiki/Radiohttp://en.wikipedia.org/wiki/File:Car_radio_antenna_extended_portrait.jpeghttp://en.wikipedia.org/wiki/Amplifierhttp://en.wikipedia.org/wiki/File:Car_radio_antenna_extended_portrait.jpeghttp://en.wikipedia.org/wiki/Electromagnetic_radiationhttp://en.wikipedia.org/wiki/File:Car_radio_antenna_extended_portrait.jpeghttp://en.wikipedia.org/wiki/Receiver_(radio)http://en.wikipedia.org/wiki/Transmission_(telecommunications)http://en.wikipedia.org/wiki/Receiver_(radio)http://en.wikipedia.org/wiki/Radio_frequencyhttp://en.wikipedia.org/wiki/File:Car_radio_antenna_extended_portrait.jpeghttp://en.wikipedia.org/wiki/Electric_powerhttp://en.wikipedia.org/wiki/File:Car_radio_antenna_extended_portrait.jpeghttp://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/Annalen_der_Physik_und_Chemiehttp://en.wikipedia.org/wiki/Parabolic_reflectorhttp://en.wikipedia.org/wiki/Dipole_antennahttp://en.wikipedia.org/wiki/James_Clerk_Maxwellhttp://en.wikipedia.org/wiki/Heinrich_Hertzhttp://en.wikipedia.org/wiki/High_gain_antennahttp://en.wikipedia.org/wiki/Directional_antennahttp://en.wikipedia.org/wiki/Omnidirectional_antennahttp://en.wikipedia.org/wiki/Radiation_patternhttp://en.wikipedia.org/wiki/Horn_antennahttp://en.wikipedia.org/wiki/Parabolic_antennahttp://en.wikipedia.org/wiki/Parasitic_elementhttp://en.wikipedia.org/wiki/Electric_fieldhttp://en.wikipedia.org/wiki/Electric_chargehttp://en.wikipedia.org/wiki/Magnetic_fieldhttp://en.wikipedia.org/wiki/Electronhttp://en.wikipedia.org/wiki/Transmission_linehttp://en.wikipedia.org/wiki/Driven_elementhttp://en.wikipedia.org/wiki/Conductor_(material)http://en.wikipedia.org/wiki/RFID_taghttp://en.wikipedia.org/wiki/Baby_monitorhttp://en.wikipedia.org/wiki/Wireless_LANhttp://en.wikipedia.org/wiki/Bluetoothhttp://en.wikipedia.org/wiki/Wireless_microphonehttp://en.wikipedia.org/wiki/Garage_door_openerhttp://en.wikipedia.org/wiki/Satellite_communicationshttp://en.wikipedia.org/wiki/Cell_phonehttp://en.wikipedia.org/wiki/Radarhttp://en.wikipedia.org/wiki/Communications_receiverhttp://en.wikipedia.org/wiki/Two-way_radiohttp://en.wikipedia.org/wiki/Broadcast_televisionhttp://en.wikipedia.org/wiki/Radio_broadcastinghttp://en.wikipedia.org/wiki/Radiohttp://en.wikipedia.org/wiki/Amplifierhttp://en.wikipedia.org/wiki/Electromagnetic_radiationhttp://en.wikipedia.org/wiki/Radio_frequencyhttp://en.wikipedia.org/wiki/Transmission_(telecommunications)http://en.wikipedia.org/wiki/Receiver_(radio)http://en.wikipedia.org/wiki/Transmitterhttp://en.wikipedia.org/wiki/Radio_wavehttp://en.wikipedia.org/wiki/Electric_powerhttp://en.wikipedia.org/wiki/Parabolic_antennahttp://en.wikipedia.org/wiki/File:Canberra_Deep_Dish_Communications_Complex_-_GPN-2000-000502.jpghttp://en.wikipedia.org/wiki/Dipole_antennahttp://en.wikipedia.org/wiki/Magnetic_fieldhttp://en.wikipedia.org/wiki/Electric_fieldhttp://en.wikipedia.org/wiki/File:Felder_um_Dipol.jpghttp://en.wikipedia.org/wiki/Whip_antennahttp://en.wikipedia.org/wiki/File:Car_radio_antenna_extended_portrait.jpeg


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Rooftop television

antennas in Israel. Yagi-

Uda antennas like these

six are widely used atVHF and UHF

frequencies.

4 Parameters

4.1 Resonant antennas

4.1.1 Current and voltage distribution

4.1.2 Bandwidth

4.2 Gain

4.3 Effective area or aperture

4.4 Radiation pattern

4.5 Field regions

4.6 Impedance

4.7 Efficiency4.8 Polarization

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

9.1 Antennas and antenna arrays

9.2 Antennas and supporting structures

9.3 Diagrams as part of a system

10 See also

11 Notes

12 References

12.1 General references

12.2 "Practical antenna" references

12.3 Theory and simulations

12.4 Patents and USPTO

13 Further reading

Terminology

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 international

technical journal, theIEEE Transactions on Antennas and Propagation.[2] In the United Kingdom and other

areas where British English is 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 pioneer Guglielmo

Marconi. In 1895, while testing early radio apparatus in the Swiss Alps at Salvan, Switzerland in the Mont Blanc

region, 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 a

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 (Italian forpole) spread among wireless researchers, and later to the general

public.[3]
http://en.wikipedia.org/wiki/Antenna_(radio)#cite_note-3http://en.wikipedia.org/wiki/Italian_languagehttp://en.wikipedia.org/wiki/Mont_Blanchttp://en.wikipedia.org/wiki/Salvan,_Switzerlandhttp://en.wikipedia.org/wiki/Swiss_Alpshttp://en.wikipedia.org/wiki/Guglielmo_Marconihttp://en.wikipedia.org/wiki/British_Englishhttp://en.wikipedia.org/wiki/United_Kingdomhttp://en.wikipedia.org/wiki/Antenna_(radio)#cite_note-2http://en.wikipedia.org/wiki/IEEE_Transactions_on_Antennas_and_Propagationhttp://en.wikipedia.org/wiki/Scientific_journalhttp://en.wikipedia.org/wiki/Antenna_(radio)#cite_note-1http://en.wikipedia.org/wiki/Antenna_(radio)#Further_readinghttp://en.wikipedia.org/wiki/Antenna_(radio)#Patents_and_USPTOhttp://en.wikipedia.org/wiki/Antenna_(radio)#Theory_and_simulationshttp://en.wikipedia.org/wiki/Antenna_(radio)#.22Practical_antenna.22_referenceshttp://en.wikipedia.org/wiki/Antenna_(radio)#General_referenceshttp://en.wikipedia.org/wiki/Antenna_(radio)#Referenceshttp://en.wikipedia.org/wiki/Antenna_(radio)#Noteshttp://en.wikipedia.org/wiki/Antenna_(radio)#See_alsohttp://en.wikipedia.org/wiki/Antenna_(radio)#Diagrams_as_part_of_a_systemhttp://en.wikipedia.org/wiki/Antenna_(radio)#Antennas_and_supporting_structureshttp://en.wikipedia.org/wiki/Antenna_(radio)#Antennas_and_antenna_arrayshttp://en.wikipedia.org/wiki/Antenna_(radio)#Antenna_galleryhttp://en.wikipedia.org/wiki/Antenna_(radio)#Mutual_impedance_and_interaction_between_antennashttp://en.wikipedia.org/wiki/Antenna_(radio)#Effect_of_groundhttp://en.wikipedia.org/wiki/Antenna_(radio)#Practical_antennashttp://en.wikipedia.org/wiki/Antenna_(radio)#Basic_antenna_modelshttp://en.wikipedia.org/wiki/Antenna_(radio)#Impedance_matchinghttp://en.wikipedia.org/wiki/Antenna_(radio)#Polarizationhttp://en.wikipedia.org/wiki/Antenna_(radio)#Efficiencyhttp://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/Ultrahigh_frequencyhttp://en.wikipedia.org/wiki/Very_High_Frequencyhttp://en.wikipedia.org/wiki/Yagi-Uda_antennahttp://en.wikipedia.org/wiki/Television_antennahttp://en.wikipedia.org/wiki/File:Antenna.jpg


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

(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 or mixer.

"Rabbit ears" dipole

antenna for television

reception

Cell phone base station

antennas

Wi-Fi WestNet Wi-Fi

base station antennas in

Calgary, Alberta

Parabolic antenna by

Himalaya Television

Nepal

Yagi antenna used for

mobile military

communications station,

Dresden, Germany, 1955

Turnstile type

transmitting antenna for

VHF low band television

broadcasting station,

Germany.

Folded dipole antenna

Large Yagi antenna

used by amateur radio

hobbyists

A mast radiator antenna

for an AM radio station

in Chapel Hill, North

Carolina

Overview
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Antennas of the Atacama Large

Millimeter submillimeter Array.[4]

Half-wave dipole antenna

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

electrical connection to the electromagnetic field. Radio waves are

electromagnetic waves which carry signals through the air (or through

space) at the speed of light with almost no transmission 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 lines and point-to-point

communications links (telephone, data networks), satellite links, many

remote controlled devices such as garage door openers, and wireless

remote sensors, among many others. Radio waves are also used directly

for measurements in technologies including RADAR, GPS, and radio

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

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 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. Directional orbeam 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 truly isotropic radiator is not even possible). A "directional" antenna usuall

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 a cell site.

One example of omnidirectional antennas is the very common vertical antenna or whip antenna consisting of a

metal rod (often, but not always, a quarter of a wavelength long). A dipole antenna is similar but consists of twosuch 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.

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 a

balanced component, with equal but opposite voltages and currents applied at its

two terminals through a balanced transmission line (or to a coaxial transmission line

through a so-called balun). The vertical antenna, on the other hand, is a monopoleantenna. It is typically connected to the inner conductor of a coaxial transmission line

(or a matching network); the shield of the transmission line is connected to ground. In

this way, the ground (or any large conductive surface) plays the role of the second

conductor of a dipole, thereby forming a complete circuit.[5] Since monopole

antennas rely on a conductive ground, a so-called grounding structure may be employed to provide a better groun

contact to the earth or which itself acts as a ground plane to perform that function regardless of (or in absence of)

an actual contact with the earth.
http://en.wikipedia.org/wiki/Ground_planehttp://en.wikipedia.org/wiki/Ground_(electricity)http://en.wikipedia.org/wiki/Antenna_(radio)#cite_note-5http://en.wikipedia.org/wiki/Circuit_theory#Open_circuit_vs._closed_circuithttp://en.wikipedia.org/wiki/Ground_(electricity)http://en.wikipedia.org/wiki/Coaxial_cablehttp://en.wikipedia.org/wiki/Balunhttp://en.wikipedia.org/wiki/Balanced_linehttp://en.wikipedia.org/wiki/Balancedhttp://en.wikipedia.org/wiki/Dipole_antennahttp://en.wikipedia.org/wiki/Whip_antennahttp://en.wikipedia.org/wiki/Cell_sitehttp://en.wikipedia.org/wiki/Isotropichttp://en.wikipedia.org/wiki/Directional_antennahttp://en.wikipedia.org/wiki/Omnidirectional_antennahttp://en.wikipedia.org/wiki/Radio_astronomyhttp://en.wikipedia.org/wiki/GPShttp://en.wikipedia.org/wiki/RADARhttp://en.wikipedia.org/wiki/Garage_door_openerhttp://en.wikipedia.org/wiki/Remote_controlledhttp://en.wikipedia.org/wiki/Trunkinghttp://en.wikipedia.org/wiki/WLANhttp://en.wikipedia.org/wiki/Wi-fihttp://en.wikipedia.org/wiki/Mobile_telephoneshttp://en.wikipedia.org/wiki/Televisionhttp://en.wikipedia.org/wiki/Absorption_(electromagnetic_radiation)http://en.wikipedia.org/wiki/Speed_of_lighthttp://en.wikipedia.org/wiki/Electromagnetic_waveshttp://en.wikipedia.org/wiki/Radiohttp://en.wikipedia.org/wiki/Dipole_antennahttp://en.wikipedia.org/wiki/File:Half_%E2%80%93_Wave_Dipole.jpghttp://en.wikipedia.org/wiki/Antenna_(radio)#cite_note-4http://en.wikipedia.org/wiki/Atacama_Large_Millimeter_Arrayhttp://en.wikipedia.org/wiki/File:The_Atacama_Large_Millimeter_submillimeter_Array_(ALMA)_by_night_under_the_Magellanic_Clouds.jpg


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ALMA array from the air[6]

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 a

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, a phased array consists 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 th

relative phase introduced by the network, the same combination of dipole antennas can 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 a log-periodic dipole array consists of a number of dipole elements ofdifferentlengths in order to obtai

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

the Yagi-Uda Antenna (or simply "Yagi"), has only one dipole element with an electrical connection; the other so-called parasitic elements interact with the electromagnetic field in order to realize a fairly directional antenna but on

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 a parabolic reflector or 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 andmicrowave 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 or loop antenna small compared to the wavelength is typically

used, with the main design challenge being that of impedance matching.

With a vertical antenna a loading coilat the base of the antenna may be

employed to cancel the reactive component of impedance; small loop

antennas are tuned with parallel capacitors for this purpose.

An antenna lead-in is the transmission line (orfeed line) which connects the antenna to a transmitter or receiver.The antenna feedmay refer to all components connecting the antenna to the transmitter or receiver, such as an

impedance matching network in addition to the transmission line. In a so-called aperture antenna, such as a horn o

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 a waveguide in lieu of a (conductive) transmission line.

An antenna counterpoise or ground plane is a structure of conductive material which improves or substitutes for th

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
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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 a coaxial cable.

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 refracto

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 an optical lens.

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

elements) used for impedance matching in between the antenna and the transmitter or receiver. This may be used timprove the standing wave ratio in 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

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

such as gain, radiation pattern, impedance, bandwidth, resonant frequency and polarization, are the same whether

the antenna is transmitting or receiving.[7][8] For example, the "receiving pattern" (sensitivity as a function of

direction) of an antenna when used for reception is identical to the radiation pattern of the antenna when it is driveand functions as a radiator. This is a consequence of the reciprocity theorem of electromagnetics. [8] Therefore in

discussions of antenna properties no distinction is usually made between receiving and transmitting terminology, an

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 are linear and 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 as isolators and circulators, made of nonreciprocal materials such as ferrite.[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 inapplications like radar.

Parameters

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's radiation 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 horizontaldirections, sacrificing power radiated toward the sky and ground. The antenna's power 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 b

built or ordered to match the frequency range of the intended application. A particular antenna design will present

particular feedpoint impedance. While this may affect the choice of an antenna, an antenna's impedance can also b

adapted to the desired impedance level of a system using a matching network while maintaining the other

characteristics (except for a possible loss of efficiency).
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Although these parameters can be measured in principle, such measurements are difficult and require very

specialized equipment. Beyond tuning a transmitting antenna using an SWR meter, 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 particular polarization which 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

While there are broadband designs for antennas, the vast majority of antennas are based on the half-wave dipole

which has a particular resonant frequency. At its resonant frequency, the wavelength (figured by dividing the speed

of light by 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 to ground or an equivalent ground plane (orcounterpoise). A Yagi-Uda array 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 a standing wave is 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 the impedance match between 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 antenn

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 be required du

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-called loading coil) the

capacitative 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) isdescribed 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 achiev

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
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becomes higher as the antenna size is reduced, and the resonant circuit formed by the antenna and the tuning coil

has a Q factor that 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

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

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

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

voltage and current largely (but not exactly) in phase quadrature, as would be obtained using a quarter wave stub 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 atf0 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 voltage is

likewise a minimum at the feed-point (and the current at a maximum there). Thus, an antenna element is alsoresonant 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 notexten

to even multiples (thus a 1/2 wavelength monopole or 1 wavelength dipole). Now the voltage standing wave is at i

eakat 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 thediameter 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

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's bandwidth specifies the

range of frequencies over which its performance does not suffer due to a poor impedance match. Also in the caseof a Yagi-Uda array, 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 poor standing wave ratio.
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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 dipol

(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.

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 frequenc

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 its Q, just as one

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

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

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

to resonant antennas where the resistance involved is the radiation 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 o

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 performequivalently at the resonant frequency, but the second antenna will perform over a bandwidth 3 times as wide as th

"hi-Q" antenna consisting of a thin conductor.

Gain

Main article: Antenna gain

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

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

defined as the ratio of the intensity (power per unit surface) radiated by the antenna in the direction of its maximumoutput, at an arbitrary distance, divided by the intensity radiated at the same distance by a hypothetical isotropic

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 particular direction. Low-

gain antennas have shorter range, but the orientation of the antenna is relatively inconsequential. For example, a dis
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antenna on a spacecraft is a high-gain device that must be pointed at the planet to be effective, whereas a typical

Wi-Fi antenna 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

Main article: Antenna effective area

The effective area or 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 of RF power to the receiver (30 microvolts rms at

75 ohms). Since the receiving antenna is not equally sensitive 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 antenna used for transmitting must be proportional to its

effective area when used for receiving. Consider an antenna with no loss, that is, one whose electrical efficiency is

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 efficiency is equal to 1. Therefore the effective area Aeffin terms of the gain G in a given direction is give

by:

For an antenna with an efficiency of 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. Aeffis especially convenient when computing the power that would be

received by an antenna of a specified gain, as illustrated by the above example.

Radiation pattern

Main article: Radiation pattern

The radiation pattern of an antenna is a plot of the relative field strength of the radio waves emitted by the antenna

at different angles. It is typically represented by a three dimensional graph, or polar plots of the horizontal and

vertical cross sections. The pattern of an ideal isotropic antenna, which radiates equally in all directions, would loo

like a sphere. Many nondirectional antennas, such as monopoles and dipoles, emit equal power in all horizontal

directions, with the power dropping off at higher and lower angles; this is called an omnidirectional pattern and

when plotted looks like a torus or donut.
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polar plots of the horizontal cross

sections of a (virtual) Yagi-Uda-

antenna. Outline connects points with

3db field power compared to an ISO

emitter.

The radiation of many antennas shows a pattern of maxima or "lobes" at various angles, separated by "nulls",

angles where the radiation falls to zero. This is because the radio waves emitted by different parts of the antenna

typically interfere, causing maxima at angles where the radio waves arrive at distant points in phase, and zero

radiation at other angles where the radio waves arrive out of phase. In a directional antenna designed to project

radio waves in a particular direction, the lobe in that direction is designed larger than the others and is called the

"main lobe". The other lobes usually represent unwanted radiation and are called "sidelobes". The axis through th

main lobe is called the "principal axis" or "boresight axis".

Field regions

The space surrounding an antenna can be divided into three concentric

regions: the reactive near-field, the radiating near-field (Fresnell region)

and the far-field (Fraunhofer) regions. These regions are useful to identify

the field structure in each, although there are no precise boundaries.

In the far-field region, we are far enough from the antenna to neglect its

size and shape. We can assume that the electromagnetic wave is purely a

radiating plane wave (electric and magnetic fields are in phase and

perpendicular to each other and to the direction of propagation). This

simplifies the mathematical analysis of the radiated field.

Impedance

As an electro-magnetic wave travels through the different 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 the impedance match, some fraction of the wave's energy will reflect

back to the source,[11] forming a standing wave in the feed line. The ratio of maximum power to minimum power in

the wave can be measured and is called the standing wave ratio (SWR). A SWR of 1:1 is ideal. A SWR of 1.5:1

considered to be marginally acceptable in low power applications where power loss is more critical, although anSWR as high as 6:1 may still be usable with the right equipment. Minimizing impedance differences at each interfac

(impedance matching) will reduce SWR and maximize power transfer through each part of the antenna system.

Complex impedance of an antenna is related to the electrical length of the antenna at the wavelength in use. The

impedance of an antenna can be matched to the feed line and radio by adjusting the impedance of the feed line,

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 matching transformer, matching networks composed of inductors and

capacitors, or matching sections such as the gamma match.

Efficiency

Main article: Antenna efficiency

Efficiency of a transmitting antenna is the ratio of power actually radiated (in all directions) to the power absorbed

by the antenna terminals. The power supplied to the antenna terminals which is not radiated is converted into heat.

This is usually through loss resistance in the antenna's conductors, but can also be due to dielectric or magnetic cor

losses in antennas (or antenna systems) using such components. Such loss effectively robs power from the

transmitter, requiring a stronger transmitter in order to transmit a signal of a given strength.
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For instance, if a transmitter delivers 100 W into an antenna having an efficiency of 80%, then the antenna will

radiate 80 W as radio waves and produce 20 W of heat. In order to radiate 100 W of power, one would need to

use a transmitter capable of supplying 125 W to the antenna. Note that antenna efficiency is a separate issue from

impedance matching, which may also reduce the amount of power radiated using a given transmitter. If an SWR

meter reads 150 W of incident power and 50 W of reflected power, that means that 100 W have actually been

absorbed by the antenna (ignoring transmission line losses). How much of that power has actually been radiated

cannot be directly determined through electrical measurements at (or before) the antenna terminals, but would

require (for instance) careful measurement of field strength. Fortunately the loss resistance of antenna conductors

such as aluminum rods can be calculated and the efficiency of an antenna using such materials predicted.

However loss resistance will generally affect the feedpoint impedance, adding to its resistive (real) component. Tha

resistance will consist of the sum of the radiation resistance Rrand the loss resistance Rloss. If an rms current I is

delivered to the terminals of an antenna, then a power of I2Rrwill be radiated and a power of I2Rloss will be lost a

heat. Therefore the efficiency of an antenna is equal to Rr/ (Rr+ Rloss). Of course only the total resistance Rr+

Rloss can be directly measured.

According to reciprocity, the efficiency of an antenna used as a receiving antenna is identical to the efficiency as

defined above. The power that an antenna will deliver to a receiver (with a proper impedance match) is reduced b

the same amount. In some receiving applications, the very inefficient antennas may have little impact on

performance. At low frequencies, for example, atmospheric or man-made noise can mask antenna inefficiency. Foexample, CCIR Rep. 258-3 indicates man-made noise in a residential setting at 40 MHz is about 28 dB above the

thermal noise floor. Consequently, an antenna with a 20 dB loss (due to inefficiency) would have little impact on

system noise performance. The loss within the antenna will affect the intended signal and the noise/interference

identically, leading to no reduction in signal to noise ratio (SNR).

This is fortunate, since antennas at lower frequencies which are not rather large (a good fraction of a wavelength in

size) are inevitably inefficient (due to the small radiation resistance Rrof small antennas). Most AM broadcast

radios (except for car radios) take advantage of this principle by including a small loop antenna for reception whic

has an extremely poor efficiency. Using such an inefficient antenna at this low frequency (5301650 kHz) thus has

little effect on the receiver's net performance, but simply requires greater amplification by the receiver's electronics

Contrast this tiny component to the massive and very tall towers used at AM broadcast stations for transmitting at

the very same frequency, where every percentage point of reduced antenna efficiency entails a substantial cost.

The definition of antenna gain orpower gain already includes the effect of the antenna's efficiency. Therefore if on

is trying to radiate a signal toward a receiver using a transmitter of a given power, one need only compare the gain

of various antennas rather than considering the efficiency as well. This is likewise true for a receiving antenna at ver

high (especially microwave) frequencies, where the point is to receive a signal which is strong compared to the

receiver's noise temperature. However in the case of a directional antenna used for receiving signals with the

intention ofrejectinginterference from different directions, one is no longer concerned with the antenna efficiency,

as discussed above. In this case, rather than quoting the antenna gain, one would be more concerned with the

directive gain which does notinclude the effect of antenna (in)efficiency. The directive gain of an antenna can be

computed from the published gain divided by the antenna's efficiency.

Polarization

Main article: Polarization (waves)
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Thepolarization of an antenna is the orientation of the electric field (E-plane) of the radio wave with respect to th

Earth's surface and is determined by the physical structure of the antenna and by its orientation. It has nothing in

common with antenna directionality terms: "horizontal", "vertical", and "circular". Thus, a simple straight wire antenn

will have one polarization when mounted vertically, and a different polarization when mounted horizontally.

"Electromagnetic wave polarization filters"[citation needed] are structures which can be employed to act directly on

the electromagnetic wave to filter out wave energy of an undesired polarization and to pass wave energy of a

desired polarization.

Reflections generally affect polarization. For radio waves the most important reflector is the ionosphere - signals

which reflect from it will have their polarization changed unpredictably. For signals which are reflected by the

ionosphere, polarization cannot be relied upon. For line-of-sight communications for which polarization can be

relied upon, it can make a large difference in signal quality to have the transmitter and receiver using the same

polarization; many tens of dB difference are commonly seen and this is more than enough to make the difference

between reasonable communication and a broken link.

Polarization is largely predictable from antenna construction but, especially in directional antennas, the polarization

of side lobes can be quite different from that of the main propagation lobe. For radio antennas, polarization

corresponds to the orientation of the radiating element in an antenna. A vertical omnidirectional WiFi antenna will

have vertical polarization (the most common type). An exception is a class of elongated waveguide antennas in

which vertically placed antennas are horizontally polarized. Many commercial antennas are marked as to the

polarization of their emitted signals.

Polarization is the sum of the E-plane orientations over time projected onto an imaginary plane perpendicular to the

direction of motion of the radio wave. In the most general case, polarization is elliptical, meaning that the

polarization of the radio waves varies over time. Two special cases are linear polarization (the ellipse collapses into

a line) and circular polarization (in which the two axes of the ellipse are equal). In linear polarization the antenna

compels the electric field of the emitted radio wave to a particular orientation. Depending on the orientation of the

antenna mounting, the usual linear cases are horizontal and vertical polarization. In circular polarization, the antenna

continuously varies the electric field of the radio wave through all possible values of its orientation with regard to th

Earth's surface. Circular polarizations, like elliptical ones, are classified as right-hand polarized or left-hand

polarized using a "thumb in the direction of the propagation" rule. Optical researchers use the same rule of thumb,but pointing it in the direction of the emitter, not in the direction of propagation, and so are opposite to radio

engineers' use.

In practice, regardless of confusing terminology, it is important that linearly polarized antennas be matched, lest the

received signal strength be greatly reduced. So horizontal should be used with horizontal and vertical with vertical.

Intermediate matchings will lose some signal strength, but not as much as a complete mismatch. Transmitters

mounted on vehicles with large motional freedom commonly use circularly polarized antennas[citation needed] so th

there will never be a complete mismatch with signals from other sources.

Impedance matching

Main article: Impedance matching

Maximum power transfer requires matching the impedance of an antenna system (as seen looking into the

transmission line) to the complex conjugate of the impedance of the receiver or transmitter. In the case of a

transmitter, however, the desired matching impedance might not correspond to the dynamic output impedance of

the transmitter as analyzed as a source impedance but rather the design value (typically 50 ohms) required for

efficient and safe operation of the transmitting circuitry. The intended impedance is normally resistive but a
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transmitter (and some receivers) may have additional adjustments to cancel a certain amount of reactance in order

to "tweak" the match. When a transmission line is used in between the antenna and the transmitter (or receiver) on

generally would like an antenna system whose impedance is resistive and near the characteristic impedance of that

transmission line in order to minimize the standing wave ratio (SWR) and the increase in transmission line losses it

entails, in addition to supplying a good match at the transmitter or receiver itself.

Antenna tuning generally refers to cancellation of any reactance seen at the antenna terminals, leaving only a resistiv

impedance which might or might not be exactly the desired impedance (that of the transmission line). Although an

antenna may be designed to have a purely resistive feedpoint impedance (such as a dipole 97% of a half waveleng

long) this might not be exactly true at the frequency that it is eventually used at. In some cases the physical length o

the antenna can be "trimmed" to obtain a pure resistance. On the other hand, the addition of a series inductance or

parallel capacitance can be used to cancel a residual capacitative or inductive reactance, respectively.

In some cases this is done in a more extreme manner, not simply to cancel a small amount of residual reactance, bu

to resonate an antenna whose resonance frequency is quite different than the intended frequency of operation. For

instance, a "whip antenna" can be made significantly shorter than 1/4 wavelength long, for practical reasons, and

then resonated using a so-called loading coil. This physically large inductor at the base of the antenna has an

inductive reactance which is the opposite of the capacitative reactance that such a vertical antenna has at the

desired operating frequency. The result is a pure resistance seen at feedpoint of the loading coil; unfortunately that

resistance is somewhat lower than would be desired to match commercial coax[citation needed].

So an additional problem beyond canceling the unwanted reactance is of matching the remaining resistive

impedance to the characteristic impedance of the transmission line. In principle this can always be done with a

transformer, however the turns ratio of a transformer is not adjustable. A general matching network with at least

two adjustments can be made to correct both components of impedance. Matching networks using discrete

inductors and capacitors will have losses associated with those components, and will have power restrictions when

used for transmitting. Avoiding these difficulties, commercial antennas are generally designed with fixed matching

elements or feeding strategies to get an approximate match to standard coax, such as 50 or 75 Ohms. Antennas

based on the dipole (rather than vertical antennas) should include a balun in between the transmission line and

antenna element, which may be integrated into any such matching network.

Another extreme case of impedance matching occurs when using a small loop antenna (usually, but not always, for

receiving) at a relatively low frequency where it appears almost as a pure inductor. Resonating such an inductor

with a capacitor at the frequency of operation not only cancels the reactance but greatly magnifies the very small

radiation resistance of such a loop[citation needed]. This is implemented in most AM broadcast receivers, with a

small ferrite loop antenna resonated by a capacitor which is varied along with the receiver tuning in order to

maintain resonance over the AM broadcast band

Basic antenna models

There are many variations of antennas. Below are a few basic models. More can be found in Category:Radio

frequency antenna types.

The isotropic radiator is a purely theoretical antenna that radiates equally in all directions. It is considered to

be a point in space with no dimensions and no mass. This antenna cannot physically exist, but is useful as a

theoretical model for comparison with all other antennas. Most antennas' gains are measured with reference

to an isotropic radiator, and are rated in dBi (decibels with respect to an isotropic radiator).

The dipole antenna is simply two wires pointed in opposite directions arranged either horizontally or

vertically, with one end of each wire connected to the radio and the other end hanging free in space. Since
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Typical US multiband TV antenna(aerial)

"Rabbit ears" set-top antenna

this is the simplest practical antenna, it is also used as a reference

model for other antennas; gain with respect to a dipole is labeled

as dBd. Generally, the dipole is considered to be omnidirectional

in the plane perpendicular to the axis of the antenna, but it has

deep nulls in the directions of the axis. Variations of the dipole

include the folded dipole, the half wave antenna, the ground plane

antenna, the whip, and the J-pole.

The Yagi-Uda antenna is a directional variation of the dipole with

parasitic elements added which are functionality similar to adding a

reflector and lenses (directors) to focus a filament light bulb.The random wire antenna is simply a very long (at least one

quarter wavelength[citation needed]) wire with one end connected

to the radio and the other in free space, arranged in any way most convenient for the space available. Foldin

will reduce effectiveness and make theoretical analysis extremely difficult. (The added length helps more tha

the folding typically hurts.) Typically, a random wire antenna will also require an antenna tuner, as it might

have a random impedance that varies non-linearly with frequency.

The horn antenna is used where high gain is needed, the wavelength is short (microwave) and space is not a

issue. Horns can be narrow band or wide band, depending on their shape. A horn can be built for any

frequency, but horns for lower frequencies are typically impractical. Horns are also frequently used as

reference antennas.

The parabolic antenna consists of an active element at the focus of a parabolic reflector to reflect the waves

into a plane wave. Like the horn it is used for high gain, microwave applications, such as satellite dishes.

The patch antenna consists mainly of a square conductor mounted over a groundplane. Another example of

planar antenna is the tapered slot antenna (TSA), as the Vivaldi-antenna.

Practical antennas

Although any circuit can radiate if driven with a signal of high enough

frequency, most practical antennas are specially designed to radiate

efficiently at a particular frequency. An example of an inefficient antenna is

the simple Hertzian dipole antenna, which radiates over wide range of

frequencies and is useful[citation needed] for its small size. A more efficient

variation of this is the half-wave dipole, which radiates with high efficiency

when the signal wavelength is twice the electrical length of the antenna.

One of the goals of antenna design is to minimize the reactance of the device

so that it appears as a resistive load. An "antenna inherent reactance"includes not only the distributed reactance of the active antenna but also the

natural reactance due to its location and surroundings (as for example, the capacity relation inherent in the position

of the active antenna relative to ground). Reactance can be eliminated by operating the antenna at its resonant

frequency, when its capacitive and inductive reactances are equal and opposite, resulting in a net zero reactive

current. If this is not possible, compensating inductors or capacitors can instead be added to the antenna to cancel

its reactance as far as the source is concerned.
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The wave reflected by earth can be

considered as emitted by the image

antenna.

Once the reactance has been eliminated, what remains is a pure resistance, which is the sum of two parts: the ohmi

resistance of the conductors, and the radiation resistance. Power absorbed by the ohmic resistance becomes wast

heat, and that absorbed by the radiation resistance becomes radiated electromagnetic energy. The greater the ratio

of radiation resistance to ohmic resistance, the more efficient the antenna.

Effect of ground

Main article: Multipath propagation

Antennas are typically used in an environment where other objects are present that may have an effect on theirperformance. Height above ground has a very significant effect on the radiation pattern of some antenna types.

At frequencies used in antennas, the ground behaves mainly as a dielectric. The conductivity of ground at these

frequencies is negligible. When an electromagnetic wave arrives at the surface of an object, two waves are created

one enters the dielectric and the other is reflected. If the object is a conductor, the transmitted wave is negligible

and the reflected wave has almost the same amplitude as the incident one. When the object is a dielectric, the

fraction reflected depends (among other things) on the angle of incidence. When the angle of incidence is small (th

is, the wave arrives almost perpendicularly) most of the energy traverses the surface and very little is reflected.

When the angle of incidence is near 90 (grazing incidence) almost all the wave is reflected.

Most of the electromagnetic waves emitted by an antenna to the ground below the antenna at moderate (say < 60

angles of incidence enter the earth and are absorbed (lost). But waves emitted to the ground at grazing angles, far

from the antenna, are almost totally reflected. At grazing angles, the ground behaves as a mirror. Quality of

reflection depends on the nature of the surface. When the irregularities of the surface are smaller than the

wavelength, reflection is good.

This means that the receptor "sees" the real antenna and, under the

ground, the image of the antenna reflected by the ground. If the

ground has irregularities, the image will appear fuzzy.

If the receiver is placed at some height above the ground, wavesreflected by ground will travel a little longer distance to arrive to the

receiver than direct waves. The distance will be the same only if the

receiver is close to ground.

In the drawing at right, the angle has been drawn far bigger than in

reality. The distance between the antenna and its image is .

The situation is a bit more complex because the reflection of

electromagnetic waves depends on the polarization of the incident

wave. As the refractive index of the ground (average value ) isbigger than the refractive index of the air ( ), the direction of the component of the electric field parallel to the

ground inverses at the reflection. This is equivalent to a phase shift of radians or 180. The vertical component o

the electric field reflects without changing direction. This sign inversion of the parallel component and the non-

inversion of the perpendicular component would also happen if the ground were a good electrical conductor.

This means that a receiving antenna "sees" the image antenna with the current in the same direction if the antenna is

vertical or with the current inverted if the antenna is horizontal.
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The vertical component of the current

reflects without changing sign. The

horizontal component reverses sign at

reflection.

Radiation patterns of antennas and their images reflected by the

ground. At left the polarization is vertical and there is always a

maximum for . If the polarization is horizontal as at right, there

is always a zero for .

For a vertical polarized emission antenna the far electric field of the electromagnetic wave produced by the direct

ray plus the reflected ray is:

The sign inversion for the parallel field case just changes a cosine to

a sine:

In these two equations:

is the electrical field radiated by the antenna if there were

no ground.

is the wave number.

is the wave length.

is the distance between antenna and its image (twice the

height of the center of the antenna).

For emitting and receiving antennas situatednear the ground (in a building or on a mast)

far from each other, distances traveled by

direct and reflected rays are nearly the same.

There is no induced phase shift. If the

emission is polarized vertically, the two fields

(direct and reflected) add and there is

maximum of received signal. If the emission

is polarized horizontally, the two signals

subtract and the received signal is minimum.

This is depicted in the image at right. In thecase of vertical polarization, there is always a

maximum at earth level (left pattern). For horizontal polarization, there is always a minimum at earth level. Note tha

in these drawings the ground is considered as a perfect mirror, even for low angles of incidence. In these drawings

the distance between the antenna and its image is just a few wavelengths. For greater distances, the number of

lobes increases.

Note that the situation is differentand more complexif reflections in the ionosphere occur. This happens over

very long distances (thousands of kilometers). There is not a direct ray but several reflected rays that add with

different phase shifts.

This is the reason why almost all public address radio emissions have vertical polarization. As public users are near

ground, horizontal polarized emissions would be poorly received. Observe household and automobile radio

receivers. They all have vertical antennas or horizontal ferrite antennas for vertical polarized emissions. In cases

where the receiving antenna must work in any position, as in mobile phones, the emitter and receivers in base

stations use circular polarized electromagnetic waves.

Classical (analog) television emissions are an exception. They are almost always horizontally polarized, because th

presence of buildings makes it unlikely that a good emitter antenna image will appear[citation needed]. However,

these same buildings reflect the electromagnetic waves and can create ghost images. Using horizontal polarization,
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reflections are attenuated because of the low reflection of electromagnetic waves whose magnetic field is parallel to

the dielectric surface near the Brewster's angle. Vertically polarized analog television has been used in some rural

areas. In digital terrestrial television reflections are less obtrusive, due to the inherent robustness of digital signalling

and built-in error correction.

Mutual impedance and interaction between antennas

Mutual impedance between parallel dipoles not staggered.

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

impedance.

Current circulating in any antenna induces currents in all others. One can postulate a mutual impedance

between two antennas that has the same significance as the in ordinary coupled inductors. The mutual

impedance between two antennas is defined as:

where is the current flowing in antenna 1 and is the voltage that would have to be applied to antenna 2with

antenna 1 removedto produce the current in the antenna 2 that was produced by antenna 1.

From this definition, the currents and voltages applied in a set of coupled antennas are:
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where:

is the voltage applied to the antenna

is the impedance of antenna

is the mutual impedance between antennas and .

As is the case for mutual inductances,

This is a consequence of Lorentz reciprocity. If some of the elements are not fed (there is a short circuit instead afeeder cable), as is the case in television antennas (Yagi-Uda antennas), the corresponding are zero. Those

elements are called parasitic elements. Parasitic elements are unpowered elements that either reflect or absorb and

reradiate RF energy.

In some geometrical settings, the mutual impedance between antennas can be zero. This is the case for crossed

dipoles used in circular polarization antennas.

Antenna gallery

Antennas and antenna arrays

A Yagi-Uda beam

antenna.

A multi-band rotary

directional antenna for

amateur radio use.

Rooftop TV antenna. It

is actually three Yagi

antennas. The longest

elements are for the low

band, while the medium

and short elements are

for the high and UHF

band.

A terrestrial microwave

radio antenna array.
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Wire dipole antenna

using open-wire ladder

line feedline for amateur

radio use.

Low cost LF time signal

receiver, antenna (left)

and receiver

Examples of US 136-

174 MHz base station

antennas.

Rotatable log-periodic

array for VHF and

UHF.

Shortwave antennas in

Delano, California.

An old VHF-band Yagi-

type television antenna.

A T2FD broadband

antenna, covering the 5-

30 MHz band.

A US multiband "aerial"

TV antenna.

"Rabbit ears" antenna

AM loop antenna

Antennas and supporting structures
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A building rooftopsupporting numerous

dish and sectored mobile

telecommunications

antennas (Doncaster,

Victoria, Australia).

Documents

Antenna (Radio) - Wikipedia, The Free Encyclopedia