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| {{More footnotes|date=November 2011}}
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| [[File:Rabbit-ears dipole antenna with UHF loop 20090204.jpg|thumb|upright|"Rabbit-ears" [[television antenna]] (the wire loop is a separate UHF loop antenna).]]
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| [[File:Dipole antenna m en.svg|thumb|right|thumb|Schematic of a (balanced) half-wave dipole antenna connected to an unbalanced [[coaxial cable]].]]
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| {{Antennas|expanded=Common Types}}
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| In [[radio]] and [[telecommunications]] a '''dipole antenna''' or '''doublet'''<ref name="Winder">{{cite book
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| | last = Winder
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| | first = Steve
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| | authorlink =
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| | coauthors = Joseph Carr
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| | title = Newnes Radio and RF Engineering Pocket Book, 3rd Ed.
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| | publisher = Newnes
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| | year = 2002
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| | location =
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| | page = 4
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| | url = http://books.google.com/books?id=3b-_InpSowcC&pg=PA4&dq=%22simplest+practical+radiator
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| | doi =
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| | id =
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| | isbn = 0080497470}}</ref> is the simplest and most widely used class of [[Antenna (radio)|antenna]].<ref>Der Dipol in Theorie und Praxis, K. Hille (DL1VU)</ref><ref name="Basu" /> It consists of two identical conductive elements<ref name="RadioElectronics">{{cite web
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| | title = Dipole Antenna / Aerial tutorial
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| | work = Resources
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| | publisher = Radio-Electronics.com, Adrio Communications, Ltd.
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| | year = 2011
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| | url = http://www.radio-electronics.com/info/antennas/dipole/dipole.php
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| | format =
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| | doi =
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| | accessdate = April 29, 2013}}</ref> such as metal wires or rods, which are usually [[bilateral symmetry|bilaterally symmetrical]].<ref name="Basu">{{cite book
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| | last = Basu
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| | first = Dipak
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| | title = Dictionary of Pure and Applied Physics, 2nd Ed.
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| | publisher = CRC Press
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| | year = 2010
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| | location =
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| | page = 21
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| | url = http://books.google.com/books?id=-QhAkBSk7IUC&pg=PA21
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| | doi =
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| | id =
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| | isbn = 1420050222}}</ref><ref name="Rouse">{{cite web
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| | last = Rouse
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| | first = Margaret
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| | title = Dipole Antenna
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| | work = Online IT Encyclopedia
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| | publisher = [http://whatis.techtarget.com/ TechTarget.com]
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| | year = 2003
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| | url = http://searchmobilecomputing.techtarget.com/definition/dipole-antenna
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| | format =
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| | doi =
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| | accessdate = April 29, 2013}}</ref><ref name="Balanis">{{cite book
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| | last = Balanis
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| | first = Constantine A.
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| | title = Modern Antenna Handbook
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| | publisher = John Wiley & Sons
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| | year = 2011
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| | location =
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| | pages = 2.3
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| | url = http://books.google.com/books?id=UYpV8L8GNCwC&pg=SA2-PA3
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| | doi =
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| | id =
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| | isbn = 1118209753}}</ref> The driving current from the [[transmitter]] is applied, or for receiving antennas the output signal to the [[radio receiver|receiver]] is taken, between the two halves of the antenna. Each side of the [[feedline]] to the transmitter or receiver is connected to one of the conductors. This contrasts with a [[monopole antenna]], which consists of a single rod or conductor with one side of the [[feedline]] connected to it, and the other side connected to some type of [[ground (electricity)|ground]].<ref name="Balanis" /> A common example of a dipole is the "rabbit ears" [[television antenna]] found on broadcast television sets.
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| The most common form of dipole is two straight rods or wires oriented end to end on the same axis, with the feedline connected to the two adjacent ends. This is the simplest type of antenna from a theoretical point of view.<ref name="Winder" /> Dipoles are ''[[resonance|resonant]]'' antennas, meaning that the elements serve as [[resonator]]s, with [[standing wave]]s of radio current flowing back and forth between their ends. So the length of the dipole elements is determined by the [[wavelength]] of the radio waves used.<ref name="Basu" /> The most common form is the '''half-wave dipole''', in which each of the two rod elements is approximately 1/4 wavelength long, so the whole antenna is a half-wavelength long.
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| Several different variations of the dipole are also used, such as the ''folded dipole'', ''short dipole'', ''cage dipole'', ''bow-tie'', and ''[[batwing antenna]]''. Dipoles may be used as standalone antennas themselves, but they are also employed as [[antenna feed|feed antenna]]s ([[driven element]]s) in many more complex antenna types,<ref name="Basu" /><ref name="RadioElectronics" /> such as the [[Yagi-Uda antenna|Yagi antenna]], [[parabolic antenna]], [[reflective array antenna|reflective array]], [[turnstile antenna]], [[log periodic antenna]], and [[phased array antenna|phased array]]. The dipole was the earliest type of antenna; it was invented by German physicist [[Heinrich Hertz]] around 1886 in his pioneering investigations of [[radio wave]]s.
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| ==Elementary doublet==
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| [[Image:elementary-doublet.jpg|right|thumb|Elementary doublet.]]
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| From a theoretical point of view, the dipole antenna is the simplest antenna type. An elementary doublet or ''Hertzian dipole'' is a small length of [[electrical conductor|conductor]] δℓ (small compared to the [[wavelength]] λ) carrying an [[alternating current]]:
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| :<math>I=I_0 e^{i\omega t}.</math>
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| Here ω = 2π''f'' is the [[angular frequency]] (and ''f'' the [[frequency]]), and ''i'' = {{sqrt|−1}} is the [[imaginary unit]], so that ''I'' is a [[phasor]].
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| It is used in, for example, analytical calculation on more complex antenna geometries. Note that this dipole cannot be physically constructed because the current needs somewhere to come from and somewhere to go to. In reality, this small length of conductor will be just one of the multiple segments into which we must divide a real antenna, in order to calculate its properties.
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| In the case of the elementary doublet it is possible to find exact, [[closed-form expression]]s for its [[electric field]], '''E''', and its [[magnetic field]], '''H'''. In [[spherical coordinates]], they are<ref name="silver92">{{cite book|last=Silver|first=Samuel|title=Microwave Antenna Theory and Design|year=1949|pages=92–94}}</ref>
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| :<math>E_r=\frac{Z \,I_0 \delta \ell}{2\pi}\left(\frac{1}{r^2}-\frac{i}{kr^3} \right) e^{i(\omega t-k\,r)}\,\cos(\theta)</math>
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| :<math>E_\theta= i\frac{Z \,I_0 \delta \ell}{4\pi} \left(\frac{k}{r} - \frac{i}{r^2} - \frac{1}{k r^3}\right) e^{i(\omega t-k\,r)}\,\sin(\theta)</math>
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| :<math>H_\phi= i \frac{I_0\delta \ell}{4\pi} \left(\frac{k}{r} - \frac{i}{r^2} \right) e^{i(\omega t-k\,r)}\,\sin(\theta)</math>
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| :<math>E_\phi = H_r = H_\theta = 0,</math>
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| where ''r'' is the distance from the doublet to the point where the fields are evaluated, ''k'' = 2π/λ is the [[wavenumber]], and ''Z'' = {{sqrt|μ/ε}} = 1/ε''c'' = μ''c'' is the [[wave impedance]] of the surrounding medium (usually [[air]] or [[vacuum]]).
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| The energy associated with the term of the near field flows alternately out of and back into the antenna. The [[exponent]] of ''e'' accounts for the [[Phase (waves)|phase]] dependence of the [[electric field]] on time and the distance from the dipole.
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| Often one is interested in the antenna's radiation pattern only in the ''[[far field]]'', when ''r'' ≫ λ/2π. In this regime, only the 1/''r'' term contributes,<ref name="silver92" /> and hence
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| :<math>E_\theta= i\frac{Z \,I_0 \delta \ell\, k}{4\pi r} e^{i(\omega t-k\,r)}\,\sin(\theta)</math>
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| :<math>H_\phi= i \frac{I_0\delta \ell\, k}{4\pi r} e^{i(\omega t-k\,r)}\,\sin(\theta)</math>
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| :<math>E_r = E_\phi = H_r = H_\theta = 0.</math>
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| The far electric field, ''E<sub>θ</sub>'', of the electromagnetic wave is co-planar with the conductor and [[perpendicular]] with the line joining the dipole to the point where the field is evaluated. If the dipole were placed in the center of a [[sphere]] with the [[Coordinate axis|axis]] south-north, the electric field would be [[Parallel (geometry)|parallel]] to geographic [[meridian (geography)|meridians]] and the [[magnetic field]] of the electromagnetic wave would be parallel to geographic [[Circle of latitude|parallels]].
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| ===Radiation resistance and aperture===
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| All antennas have a ''[[radiation resistance]]'', which is the [[Electrical resistance|resistance]] the antenna presents to its circuit due to radiation. The radiation resistance of the elementary doublet in free space is
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| :<math>R_\mathrm{rad} = \frac{2 \pi}{3} Z_{0} \left( \frac{\delta\ell}{\lambda}\right)^{2},</math>
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| where ''Z''<sub>0</sub> is the [[impedance of free space]]. This is precisely four times the radiation resistance of the real short dipole with the linearly tapered current distribution.
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| The radiation resistance is typically a fraction of an ohm, making the elementary doublet an inefficient radiator.
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| The [[directivity]] of the elementary doublet—that is, the theoretical [[antenna gain]] assuming no [[ohmic loss]]es—is 1.5, which corresponds to 1.76 dBi. The actual gain will be much less due to the ohmic losses (because of the very high currents) and the loss inherent in connecting a transmission line to the antenna, which is very hard to do efficiently because of the low radiation resistance.
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| The maximum [[Antenna aperture|effective aperture]] of the elementary doublet is
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| :<math>A_\mathrm{e} = \frac{3 \lambda ^2 }{8 \pi} = \frac{G \lambda ^2 }{4 \pi},</math>
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| where ''G'' = 1.5 is the antenna gain.
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| A surprising result is that even though the elementary doublet is minute, its effective aperture is comparable to antennas many times its size. A real small antenna will have a smaller effective aperture because of its lower gain.
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| ==Dipole characteristics==
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| ===Frequency versus length===
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| Dipoles that are much smaller than the wavelength of the signal are called ''Hertzian, short, or infinitesimal dipoles''. These have a very low [[radiation resistance]] and a high capacitive [[Reactance (electronics)|reactance]], so they are inefficient antennas; though inefficient, they can be practical antennas for long wavelengths. Dipoles whose length is half the wavelength of the signal are called ''half-wave dipoles'', and are more efficient. In general radio engineering, the term ''dipole'' usually means a half-wave dipole (center-fed).
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| A half-wave dipole is cut to length ''l'' for frequency ''f'' in hertz according to the formula
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| :<math>l= \frac{1}{2} \lambda_d = \frac{1}{2} k \lambda_0 = \frac{1}{2} k \frac{c}{f}</math>
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| where ''λ<sub>d</sub>'' is the wavelength on the dipole elements, ''λ''<sub>0</sub> is the free-space wavelength, ''c'' is the [[speed of light]] in free space ({{convert|299792458|m/s|ft/s}}), and ''k'' is an adjustment factor. The adjustment factor compensates for propagation speed being somewhat less than the speed of light. The dipole elements will have distributed [[inductance]] and [[capacitance]]. The value of ''k'' is typically 0.95. For thin wires (radius = 0.000001 wavelengths), ''k'' is approximately 0.981; for thick wires (radius = 0.01 wavelengths), ''k'' drops to about 0.915.
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| The above formula is often shortened to the length in metres = 143/''f''<sub>MHz</sub> or the length in feet = 468/''f''<sub>MHz</sub>; ''f''<sub>MHz</sub> is the frequency in megahertz.<ref>[http://www.ycars.org/EFRA/Module%20C/AntDip.htm ycars.org - Reflections and standing wave ratio], 2011-01-30</ref>
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| ===Radiation pattern and gain===
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| [[File:Felder um Dipol.jpg|thumb|Electric fields ''(<span style="color:blue;">blue</span>)'' and magnetic fields ''(<span style="color:red;">red</span>)'' radiated by a dipole antenna]]
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| Dipoles have a [[radiation pattern]], shaped like a [[toroid]] ([[doughnut]]) symmetrical about the axis of the dipole. The radiation is maximum at right angles to the dipole, dropping off to zero on the antenna's axis. The theoretical maximum gain of a Hertzian dipole is 10 log 1.5 or 1.76 dBi. The maximum theoretical gain of a λ/2-dipole is 10 log 1.64 or 2.15 dBi.
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| {|
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| |[[File:RadPatt-lin.png|thumb|Radiation pattern of a half-wave dipole antenna. The scale is linear.]]
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| |[[File:RadPatt-dB.png|thumb|Gain of a half-wave dipole (same as left). The scale is in dBi (decibels over isotropic).]]
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| |}
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| ==Feeding a Dipole Antenna==
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| A folded dipole has a central impedance of about 300 ohms. Therefore the simplest way of feeding a folded dipole antenna is using a 300-ohm ladder line.<ref>Practical Wire Antennas 2 (I. Poole, G3YWX)</ref> Ideally, a half-wave (λ/2) dipole should be fed with a balanced line matching the theoretical 73-ohm impedance of the antenna. A folded dipole uses a 300-ohm balanced feeder line. Many people have had success in feeding a dipole directly with a coaxial cable feed rather than a [[Ladder line|ladder-line]]{{citation needed|date=December 2012}}. However, coax is not symmetrical and thus not a balanced feeder. It is unbalanced because the outer shield is connected to earth potential at the other end. When a balanced antenna such as a dipole is fed with an unbalanced feeder, [[common mode currents]] can cause the coax line to radiate in addition to the antenna itself,<ref name="w7el">Baluns: That They Do And How They Do It (W7EL) http://www.eznec.com/Amateur/Articles/Baluns.pdf</ref> and the radiation pattern may be asymmetrically distorted. This can be remedied with the use of a [[balun]].
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| ===Feeding a Dipole with Baluns===
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| {{multiple image
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| | direction = horizontal
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| | header = Feeding a Dipole Antenna with Coax Cable
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| | width = 200
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| | image1 = dipolefeedrad.png
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| | alt1 = Coax and antenna both acting as radiators instead of only the antenna.
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| | caption1 = Coax and antenna both acting as radiators instead of only the antenna.
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| | image2 = dipolewidebandbalun.png
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| | alt2 = Dipole with a current balun.
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| | caption2 = Dipole with a current balun.
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| | image3 = Dipolehalfwavebalun.png
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| | alt3 = A folded dipole (300Ω) to coax (75Ω) 4:1 balun.
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| | caption3 = A folded dipole (300Ω) to coax (75Ω) 4:1 balun.
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| | image4 = dipolesleevebalun.png
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| | alt4 = Dipole using a sleeve balun.
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| | caption4 = Dipole using a sleeve balun.
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| }}
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| A dipole is a symmetrical antenna, as it is composed of two symmetrical ungrounded elements. Therefore it works best when fed by a [[Transmission line|balanced transmission line]], such as a [[ladder line]]. This is because in that case the symmetry (one aspect of the impedance complex, which is a complex number) matches and therefore the power transfer is extremal.
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| When a dipole with an unbalanced feedline such as [[coaxial cable]] is used for [[Transmission (telecommunications)|transmitting]], the shield side of the cable, in addition to the antenna, radiates.<ref name="w7el"/> This can induce [[Radio Frequency|RF]] currents into other electronic equipment near the radiating feedline, causing RF interference. Furthermore, the antenna is not as efficient as it could be because it is radiating closer to the ground and its radiation (and reception) pattern may be distorted asymmetrically. At higher frequencies, where the length of the dipole becomes significantly shorter than the diameter of the feeder coax, this becomes a more significant problem. To prevent this, dipoles fed by coaxial cables have a [[balun]] between the cable and the antenna, to convert the unbalanced signal provided by the coax to a balanced symmetrical signal for the antenna.
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| Several types of [[balun]]s are commonly used to feed a dipole antenna: current baluns and coax baluns. Baluns can be made using ferrite toroid cores or even from the coax feedline itself.<ref>Baluns for 88–108 MHz B. Beezely (K6STI) http://www.ham-radio.com/k6sti/balun.htm</ref> The choice of the toroid core is crucial. A rule of thumb is: the more power the bigger the core.<ref>Toroid Cores for 1:4 Baluns (DG3OBK) http://www.aroesner.homepage.t-online.de/balun.html</ref>
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| ====Current balun====
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| A current balun consists of two windings that are closely coupled.<ref name="w7el"/><ref>A Cost Effective Current-mode 1:1 Balun (R. Holland) http://www.arising.com.au/people/Holland/Ralph/CMBalun.htm</ref>
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| ====Coax balun====
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| A coax balun is a cost-effective method of eliminating feeder radiation, but is limited to a narrow set of operating frequencies.
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| *One easy way to make a balun is a (''λ''/2) length of coaxial cable. The inner core of the cable is linked at each end to one of the balanced connections for a feeder or dipole. One of these terminals should be connected to the inner core of the coaxial feeder. All three braids should be connected together. This then forms a 4:1 balun which works correctly at only a narrow band of frequencies.
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| ====Sleeve balun====
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| At [[VHF]] frequencies, a sleeve balun can also be built to remove feeder radiation.<ref>[http://www.w8ji.com/sleeve_baluns.htm Sleeve Baluns]</ref>
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| * Another narrow band design is to use a ''λ''/4 length of metal pipe. The coaxial cable is placed inside the pipe; at one end the braid is wired to the pipe while at the other end no connection is made to the pipe. The balanced end of this balun is at the end where the pipe is wired to the braid. The ''λ''/4 conductor acts as a transformer converting the infinite impedance at the unconnected end into a zero impedance at the end connected to the braid. Hence any current entering the balun through the connection, which goes to the braid at the end with the connection to the pipe, will flow into the pipe. This balun design is impractical for low frequencies because of the long length of pipe that will be needed.
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| ==Dipole types==
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| ===Short dipole===
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| [[Image:short-dipole.svg|left|Diagram of a short dipole antenna.]] A short dipole is a physically feasible dipole formed by two conductors with a total length ''L'' very small compared with the wavelength λ. The two elements are fed at the center of the dipole. The current profile in each element, actually the tail end of a sinusoidal standing wave, is approximately a triangular distribution, declining linearly from a maximum at the center feed point to zero at the ends. At any instant the direction of the current is the same in both the dipole branches: to the right in both or to the left in both. The far field ''E<sub>θ</sub>'' of the electromagnetic wave radiated by this dipole is
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| :<math>E_\theta={-iI_0\sin\theta\over 4\varepsilon_0 c r}{L\over\lambda}e^{i\left(\omega t-kr\right)}.</math>
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| [[Image:elem-doubl-rad-pat.jpg|thumb|right|Radiation pattern of an elementary doublet, shown in profile.]]
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| [[Image:Elem-doub-rad-pat-pers.jpg|thumb|left|Three-dimensional perspective of the radiation pattern of an elementary doublet.]]
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| Field strength is maximal in the plane perpendicular to the dipole axis, declining monotonically to zero on the antenna's axis. The 3 dimensional [[radiation pattern]] ''(right)'' of a vertical dipole is [[torus]]-shaped, with equal radiation in all horizontal directions.
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| Knowing the radiated electric field, we can compute the total emitted power and then compute the resistive part of the series impedance of this dipole due to the radiated field, known as the [[radiation resistance]]:
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| :<math>R_\text{series}={\pi\over6}Z_0 \left({L\over\lambda}\right)^2 \qquad \text{ for } L \ll \lambda,</math>
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| where <math>Z_0</math> is the [[impedance of free space]]. Using a common approximation of <math>Z_0 \approx 120 \pi</math> ohms, we get
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| :<math>R_\text{series}\approx 20\pi^2\left({L\over\lambda}\right)^2 \qquad \text{(in ohms)}.</math>
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| <br style="clear:both" />
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| ====Antenna gain====
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| [[Antenna gain]], ''G'', is the ratio of surface power radiated by the antenna to the surface power radiated by a hypothetical [[isotropic antenna]]:
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| :<math>G=\frac{(P/S)_\text{ant}}{(P/S)_\text{iso}}.</math>
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| The surface power carried by an electromagnetic wave is
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| :<math>\left(\frac{P}{S}\right)_\text{ant} = \frac{1}{2}c \varepsilon_0 E_\theta^{\,2} \simeq \frac{E_\theta^{\,2}}{240\pi},</math>
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| while the surface power radiated by an isotropic antenna feed with the same power is
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| :<math> \left(\frac{P}{S}\right)_\text{iso} = \frac{\tfrac{1}{2} R_\text{series} I_0^{\,2}}{4\pi r^2}.</math>
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| Combining these expressions with the far-field expression for ''E<sub>θ</sub>'' for a short dipole gives
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| :<math> G = \frac{3}{2} = \mathrm{1.76\ dBi},</math>
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| where dBi means [[decibels]] gain relative to an isotropic antenna.
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| ===Half-wave dipole{{anchor|Half-wave antenna}}===
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| [[File:Half – Wave Dipole.jpg|thumb|UHF–Half–Wave Dipole, 1.0–4 GHz]]
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| [[File:Dipole Antenna.svg|left|thumb|The instantaneous voltage distribution across a dipole antenna of total length λ/2.]]
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| Typically a dipole antenna is formed by two quarter-wavelength conductors or elements placed back to back for a total length of ''L'' = λ/2. A standing wave on an element of length approximately λ/2 yields the greatest voltage differential, as one end of the element is at a node while the other is at an antinode of the wave. The larger the differential voltage, the greater the current between the elements.
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| [[File:lambdaover2-antenna.jpg|right]]
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| The current distribution is assumed to be approximately sinusoidal along the length of the dipole, with a node at each end and an antinode in the center:<ref name="silver_halfwave">{{cite book|last=Silver|first=Samuel|title=Microwave Antenna Theory and Design|year=1984|pages=98–99}}</ref>
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| :<math> I(z) = I_0 e^{i\omega t} \cos kz,</math>
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| where ''k'' = 2π/λ and ''z'' runs from −''L'' /2 to ''L'' /2.
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| In the far field, this produces a radiation pattern whose electric field is given by<ref name="silver_halfwave" />
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| :<math>E_\theta = \frac{-i Z I_0}{2\pi r} \frac{\cos\left(\frac{\pi}{2}\cos\theta\right)}{\sin\theta} e^{i(\omega t - kr)},</math>
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| where again ''Z'' = {{sqrt|''μ''/''ε''}}. The trigonometric factor cos[(''π''/2)cos ''θ'']/sin ''θ'' is approximately equal to the factor sin ''θ'' appearing in the far-field radiation pattern for the elementary doublet, so the radiation pattern of a half-wave antenna is a slightly flattened [[torus]].<ref name="silver_halfwave" />
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| [[File:L-over2-rad-pat.svg|right|thumb|Cross-section of the far-field radiation pattern of the half-wave antenna (solid line) compared to the far-field radiation pattern of the elementary doublet (dashed line).]]
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| [[File:L-over2-rad-pat-per.jpg|left|thumb|Three-dimensional view of the far-field radiation pattern of the half-wave antenna.]]
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| This time it is not possible to compute analytically the total power emitted by the antenna (the last formula does not allow), though a simple numerical integration or series expansion leads to the more precise, actual value of the half-wave resistance:
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| <math>\begin{align}R_{\frac{\lambda}{2}}
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| &= \frac{Z_0}{2\pi} \left[\ln(2\pi\gamma)-\operatorname{Ci}(2\pi)\right] = \frac{Z_0}{4 \pi} \operatorname{Cin}(2\pi) = 29.9792458 \int_{0}^{2\pi} \frac{ 1-\cos(\theta)}{\theta} d \theta,\\
| |
| &\approx 73.0790102 \ \Omega;
| |
| \end{align}\,\!</math>
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| | |
| This leads to the gain of a dipole antenna, <math>G_{\frac{\lambda}{2}}\,\!</math>:
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| | |
| :::<math>\begin{align}G_{\frac{\lambda}{2}}
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| &=\frac{60^2}{30R_{\frac{\lambda}{2}}}=\frac{3600}{30R_{\frac{\lambda}{2}}} = \frac{120}{R_{\frac{\lambda}{2}}} = \frac{4}{\operatorname{Cin}(2\pi)} \approx 1.64 \approx 2.15 \,\mathrm{dBi}. \end{align}\,\!</math>
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| | |
| The resistance, however, is not enough to characterize the dipole impedance, as there is also an imaginary part—it is better to measure the impedance.
| |
| | |
| In the image below, the real and imaginary parts of a dipole's impedance are drawn for lengths going from <math>\scriptstyle{0.4\,\lambda}\,\!</math> to <math>\scriptstyle{0.6\,\lambda}\,\!</math>, accompanied by a chart comparing the gains of dipole antennas of other lengths, both as a number and in dBi:
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| [[File:Dipole antenna impedance.svg|right|thumb|Dipole impedance]]
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| | |
| {| class="wikitable"
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| |colspan="3"|Gain of dipole antennas
| |
| |-
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| | length '''L''' in <math>\scriptstyle{\lambda}</math>
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| | Gain
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| | Gain(dBi)
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| |-
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| | <math>\scriptstyle{\ll}</math> 0.1
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| | 1.50
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| |1.76
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| |-
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| |'''0.5'''
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| |'''1.64'''
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| |'''2.15'''
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| |-
| |
| |}
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| <br style="clear:both" /> | |
| | |
| ====Ideal half-wavelength dipole====
| |
| This type of antenna is a special case where each wire is exactly one quarter of the wavelength, for a total of a half wavelength. The [[radiation resistance]] is about 73 ohms if wire diameter is ignored, making it easily matched to a coaxial transmission line. The [[directivity]] is a constant 1.64, or 2.15 dB. Actual gain will be slightly lower due to ohmic losses.
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| | |
| If the dipole is not driven at the center, then the feed point resistance will be higher. If the feed point is distance ''x'' from one end of a half wave (''λ''/2) dipole, the resistance will be described by the following equation.
| |
| | |
| :<math>R_r = \frac{75\ \Omega}{\sin^2(2 \pi x / \lambda)}</math>
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| | |
| If taken to the extreme then the feed point resistance of a ''λ''/2 long rod is infinite, but it is possible to use a ''λ''/2 pole as an aerial; the right way to drive it is to connect it to one terminal of a parallel LC [[resonant circuit]]. The other side of the circuit must be connected to the braid of a [[coaxial cable]] lead and the core of the coaxial cable can be connected part-way up the coil from the RF ground side. An alternative means of feeding this system is to use a second coil that is magnetically coupled to the [[coil]] attached to the aerial.
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| | |
| ===Quarter-wave monopole===
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| [[File:A6-3EN.jpg|right|frame| The antenna and its image form a <math>\scriptstyle{{\lambda\over 2}}</math> dipole that radiates only upward.]]
| |
| The quarter-wave [[Monopole antenna|monopole]] antenna is a single-element antenna fed at one end, that behaves as a dipole antenna. It is formed by a conductor <math>\scriptstyle{{\lambda\over 4}}</math> in length, fed in the lower end, which is near a conductive surface which works as a reflector (see [[Antenna (radio)#Effect of ground|effect of ground]]) and is an example of a Marconi antenna. The current in the reflected image has the same direction and phase as the current in the real antenna. The quarter-wave conductor and its image together form a half-wave dipole that radiates only in the upper half of space.
| |
| | |
| In this upper side of space, the emitted field has the same amplitude of the field radiated by a half-wave dipole fed with the same current. Therefore, the total emitted power is half the emitted power of a half-wave dipole fed with the same current. As the current is the same, the radiation resistance (real part of series impedance) will be half of the series impedance of a half-wave dipole. As the reactive part is also divided by 2, the impedance of a quarter-wave antenna is <math>\scriptstyle{{73+i43\over 2}=36+i21}</math> ohms. Since the fields above ground are the same as for the dipole, but only half the power is applied, the gain is twice (3dB over) that of a half-wave dipole (<math>\scriptstyle{{\lambda\over 2}}</math>), that is, 5.14 dBi.
| |
| | |
| The earth can be used as ground plane, but it is a poor conductor. The reflected antenna image is only clear at glancing angles (far from the antenna). At these glancing angles, electromagnetic fields and radiation patterns are the same as for a half-wave dipole.
| |
| | |
| Naturally, the impedance of the earth is far inferior to that of a good conductor ground plane. This can be improved (at cost) by laying a copper mesh.
| |
| | |
| When ground is not available (such as in a vehicle) other metallic surfaces can serve as a ground plane (typically the vehicle's roof). Alternatively, radial wires placed at the base of the antenna can simulate a ground plane. For VHF bands, the radiating and ground plane elements can be constructed from rigid rods or tubes.
| |
| | |
| ===Folded dipole===
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| [[File:Folded dipole.jpg|thumb|upright|left|Folded dipole antenna]]
| |
| A folded dipole is a half-wave dipole with an additional wire connecting its two ends. If the additional wire has the same diameter and cross-section as the dipole, two nearly identical radiating currents are generated. The resulting far-field emission pattern is nearly identical to the one for the single-wire dipole described above; however, at resonance its input (feedpoint) impedance <math>R_{fd}</math> is four times the radiation resistance of a single-wire dipole. This is because for a fixed amount of power, the total radiating current <math>I_0</math> is equal to twice the current in each wire and thus equal to twice the current at the feed point. Equating the average radiated power to the average power delivered at the feedpoint, we may write
| |
| | |
| :<math> \frac{1}{2} R_{\frac{\lambda}{2}} I_0^2 = \frac{1}{2} R_{fd}\left( I_0/2 \right)^2. </math>
| |
| | |
| It follows that
| |
| | |
| :<math> R_{fd}= 4 R_{\frac{\lambda}{2}} \approx 292.32\ \Omega .</math>
| |
| | |
| The folded dipole is therefore well matched to 300-ohm balanced transmission lines. The [[T2FD]] antenna is a folded dipole.
| |
| | |
| Another common place one can see dipoles is as antennas for the FM band; these are folded dipoles. The tips of the antenna are folded back until they almost meet at the feedpoint, such that the antenna comprises one entire wavelength. This arrangement has a greater bandwidth than a standard half-wave dipole. If the conductor has a constant radius and cross-section, at resonance the input impedance is four times that of a half-wave dipole. Moreover, the folded dipole can be used for transforming the value of input impedance of the dipole over a broad range of step-up ratios by changing the thicknesses of the wire conductors for the fed- and folded-sides.<ref>{{cite journal |first=Yasuto |last=Mushiake |title=An Exact Impedance Step-Up Impedance-Ratio Chart of a Folded Antenna |journal=IRE. Trans. Ant. Prop. |volume=AP-3 |issue=4 |page=163 |date=October 1954 |url=http://www.sm.rim.or.jp/~ymushiak/sub.ire.chart.htm |accessdate=2014-01-10 |doi=}}</ref>
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| [[File:AmateurRadioAntenna2.JPG|thumb|right|upright|A self-made dipole antenna with mast]]
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| | |
| ===Other dipole antenna types===
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| There is a variety of other important dipole antennas.
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| * The ''bow-tie antenna'' is a dipole with flaring, triangular shaped arms. The shape gives it a much wider bandwidth than an ordinary dipole. It is widely used in UHF [[television antenna]]s.
| |
| * The [[G5RV Antenna]] is a dipole antenna with a symmetric feeder line, which also serves as a 1:1 impedance transformer allowing the transceiver to see the impedance of the antenna (it does not match the antenna to the 50-ohm transceiver. In fact the impedance will be somewhere around 90 ohms at the resonant frequency but significantly different at other frequencies).
| |
| * The [[Doublet Antenna]] is a dipole antenna with a resonant symmetric feeder line.
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| * The [[Sloper antenna]] is a slanted dipole antenna used for long-range communications or in limited space.
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| * The [[Near Vertical Incidence Skywave#The AS-2259 Antenna|AS-2259 Antenna]] is an inverted-V dipole antenna used for [[Near Vertical Incidence Skywave|NVIS]] communications.
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| | |
| ===General impedance formulas===
| |
| The complex radiation impedance of a dipole antenna is the sum of the real resistance ''R''<sub>dipole</sub> and the imaginary [[Electrical reactance|reactance]] ''X''<sub>dipole</sub>. In practice numerical solutions are required to get useful results but several attempts to solve the problem analytically has been done.
| |
| | |
| ====Induced EMF method====
| |
| Assuming sinusoidal current distribution, the Induced EMF method gives a rough estimate of reactance ''X'' and radiation resistance ''R'' for a dipole of length ''L'' and radius ''a'' operating at a frequency with wavenumber ''k'' in a medium with impedance ''Z'':
| |
| | |
| :<math>\begin{align}
| |
| R_\mathrm{dipole} &= \frac{Z}{2 \pi \sin^2(kL/2)} \Big\{
| |
| \gamma + \ln(kL) - \operatorname{Ci}(kL) + \tfrac{1}{2}\sin(kL) \big[\operatorname{Si}(2kL)- 2\operatorname{Si}(kL)\big] \\
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| &\qquad\qquad\qquad\qquad + \tfrac{1}{2}\cos(kL)\big[ \gamma + \ln(kL/2) + \operatorname{Ci}(2kL) - 2\operatorname{Ci}(kL) \big]
| |
| \Big\}
| |
| \end{align}</math>
| |
| | |
| :<math>\begin{align}
| |
| X_\mathrm{dipole} &= \frac{Z}{ 4 \pi \sin^2(kL/2)} \Big\{
| |
| 2 \operatorname{Si}(kL) + \cos(kL)\big[ 2 \operatorname{Si}(kL) - \operatorname{Si}(2kL) \big] \\
| |
| &\qquad\qquad\qquad\qquad - \sin(kL)\big[ 2 \operatorname{Ci}(kL) - \operatorname{Ci}(2kL) - \operatorname{Ci}(2ka^2/L) \big]
| |
| \Big\},
| |
| \end{align}</math>
| |
| where Ci and Si are the [[trigonometric integral|cosine and sine integral functions]] and ''γ'' is the [[Euler constant]].<ref>Chaotic behavior in receiver front-end limiters, F Caudron & A Ouslimani, Progress in Electromagnetics Research Letters, Vol 23 19-28 2011, pp 23-24</ref>
| |
| | |
| The Induced EMF method is inaccurate for dipoles longer than a half wavelength (''kL''>π) and verticals longer than quarter wavelength.<ref>[http://www.mikrocontroller.net/attachment/65931/antenna-impedance-models.pdf Steve Stearns, K6OIK - Antenna Impedance Models – Old and New], 2013-08-08</ref> Halléns integral solution and similar give more successful results.
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| | |
| ==Dipole as a reference standard==
| |
| Antenna [[gain]] is sometimes measured as dB relative to a dipole, which means that the antenna in question is being compared to a dipole, and has a certain amount of gain relative to a dipole antenna tuned to the same operating [[frequency]]. In this case, one says the antenna has a [[gain]] of "''x'' dBd" (see [[decibel]]). More often, gains are expressed relative to an [[isotropic radiator]], which is an imaginary aerial that radiates equally in all directions. In this case one uses dBi instead of dBd (see [[decibel]]). As it is impossible to build an isotropic radiator, gain measurements expressed relative to a dipole are more practical when a reference dipole aerial is used for experimental measurements. 0 dBd is often considered equal to 2.15 dBi.
| |
| | |
| From [[Babinet's principle]], a dipole antenna is complementary to a [[slot antenna]] consisting of a slot the same size and shape as a dipole cut from an infinite sheet of metal; both give the same radiation pattern.
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| | |
| ==Common applications==
| |
| | |
| ===Set-top TV antenna===
| |
| The most common dipole antenna is the type used with [[television]]s, often colloquially referred to as ''rabbit ears'' or ''bunny ears''. While in most applications the dipole elements are arranged along the same line, rabbit ears are adjustable in length and angle. Larger dipoles are sometimes hung in a V shape with the center near the radio equipment on the ground or the ends on the ground with the center supported. Shorter dipoles can be hung vertically. Some have extra elements to get better reception such as loops (especially for UHF transmissions), which can be turnable around a vertical axis, or a dial, which modifies the electrical properties of the antenna at each dial position.
| |
| | |
| ===Shortwave antenna===
| |
| Horizontal wire dipole antennas are popular for use on the [[high frequency|HF]] [[shortwave band]]s, both for transmitting and [[shortwave listening]]. They are usually constructed of two lengths of wire joined by a [[strain insulator]] in the center at which a [[ladder line]] or [[coaxial cable|coaxial]] [[feedline]] is attached, with the ends supported by buildings, towers, or trees. These are simple to put up for temporary or field use. For transmitting antennas, it is essential that the ends of the antenna be attached to supports through strain insulators with a sufficiently high [[electric arc|flashover voltage]], since the antenna's high voltage [[antinode]]s occur there.
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| | |
| ====Dipoles versus whip antennas====
| |
| Dipoles are generally more efficient than whip antennas (quarter-wave monopoles). The total radiated power and the radiation resistance are twice that of a quarter-wave monopole. Thus, if a whip antenna were used with an infinite perfectly conducting [[ground plane]], then it would be as efficient in half-space as a dipole in free space an infinite distance from any [[conductive]] surfaces such as the [[Ground (electricity)|earth]]'s surface. However, in real life situations, if considering the antenna height, a monopole may have an advantage at certain radiating angles, especially at low heights.
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| | |
| ===Dipole towers===
| |
| Large constructed half-wavelength dipole towers include the [[Warsaw radio mast]] — the only half-wave dipole for [[longwave]] ever built.
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| | |
| ===Collinear dipole arrays===
| |
| Vertical dipoles can be stacked end to end to make [[collinear antenna array]]s, to give a higher gain than a single dipole. The radiation pattern of the array is [[omnidirectional antenna|omnidirectional]] like a dipole, but the toroidal-shaped pattern is "flattened" so more of the power is radiated in horizontal directions and less is radiated up into the sky and down toward the ground and wasted. Collinear arrays are a higher gain alternative to [[whip antenna]]s for fixed base station antennas for mobile [[two-way radio]]s, such as police, fire, or taxi dispatchers.
| |
| | |
| ==See also==
| |
| *[[Electronic symbol]]
| |
| *[[Isotropic antenna]]
| |
| *[[Omnidirectional antenna]]
| |
| *[[Whip antenna]]
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| *[[Driven element]]
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| *[[Balun]]
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| *[[Coaxial antenna]]
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| *[[Amateur radio]]
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| *[[Shortwave listening]]
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| *[[T-aerial]]
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| *[[AM broadcasting]]
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| *[[FM broadcasting]]
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| | |
| ==References==
| |
| {{Reflist|33em}}
| |
| | |
| Elementary, short and half-wave dipoles:
| |
| {{refbegin|colwidth=33em}}
| |
| *{{Citation
| |
| |title= Electronic Radio and Engineering
| |
| |first= Federick R.
| |
| |last= Terman
| |
| |first2= Robert
| |
| |last2= Helliwell
| |
| |publisher= MacGraw-Hill
| |
| |year= 1955
| |
| |edition= 4th
| |
| |isbn= 978-0-07-085795-7
| |
| |doi= }}
| |
| *{{Citation
| |
| |title=Lectures on Physics
| |
| |last=Feynman
| |
| |last2= Leighton
| |
| |last3= Sands
| |
| |publisher= Addison-Wesley
| |
| |year=
| |
| |volume=
| |
| |isbn=
| |
| |doi= }}
| |
| *{{Citation
| |
| |title= Classical Electricity and Magnetism
| |
| |first= W.
| |
| |last= Panofsky
| |
| |first2= M.
| |
| |last2= Phillips
| |
| |publisher= Addison-Wesley
| |
| |year=
| |
| |isbn=
| |
| |doi= }}
| |
| * http://www.ece.rutgers.edu/~orfanidi/ewa/ Electromagnetic Waves and Antennas, Sophocles J. Orfanidis.
| |
| * [http://www.n0hr.com/hamradio/73/10/ham_radio10.htm Wire Antenna Resources for Ham Radio] Wire Antenna Resources including off center fed dipole (OCFD), dipole calculators and construction sites
| |
| * http://stewks.ece.stevens-tech.edu/sktpersonal.dir/sktwireless/lin-ant.pdf
| |
| * http://www.nt.hs-bremen.de/peik/asc/asc_antenna_slides.pdf
| |
| * [http://farside.ph.utexas.edu/teaching/em/lectures/node94.html The Hertzian dipole]
| |
| * {{Citation
| |
| |url=http://www.arrl.org/catalog/?category=Antennas,+Transmission+Lines+%26+Propagation#6133
| |
| |title= The ARRL Antenna Book
| |
| |edition= 21st
| |
| |year= 2007
| |
| |publisher= The American Radio Relay League, Inc.
| |
| |isbn= 0-87259-987-6
| |
| |doi=}}
| |
| * {{Citation
| |
| |url= http://www.arrl.org/catalog/?category=Antennas,+Transmission+Lines+%26+Propagation#7075
| |
| |title= ARRL's Wire Antenna Classics - A collection of the best articles from ARRL publications
| |
| |volume= 1
| |
| |edition= First
| |
| |year= 2005
| |
| |publisher= The American Radio Relay League, Inc.
| |
| |isbn= 0-87259-707-5
| |
| |doi= }}
| |
| * [http://web.archive.org/web/20050526142614/http://dibinst.mit.edu/DIBNER/DIConferences/OldConferences/Sloan/reflecti.htm Reflections on Hertz and the Hertzian Dipole] [[Jed Z. Buchwald]], MIT and the Dibner Institute for the History of Science and Technology (link inactive February 2, 2007; archive accessed from Wayback, March 13, 2011)
| |
| {{refend}}
| |
| | |
| ==External links==
| |
| *[http://www.emtalk.com/designer_tut_2.htm Dipole Antenna Tutorial] EM Talk
| |
| *[http://www.antenna-theory.com/antennas/broaddipole.php Broadband Dipoles] Antenna-Theory.com
| |
| *[http://www.ac6v.com/antprojects.htm AC6V's Homebrew Antennas Links]
| |
| *[http://www.eham.net/articles/24060 Your First HF Dipole] - simple yet complete tutorial from eham.net
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| | |
| {{Antenna Types}}
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| {{Analogue TV transmitter topics}}
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| {{DEFAULTSORT:Dipole Antenna}}
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| [[Category:Antennas (radio)]]
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| [[Category:Radio frequency antenna types]]
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| [[Category:Radio technology]]
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