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radio Radio is the technology of signaling and communicating using radio waves. Radio waves are electromagnetic waves of frequency between 30 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device called a transmit ...
and
telecommunications Telecommunication is the transmission of information by various types of technologies over wire, radio, optical, or other electromagnetic systems. It has its origin in the desire of humans for communication over a distance greater than that fe ...
a dipole antenna or doublet is the simplest and most widely used class of antenna. The dipole is any one of a class of antennas producing a radiation pattern approximating that of an elementary electric dipole with a radiating structure supporting a line current so energized that the current has only one node at each end. A dipole antenna commonly consists of two identical conductive elements such as metal wires or rods. The driving current from the
transmitter In electronics and telecommunications, a radio transmitter or just transmitter is an electronic device which produces radio waves with an antenna. The transmitter itself generates a radio frequency alternating current, which is applied to th ...
is applied, or for receiving antennas the output signal to the receiver is taken, between the two halves of the antenna. Each side of the
feedline In a radio antenna, the feed line (feedline), or feeder, is the cable or other transmission line that connects the antenna with the radio transmitter or receiver. In a transmitting antenna, it feeds the radio frequency (RF) current from th ...
to the transmitter or receiver is connected to one of the conductors. This contrasts with a
monopole antenna A monopole antenna is a class of radio antenna consisting of a straight rod-shaped conductor, often mounted perpendicularly over some type of conductive surface, called a ground plane. The driving signal from the transmitter is applied, or ...
, 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. A common example of a dipole is the "rabbit ears"
television antenna A television antenna (TV aerial) is an antenna specifically designed for use with a television receiver (TV) to receive over-the-air broadcast television signals from a television station. Television reception is dependent upon the antenna as ...
found on broadcast television sets. The dipole is the simplest type of antenna from a theoretical point of view. Most commonly it consists of two conductors of equal length oriented end-to-end with the feedline connected between them. Dipoles are frequently used as resonant antennas. If the feedpoint of such an antenna is shorted, then it will be able to
resonate Resonance describes the phenomenon of increased amplitude that occurs when the frequency of an applied periodic force (or a Fourier component of it) is equal or close to a natural frequency of the system on which it acts. When an oscill ...
at a particular frequency, just like a guitar
string String or strings may refer to: *String (structure), a long flexible structure made from threads twisted together, which is used to tie, bind, or hang other objects Arts, entertainment, and media Films * ''Strings'' (1991 film), a Canadian anim ...
that is plucked. Using the antenna at around that frequency is advantageous in terms of feedpoint impedance (and thus
standing wave ratio In radio engineering and telecommunications, standing wave ratio (SWR) is a measure of impedance matching of loads to the characteristic impedance of a transmission line or waveguide. Impedance mismatches result in standing waves along the transmi ...
), so its length is determined by the intended
wavelength In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tro ...
(or frequency) of operation. The most commonly used is the center-fed half-wave dipole which is just under a half-wavelength long. The
radiation pattern In the field of antenna design the term radiation pattern (or antenna pattern or far-field pattern) refers to the ''directional'' (angular) dependence of the strength of the radio waves from the antenna or other source.Constantine A. Balanis: “ ...
of the half-wave dipole is maximum perpendicular to the conductor, falling to zero in the axial direction, thus implementing an
omnidirectional antenna In radio communication, an omnidirectional antenna is a class of antenna which radiates equal radio power in all directions perpendicular to an axis (azimuthal directions), with power varying with angle to the axis (elevation angle), declining ...
if installed vertically, or (more commonly) a weakly directional antenna if horizontal. Although they may be used as standalone low-gain antennas, dipoles are also employed as
driven element {{Unreferenced, date=April 2016 In a multielement antenna array (such as a Yagi–Uda antenna), the driven element or active element is the element in the antenna (typically a metal rod) which is electrically connected to the receiver or transm ...
s in more complex antenna designs such as the
Yagi antenna Yagi may refer to: Places * Yagi, Kyoto, in Japan * Yagi (Kashihara), in Nara Prefecture, Japan * Yagi-nishiguchi Station, in Kashihara, Nara, Japan * Kami-Yagi Station, a JR-West Kabe Line station located in 3-chōme, Yagi, Asaminami-ku, Hiroshima ...
and driven
arrays An array is a systematic arrangement of similar objects, usually in rows and columns. Things called an array include: {{TOC right Music * In twelve-tone and serial composition, the presentation of simultaneous twelve-tone sets such that the ...
. Dipole antennas (or such designs derived from them, including the monopole) are used to feed more elaborate
directional antenna A directional antenna or beam antenna is an antenna which radiates or receives greater power in specific directions allowing increased performance and reduced interference from unwanted sources. Directional antennas provide increased performanc ...
s such as a
horn antenna A horn antenna or microwave horn is an antenna that consists of a flaring metal waveguide shaped like a horn to direct radio waves in a beam. Horns are widely used as antennas at UHF and microwave frequencies, above 300 MHz. They are ...
,
parabolic reflector A parabolic (or paraboloid or paraboloidal) reflector (or dish or mirror) is a reflective surface used to collect or project energy such as light, sound, or radio waves. Its shape is part of a circular paraboloid, that is, the surface generated ...
, or
corner reflector A corner reflector is a retroreflector consisting of three mutually perpendicular, intersecting flat surfaces, which reflects waves directly towards the source, but translated. The three intersecting surfaces often have square shapes. Radar co ...
. Engineers analyze vertical (or other monopole) antennas on the basis of dipole antennas of which they are one half.


History

German physicist
Heinrich Hertz Heinrich Rudolf Hertz ( ; ; 22 February 1857 – 1 January 1894) was a German physicist who first conclusively proved the existence of the electromagnetic waves predicted by James Clerk Maxwell's equations of electromagnetism. The unit o ...
first demonstrated the existence of
radio wave Radio waves are a type of electromagnetic radiation with the longest wavelengths in the electromagnetic spectrum, typically with frequencies of 300 gigahertz (GHz) and below. At 300 GHz, the corresponding wavelength is 1 mm (shor ...
s in 1887 using what we now know as a dipole antenna (with capacitative end-loading). On the other hand,
Guglielmo Marconi Guglielmo Giovanni Maria Marconi, 1st Marquis of Marconi (; 25 April 187420 July 1937) was an Italian inventor and electrical engineer, known for his creation of a practical radio wave-based wireless telegraph system. This led to Marconi bei ...
empirically found that he could just ground the transmitter (or one side of a transmission line, if used) dispensing with one half of the antenna, thus realizing the ''vertical'' or
monopole antenna A monopole antenna is a class of radio antenna consisting of a straight rod-shaped conductor, often mounted perpendicularly over some type of conductive surface, called a ground plane. The driving signal from the transmitter is applied, or ...
. For the low frequencies Marconi employed to achieve long-distance communications, this form was more practical; when radio moved to higher frequencies (especially VHF transmissions for FM radio and TV) it was advantageous for these much smaller antennas to be entirely atop a tower thus requiring a dipole antenna or one of its variations. In the early days of radio, the thus-named Marconi antenna (monopole) and the doublet (dipole) were seen as distinct inventions. Now, however, the "monopole" antenna is understood as a special case of a dipole which has a virtual element "underground".


Dipole variations


Short dipole

A short dipole is a dipole formed by two conductors with a total length substantially less than a half wavelength (). Short dipoles are sometimes used in applications where a full half-wave dipole would be too large. They can be analyzed easily using the results obtained
below Below may refer to: *Earth * Ground (disambiguation) *Soil *Floor * Bottom (disambiguation) *Less than *Temperatures below freezing *Hell or underworld People with the surname *Ernst von Below (1863–1955), German World War I general *Fred Below ...
for the Hertzian dipole, a fictitious entity. Being shorter than a resonant antenna (half wavelength long) its feedpoint impedance includes a large
capacitive reactance In electrical circuits, reactance is the opposition presented to alternating current by inductance or capacitance. Greater reactance gives smaller current for the same applied voltage. Reactance is similar to resistance in this respect, but does ...
requiring a
loading coil A loading coil or load coil is an inductor that is inserted into an electronic circuit to increase its inductance. The term originated in the 19th century for inductors used to prevent signal distortion in long-distance telegraph transmission c ...
or other matching network in order to be practical, especially as a transmitting antenna. To find the far-field electric and magnetic fields generated by a short dipole we use the result shown below for the Hertzian dipole (an infinitesimal current element) at a distance from the current and at an angle to the direction of the current, as being: :\begin H_\phi &= j\ \frac\ e^\ \sin(\theta) \\ E_\theta &= \zeta_0\ H_\phi = j\ \frac\ e^\ \sin(\theta) ~. \end where the radiator consists of a current of \ I_h\ e^\ over a short length and \ j^2 = -1\ in electronics replaces the customary mathematical symbol for the "square root of −1". is the radian frequency (\omega \equiv 2 \pi f\ ) and is the wavenumber (\ k \equiv 2\pi / \lambda\ ). 0 is the
impedance of free space The impedance of free space, , is a physical constant relating the magnitudes of the electric and magnetic fields of electromagnetic radiation travelling through free space. That is, , where is the electric field strength and is the magnetic field ...
(\zeta_0 \approx 377\text), which is the ratio of a free space plane wave's electric to magnetic field strength. The feedpoint is usually at the center of the dipole as shown in the diagram. The current along dipole arms are approximately described as proportional to \ \sin(k\ z)\ where is the distance to the end of the arm. In the case of a short dipole, that is essentially a linear drop from \ I_0\ at the feedpoint to zero at the end. Therefore, this is comparable to a Hertzian dipole with an ''effective'' current h equal to the average current over the conductor, so \ I_h = \frac I_0\ . With that substitution, the above equations closely approximate the fields generated by a short dipole fed by current \ I_0\ . From the fields calculated above, one can find the radiated ''flux'' (power per unit area) at any point as the magnitude of the real part of the
Poynting vector In physics, the Poynting vector (or Umov–Poynting vector) represents the directional energy flux (the energy transfer per unit area per unit time) or '' power flow'' of an electromagnetic field. The SI unit of the Poynting vector is the watt ...
, , which is given by \ \frac \mathbf \times \mathbf^*\ . With and being at right angles and in phase, there is no imaginary part and is simply equal to \ \frac E_\theta H_\phi^*\ with the phase factors (the exponentials) cancelling out leaving: : S = \frac E_\theta H_\phi^* = \frac\ \frac\ \sin^2(\theta) = \frac\ I_0^2\ \left(\frac\right)^2\ \frac\ \sin^2(\theta) ~. We have now expressed the flux in terms of the feedpoint current 0 and the ratio of the short dipole's length to the wavelength of radiation . The radiation pattern given by \ \sin^2(\theta)\ is seen to be similar to and only slightly less directional than that of the half-wave dipole. Using the above expression for the radiation in the far field for a given feedpoint current, we can integrate over all
solid angle In geometry, a solid angle (symbol: ) is a measure of the amount of the field of view from some particular point that a given object covers. That is, it is a measure of how large the object appears to an observer looking from that point. The poi ...
to obtain the total radiated power. :P_\text = \frac\ \zeta_0\ I_0^2\ \left( \frac \right)^2 ~. From that, it is possible to infer the
radiation resistance Radiation resistance, \ R_\mathsf\ or \ R_\mathsf\ , is proportional to the part of an antenna's feedpoint electrical resistance that is caused by power loss from the emission of radio waves from the antenna. Radiation resistance is an ''effect ...
, equal to the resistive (real) part of the feedpoint impedance, neglecting a component due to ohmic losses. By setting total to the power supplied at the feedpoint \ \frac\ I_0^2\ R_\text\ we find: :R_\text = \frac\ \zeta_0\ \left(\frac\right)^2 \approx \left(\frac\right)^2 (197\ \Omega) ~. Again, these become exact for Setting despite its use not quite being valid for so large a fraction of the wavelength, the formula would predict a radiation resistance of 49 Ω, instead of the actual value of 73 Ω produced by a half-wave dipole when more correct quarter-wave sinusoidal currents are used.


Dipole antennas of various lengths

The fundamental resonance of a thin linear conductor occurs at a frequency whose free-space wavelength is ''twice'' the wire's length; i.e. where the conductor is 1/2 wavelength long. Dipole antennas are frequently used at around that frequency and thus termed ''half-wave dipole'' antennas. This important case is dealt with in the next section. Thin linear conductors of length ''l'' are in fact resonant at any integer multiple of a half-wavelength: :l = n \frac where ''λ'' = ''c''/''f'' is the wavelength and ''n'' is an integer. For a center-fed dipole, however, there is a great dissimilarity between ''n'' being odd or being even. Dipoles which are an ''odd'' number of half-wavelengths in length have reasonably low driving point impedances (which are purely resistive at that resonant frequency). However ones which are an ''even'' number of half-wavelengths in length, that is, an integer number of wavelengths in length, have a ''high'' driving point impedance (albeit purely resistive at that resonant frequency). For instance, a full-wave dipole antenna can be made with two half-wavelength conductors placed end to end for a total length of approximately ''L'' = λ. This results in an additional gain over a half-wave dipole of about 2 dB. Full wave dipoles can be used in short wave broadcasting only by making the effective diameter very large and feeding from a high impedance balanced line. Cage dipoles are often used to get the large diameter. A 5/4-wave dipole antenna has a much lower but not purely resistive feedpoint impedance, which requires a
matching network In electronics, impedance matching is the practice of designing or adjusting the input impedance or output impedance of an electrical device for a desired value. Often, the desired value is selected to maximize power transfer or minimize signal ...
to the impedance of the transmission line. Its gain is about 3 dB greater than a half-wave dipole, the highest gain of any dipole of any similar length. Other reasonable lengths of dipole do not offer advantages and are seldom used. However the overtone resonances of a half-wave dipole antenna at odd multiples of its fundamental frequency are sometimes exploited. For instance,
amateur radio Amateur radio, also known as ham radio, is the use of the radio frequency spectrum for purposes of non-commercial exchange of messages, wireless experimentation, self-training, private recreation, radiosport, contesting, and emergency communica ...
antennas designed as half-wave dipoles at 7 MHz can also be used as 3/2-wave dipoles at 21 MHz; likewise VHF television antennas resonant at the low VHF television band (centered around 65 MHz) are also resonant at the high VHF television band (around 195 MHz).


Half-wave dipole

A half-wave dipole antenna consists of two quarter-wavelength conductors placed end to end for a total length of approximately ''L'' = λ/2. The current distribution is that of a
standing wave In physics, a standing wave, also known as a stationary wave, is a wave that oscillates in time but whose peak amplitude profile does not move in space. The peak amplitude of the wave oscillations at any point in space is constant with respect ...
, approximately sinusoidal along the length of the dipole, with a node at each end and an antinode (peak current) at the center (feedpoint): :I(z) = I_0 \cos \cos kz, where ''k'' = 2''π''/''λ'' and ''z'' runs from −''L''/2 to ''L''/2. In the far field, this produces a radiation pattern whose electric field is given by :E_\theta = \frac \frac \sin. The directional factor cos ''π''/2)cos ''θ''sin ''θ'' is barely different from sin ''θ'' applying to the short dipole, resulting in a very similar radiation pattern as noted above. A numerical integration of the radiated power / over all solid angle, as we did for the short dipole, obtains a value for the total power Ptotal radiated by the dipole with a current having a peak value of ''I''0 as in the form specified above. Dividing ''P''total by 4π''R''2 supplies the flux at a distance ''R'' ''averaged'' over all directions. Dividing the flux in the direction (where it is at its peak) at distance R by that average flux, we find the directive gain to be 1.64. This can also be directly computed using the
cosine integral In mathematics, trigonometric integrals are a family of integrals involving trigonometric functions. Sine integral The different sine integral definitions are \operatorname(x) = \int_0^x\frac\,dt \operatorname(x) = -\int_x^\infty\frac ...
: :G = \frac \approx 1.64 \; (2.15 dBi) (Note that the
cosine integral In mathematics, trigonometric integrals are a family of integrals involving trigonometric functions. Sine integral The different sine integral definitions are \operatorname(x) = \int_0^x\frac\,dt \operatorname(x) = -\int_x^\infty\frac ...
Cin(''x'') is not the same as the cosine integral Ci(''x''). Both
MATLAB MATLAB (an abbreviation of "MATrix LABoratory") is a proprietary multi-paradigm programming language and numeric computing environment developed by MathWorks. MATLAB allows matrix manipulations, plotting of functions and data, implementatio ...
and
Mathematica Wolfram Mathematica is a software system with built-in libraries for several areas of technical computing that allow machine learning, statistics, symbolic computation, data manipulation, network analysis, time series analysis, NLP, optimizat ...
have inbuilt functions which compute Ci(''x''), but not Cin(''x''). See the Wikipedia page on
cosine integral In mathematics, trigonometric integrals are a family of integrals involving trigonometric functions. Sine integral The different sine integral definitions are \operatorname(x) = \int_0^x\frac\,dt \operatorname(x) = -\int_x^\infty\frac ...
for the relationship between these functions.) We can now also find the radiation resistance as we did for the short dipole by solving: :P_\text = \frac I_0^2 R_\text to obtain: :R_\text \approx 73.1\ \Omega. Using the induced EMF method, the real part of the driving point impedance can also be written in terms of the cosine integral, obtaining the same result: :R_\text = \frac \operatorname(2\pi) = \frac \int_0^ \frac d \theta \approx 73.1\ \Omega. If a half-wave dipole is driven at a point other the center, then the feed point resistance will be higher. The radiation resistance is ''usually'' expressed relative to the maximum current present along an antenna element, which for the half-wave dipole (and most other antennas) is also the current at the feedpoint. However, if the dipole is fed at a different point at a distance ''x'' from a current maximum (the center in the case of a half-wave dipole), then the current there is not ''I''0 but only ''I''0 cos(''kx''). In order to supply the same power, the voltage at the feedpoint has to be similarly ''increased'' by the factor 1/cos(''kx''). Consequently, the resistive part of the feedpoint impedance Re(''V''/''I'') is increased by the factor 1/cos2(''kx''): :R_\text = \frac \approx \frac This equation can also be used for dipole antennas of other lengths, provided that ''R''radiation has been computed relative to the current maximum, which is ''not'' generally the same as the feedpoint current for dipoles longer than half-wave. Note that this equation breaks down when feeding an antenna near a current node, where cos(kx) approaches zero. Indeed, the driving point impedance rises greatly, but is nevertheless limited due to quadrature components of the elements' current which is ignored in the above model for the current distribution.


Folded dipole

A folded dipole is a half-wave dipole with an additional parallel 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, but at resonance its feedpoint impedance R_\text is four times the radiation resistance of a single-wire dipole. A folded "dipole" is, technically, a folded full-wave
loop antenna A loop antenna is a radio antenna consisting of a loop or coil of wire, tubing, or other electrical conductor, that is usually fed by a balanced source or feeding a balanced load. Within this physical description there are two (possibly three) di ...
, where the loop has been bent at opposing ends and squashed into two parallel wires in a flat line. Although the broad bandwidth, high feedpoint impedance, and high efficiency are characteristics more similar to a full loop antenna, the folded dipole's radiation pattern is more like an ordinary dipole. Since the operation of a single halfwave dipole is easier to understand, both full loops and folded dipoles are often described as two halfwave dipoles in parallel, connected at the ends. The high feedpoint impedance R_\text at resonance is because for a fixed amount of power, the total radiating current I_0 is equal to twice the current in each wire separately and thus equal to twice the current at the feed point. We equate the average radiated power to the average power delivered at the feedpoint, we may write :\frac R_\text I_0^2 = \frac R_\text\left(\frac\right)^2 , where R_\text is the lower feedpoint impedance of the resonant halfwave dipole. It follows that :R_\text = 4 R_\text \approx 292\ \Omega. The folded dipole is therefore well matched to 300 ohm balanced transmission lines, such as twin-feed ribbon cable. The folded dipole has a wider bandwidth than a single dipole. They 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. Instead of altering thickness or spacing, one can add a third parallel wire to increase the antenna impedance to 9 times that of a single-wire dipole, raising the impedance to 658 Ω, making a good match for open wire feed cable, and further broadening the resonant frequency band of the antenna. Half-wave folded dipoles are often used for
FM radio FM broadcasting is a method of radio broadcasting using frequency modulation (FM). Invented in 1933 by American engineer Edwin Armstrong, wide-band FM is used worldwide to provide high fidelity sound over broadcast radio. FM broadcasting is capa ...
antennas; versions made with
twin lead Twin-lead cable is a two-conductor flat cable used as a balanced transmission line to carry radio frequency (RF) signals. It is constructed of two stranded or solid copper or copper-clad steel wires, held a precise distance apart by a plastic ( ...
which can be hung on an inside wall often come with FM tuners. The
T2FD The ''Tilted Terminated Folded Dipole'' (T²FD, T2FD, or TTFD) or ''Balanced Termination, Folded Dipole'' (BTFD) - also known as W3HH antenna - is a general-purpose shortwave antenna developed in the late 1940s by the United States Navy. It per ...
antenna is a folded dipole. They are also widely used as
driven element {{Unreferenced, date=April 2016 In a multielement antenna array (such as a Yagi–Uda antenna), the driven element or active element is the element in the antenna (typically a metal rod) which is electrically connected to the receiver or transm ...
s for rooftop Yagi
television antenna A television antenna (TV aerial) is an antenna specifically designed for use with a television receiver (TV) to receive over-the-air broadcast television signals from a television station. Television reception is dependent upon the antenna as ...
s.


Other variants

There are numerous modifications to the shape of a dipole antenna which are useful in one way or another but result in similar radiation characteristics (low gain). This is not to mention the many
directional antenna A directional antenna or beam antenna is an antenna which radiates or receives greater power in specific directions allowing increased performance and reduced interference from unwanted sources. Directional antennas provide increased performanc ...
s which include one or more dipole elements in their design as
driven element {{Unreferenced, date=April 2016 In a multielement antenna array (such as a Yagi–Uda antenna), the driven element or active element is the element in the antenna (typically a metal rod) which is electrically connected to the receiver or transm ...
s, many of which are linked to in the information box at the bottom of this page. * 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 A television antenna (TV aerial) is an antenna specifically designed for use with a television receiver (TV) to receive over-the-air broadcast television signals from a television station. Television reception is dependent upon the antenna as ...
s. * The '' cage dipole'' is a similar modification in which the bandwidth is increased by using fat cylindrical dipole elements made of a "cage" of wires (see photo). These are used in a few broadband array antennas in the
medium wave Medium wave (MW) is the part of the medium frequency (MF) radio band used mainly for AM radio broadcasting. The spectrum provides about 120 channels with more limited sound quality than FM stations on the FM broadcast band. During the daytime ...
and
shortwave Shortwave radio is radio transmission using shortwave (SW) radio frequencies. There is no official definition of the band, but the range always includes all of the high frequency band (HF), which extends from 3 to 30 MHz (100 to 10 me ...
bands for applications such as
over-the-horizon radar Over-the-horizon radar (OTH), sometimes called beyond the horizon radar (BTH), is a type of radar system with the ability to detect targets at very long ranges, typically hundreds to thousands of kilometres, beyond the radar horizon, which is ...
and
radio telescope A radio telescope is a specialized antenna and radio receiver used to detect radio waves from astronomical radio sources in the sky. Radio telescopes are the main observing instrument used in radio astronomy, which studies the radio frequency ...
s. * A ''
halo antenna A halo antenna, or halo, is a center-fed dipole antenna, which has been bent into a circle, with a break directly opposite the feed point. The dipole ends are close, but do not touch, and may be widened to form an air capacitor, whose spacing is ...
'' is a half-wave dipole bent into a circle for a nearly uniform radiation pattern in the plane of the circle. When the halo's circle is horizontal, it produces horizontally polarized radiation in a nearly omnidirectional pattern with little power wasted toward the sky, compared to a straight horizontal dipole. In practice, it is categorized both as a bent dipole and as a loop antenna, depending on author preference. * A ''
turnstile antenna A turnstile antenna, or crossed-dipole antenna, is a radio antenna consisting of a set of two identical dipole antennas mounted at right angles to each other and fed in phase quadrature; the two currents applied to the dipoles are 90° out of pha ...
'' comprises two dipoles crossed at a right angle and feed system which introduces a quarter-wave phase difference between the currents along the two. With that geometry, the two dipoles do not interact electrically but their fields add in the far-field producing a net radiation pattern which is rather close to
isotropic Isotropy is uniformity in all orientations; it is derived . Precise definitions depend on the subject area. Exceptions, or inequalities, are frequently indicated by the prefix ' or ', hence ''anisotropy''. ''Anisotropy'' is also used to describe ...
, with horizontal polarization in the plane of the elements and
circular Circular may refer to: * The shape of a circle * ''Circular'' (album), a 2006 album by Spanish singer Vega * Circular letter (disambiguation) ** Flyer (pamphlet), a form of advertisement * Circular reasoning, a type of logical fallacy * Circular ...
or elliptical polarization at other angles. Turnstile antennas can be stacked and fed in phase to realize an omnidirectional broadside array or phased for an end-fire array with circular polarization. * The ''
batwing antenna A batwing or super turnstile antenna is a type of broadcasting antenna used at VHF and UHF frequencies, named for its distinctive shape which resembles a bat wing or bow tie. Stacked arrays of batwing antennas are used as television broadcast ...
'' is a
turnstile antenna A turnstile antenna, or crossed-dipole antenna, is a radio antenna consisting of a set of two identical dipole antennas mounted at right angles to each other and fed in phase quadrature; the two currents applied to the dipoles are 90° out of pha ...
with its linear elements widened as in a bow-tie antenna, again for the purpose of widening its resonant frequency and thus usable over a larger bandwidth, without re-tuning. When stacked to form an array the radiation is omnidirectional, horizontally polarized, and with increased gain at low elevations, making it ideal for television broadcasting. * A V (or "Vee") antenna is a dipole with a bend in the middle so its arms are at an angle instead of co-linear. * A ''quadrant'' antenna is a 'V' antenna with an unusual overall length of a ''full'' wavelength, with two half-wave horizontal elements meeting at a right angle where it is fed. Quadrant antennas produce mostly
horizontal polarization Polarization ( also polarisation) is a property applying to transverse waves that specifies the geometrical orientation of the oscillations. In a transverse wave, the direction of the oscillation is perpendicular to the direction of motion of th ...
at low to intermediate elevation angles and have nearly
omnidirectional Omnidirectional refers to the notion of existing in every direction. Omnidirectional devices include: * Omnidirectional antenna, an antenna that radiates equally in all directions * VHF omnidirectional range, a type of radio navigation system for ...
radiation patterns. One implementation uses "cage" elements (see above); the thickness of the resulting elements lowers the high driving point impedance of a full-wave dipole to a value that accommodates a reasonable match to open wire lines and increases the bandwidth (in terms of SWR) to a full octave. They are used for HF band transmissions. * The '' G5RV antenna'' is a dipole antenna fed indirectly, through a carefully chosen length of 300 Ω or 450 Ω
twin lead Twin-lead cable is a two-conductor flat cable used as a balanced transmission line to carry radio frequency (RF) signals. It is constructed of two stranded or solid copper or copper-clad steel wires, held a precise distance apart by a plastic ( ...
, which acts as an impedance
matching network In electronics, impedance matching is the practice of designing or adjusting the input impedance or output impedance of an electrical device for a desired value. Often, the desired value is selected to maximize power transfer or minimize signal ...
to connect (through a
balun A balun (from "balanced to unbalanced", originally, but now dated from "balancing unit") is an electrical device that allows balanced and unbalanced lines to be interfaced without disturbing the impedance arrangement of either line. A balun ...
) to a standard 50 Ω coaxial transmission line. * The '' sloper antenna'' is a slanted vertical dipole antenna attached to the top of a single tower. The element can be center-fed or can be end-fed as an unbalanced monopole antenna from a transmission line at the top of the tower, in which case the monopole's "ground" connection can better be viewed as a second element comprising the tower and/or transmission line shield. * The '' inverted "V" antenna'' is likewise supported using a single tower but is a balanced antenna with two symmetric elements angled toward the ground. It is thus a half-wave dipole with a bend in the middle. Like the sloper, this has the practical advantage of elevating the antenna but requiring only a ''single'' tower. * The ''AS-2259 antenna'' is an inverted-‘V’ dipole antenna used for local communications via Near Vertical Incidence Skywave (NVIS).


Vertical (monopole) antennas

The "vertical", "Marconi", or
monopole antenna A monopole antenna is a class of radio antenna consisting of a straight rod-shaped conductor, often mounted perpendicularly over some type of conductive surface, called a ground plane. The driving signal from the transmitter is applied, or ...
is a single-element antenna usually fed at the bottom (with the shield side of its unbalanced transmission line connected to ground). It behaves essentially as a dipole antenna. The ground (or
ground plane In electrical engineering, a ground plane is an electrically conductive surface, usually connected to electrical ground. The term has two different meanings in separate areas of electrical engineering. *In antenna theory, a ground plane is a ...
) is considered to be a conductive surface which works as a reflector (see effect of ground). Vertical currents in the reflected image have the same direction (thus are ''not'' reflected about the ground) and phase as the current in the real antenna. The conductor and its image together act as a dipole in the upper half of space. Like a dipole, in order to achieve resonance (resistive feedpoint impedance) the conductor must be close to a quarter wavelength in height (like each conductor in a half-wave dipole). In this upper side of space, the emitted field has the same amplitude of the field radiated by a similar dipole fed with the same current. Therefore, the total emitted power is half the emitted power of a 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 the comparable dipole. A quarter-wave monopole, then, has an impedance of \frac = 36 + i21 Ω. Another way of seeing this, is that a true dipole receiving a current I has voltages on its terminals of +V and −V, for an impedance across the terminals of 2V/I, whereas the comparable vertical antenna has the current I but an applied voltage of only V. Since the fields above ground are the same as for the dipole, but only half the power is applied, the gain is doubled to 5.14 dBi. This is not an actual performance advantage per se, since in practice a dipole also reflects half of its power off the ground which (depending on the antenna height and sky angle) can augment (or cancel!) the direct signal. The vertical polarization of the monopole (as for a vertically oriented dipole) is advantageous at low elevation angles where the ground reflection combines with the direct wave approximately in phase. The earth acts as a ground plane, but it can be a poor conductor leading to losses. Its conductivity can be improved (at cost) by laying a copper mesh. When an actual 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 form a ground plane. For VHF and UHF bands, the radiating and ground plane elements can be constructed from rigid rods or tubes. Using such an artificial ground plane allows for the entire antenna and "ground" to be mounted at an arbitrary height. One common modification has the radials forming the ground plane sloped down, which has the effect of raising the feedpoint impedance to around 50 Ω, matching common coaxial cable. No longer being a true ground, a
balun A balun (from "balanced to unbalanced", originally, but now dated from "balancing unit") is an electrical device that allows balanced and unbalanced lines to be interfaced without disturbing the impedance arrangement of either line. A balun ...
(such as a simple choke balun) is then recommended.


Dipole characteristics


Impedance of dipoles of various lengths

The feedpoint impedance of a dipole antenna is sensitive to its
electrical length In electrical engineering, electrical length is a dimensionless parameter equal to the physical length of an electrical conductor such as a cable or wire, divided by the wavelength of alternating current at a given frequency traveling through th ...
and feedpoint position. Therefore, a dipole will generally only perform optimally over a rather narrow bandwidth, beyond which its impedance will become a poor match for the transmitter or receiver (and transmission line). The real (resistive) and imaginary (reactive) components of that impedance, as a function of electrical length, are shown in the accompanying graph. The detailed calculation of these numbers are described
below Below may refer to: *Earth * Ground (disambiguation) *Soil *Floor * Bottom (disambiguation) *Less than *Temperatures below freezing *Hell or underworld People with the surname *Ernst von Below (1863–1955), German World War I general *Fred Below ...
. Note that the value of the reactance is highly dependent on the diameter of the conductors; this plot is for conductors with a diameter of 0.001 wavelengths. Dipoles that are much smaller than one half the wavelength of the signal are called ''short dipoles''. These have a very low
radiation resistance Radiation resistance, \ R_\mathsf\ or \ R_\mathsf\ , is proportional to the part of an antenna's feedpoint electrical resistance that is caused by power loss from the emission of radio waves from the antenna. Radiation resistance is an ''effect ...
(and a high capacitive reactance) making them inefficient antennas. More of a transmitter's current is dissipated as heat due to the finite resistance of the conductors which is greater than the radiation resistance. However they can nevertheless be practical receiving antennas for longer wavelengths. Dipoles whose length is approximately half the wavelength of the signal are called ''half-wave dipoles'' and are widely used as such or as the basis for derivative antenna designs. These have a radiation resistance which is much greater, closer to the characteristic impedances of available
transmission line In electrical engineering, a transmission line is a specialized cable or other structure designed to conduct electromagnetic waves in a contained manner. The term applies when the conductors are long enough that the wave nature of the transmis ...
s, and normally much larger than the resistance of the conductors, so that their
efficiency Efficiency is the often measurable ability to avoid wasting materials, energy, efforts, money, and time in doing something or in producing a desired result. In a more general sense, it is the ability to do things well, successfully, and without ...
approaches 100%. In general radio engineering, the term ''dipole'', if not further qualified, is taken to mean a center-fed half-wave dipole. A true half-wave dipole is one half of the wavelength λ in length, where in free space. Such a dipole has a feedpoint impedance consisting of 73Ω resistance and +43Ω reactance, thus presenting a slightly inductive reactance. To cancel that reactance, and present a pure resistance to the feedline, the element is shortened by the factor ''k'' for a net length ''\ell'' of: :\ell = \frac k \lambda = \frac k \frac where ''λ'' is the free-space wavelength, ''c'' is the
speed of light The speed of light in vacuum, commonly denoted , is a universal physical constant that is important in many areas of physics. The speed of light is exactly equal to ). According to the special theory of relativity, is the upper limit for ...
in free space, and ''f'' is the frequency. The adjustment factor ''k'' which causes feedpoint reactance to be eliminated, depends on the diameter of the conductor, as is plotted in the accompanying graph. ''k'' ranges from about 0.98 for thin wires (diameter, 0.00001 wavelengths) to about 0.94 for thick conductors (diameter, 0.008 wavelengths). This is because the effect of antenna length on reactance (upper graph) is much greater for thinner conductors, so that a smaller deviation from the exact half wavelength is required in order to cancel the 43 Ω inductive reactance it has when exactly λ/2. For the same reason, antennas with thicker conductors have a wider operating bandwidth over which they attain a practical
standing wave ratio In radio engineering and telecommunications, standing wave ratio (SWR) is a measure of impedance matching of loads to the characteristic impedance of a transmission line or waveguide. Impedance mismatches result in standing waves along the transmi ...
which is degraded by any remaining reactance. For a typical ''k'' of about 0.95, the above formula for the corrected antenna length can be written, for a length in metres as 143/''f'', or a length in feet as 468/''f'' where ''f'' is the frequency in megahertz. Dipole antennas of lengths approximately equal to any ''odd'' multiple of  λ are also resonant, presenting a small reactance (which can be cancelled by a small length adjustment). However these are rarely used. One size that is more practical though is a dipole with a length of wavelengths. Not being close to wavelengths, this antenna's impedance has a large (negative) reactance and can only be used with an
impedance matching In electronics, impedance matching is the practice of designing or adjusting the input impedance or output impedance of an electrical device for a desired value. Often, the desired value is selected to maximize power transfer or minimize signal ...
network (a so-called
antenna tuner An antenna tuner (and any of the names in the list below) is a device that is inserted between a radio transmitter and its antenna; when placed close by the antenna and properly adjusted (tuned) it optimizes power transfer by matching the im ...
). It is a desirable length because such an antenna has the highest gain for any dipole which isn't a great deal longer.


Radiation pattern and gain

A dipole is
omnidirectional Omnidirectional refers to the notion of existing in every direction. Omnidirectional devices include: * Omnidirectional antenna, an antenna that radiates equally in all directions * VHF omnidirectional range, a type of radio navigation system for ...
in the plane perpendicular to the wire axis, with the radiation falling to zero on the axis (off the ends of the antenna). In a half-wave dipole the radiation is maximum perpendicular to the antenna, declining as (\sin\theta)^2 to zero on the axis. Its
radiation pattern In the field of antenna design the term radiation pattern (or antenna pattern or far-field pattern) refers to the ''directional'' (angular) dependence of the strength of the radio waves from the antenna or other source.Constantine A. Balanis: “ ...
in three dimensions (see figure) would be plotted approximately as a
toroid In mathematics, a toroid is a surface of revolution with a hole in the middle. The axis of revolution passes through the hole and so does not intersect the surface. For example, when a rectangle is rotated around an axis parallel to one of its ...
(doughnut shape) symmetric about the conductor. When mounted vertically this results in maximum radiation in horizontal directions. When mounted horizontally, the radiation peaks at right angles (90°) to the conductor, with nulls in the direction of the dipole. Neglecting electrical inefficiency, the
antenna gain In electromagnetics, an antenna's gain is a key performance parameter which combines the antenna's directivity and radiation efficiency. The term ''power gain'' has been deprecated by IEEE. In a transmitting antenna, the gain describes how ...
is equal to the
directive gain In electromagnetics, directivity is a parameter of an antenna or optical system which measures the degree to which the radiation emitted is concentrated in a single direction. It is the ratio of the radiation intensity in a given direction fro ...
, which is 1.5 (1.76 dBi) for a short dipole, increasing to 1.64 (2.15 dBi) for a half-wave dipole. For a 5/4 wave dipole the gain further increases to about 5.2 dBi, making this length desirable for that reason even though the antenna is then off-resonance. Longer dipoles than that have radiation patterns that are multi-lobed, with poorer gain (unless they are ''much'' longer) even along the strongest lobe. Other enhancements to the dipole (such as including a
corner reflector A corner reflector is a retroreflector consisting of three mutually perpendicular, intersecting flat surfaces, which reflects waves directly towards the source, but translated. The three intersecting surfaces often have square shapes. Radar co ...
or an
array An array is a systematic arrangement of similar objects, usually in rows and columns. Things called an array include: {{TOC right Music * In twelve-tone and serial composition, the presentation of simultaneous twelve-tone sets such that the ...
of dipoles) can be considered when more substantial directivity is desired. Such antenna designs, although based on the half-wave dipole, generally acquire their own names.


Feeding a dipole antenna

Ideally, a half-wave dipole should be fed using a balanced transmission line matching its typical 65–70 Ω input impedance.
Twin lead Twin-lead cable is a two-conductor flat cable used as a balanced transmission line to carry radio frequency (RF) signals. It is constructed of two stranded or solid copper or copper-clad steel wires, held a precise distance apart by a plastic ( ...
with a similar impedance is available but seldom used and does not match the balanced antenna terminals of most radio and television receivers. Much more common is the use of common 300 Ω twin lead in conjunction with a ''folded dipole''. The driving point impedance of a half-wave folded dipole is 4 times that of a simple half-wave dipole, thus closely matching that 300 Ω
characteristic impedance The characteristic impedance or surge impedance (usually written Z0) of a uniform transmission line is the ratio of the amplitudes of voltage and current of a single wave propagating along the line; that is, a wave travelling in one direction in ...
. Most FM broadcast band tuners and older analog televisions include balanced 300 Ω antenna input terminals. However twin lead has the drawback that it is electrically disturbed by any other nearby conductor (including earth); when used for transmitting, care must be taken not to place it near other conductors. Many types of
coaxial cable Coaxial cable, or coax (pronounced ) is a type of electrical cable consisting of an inner Electrical conductor, conductor surrounded by a concentric conducting Electromagnetic shielding, shield, with the two separated by a dielectric (Insulato ...
(or "coax") have a characteristic impedance of 75 Ω, which would otherwise be a good match for a half-wave dipole. However coax is a single-ended line whereas a center-fed dipole expects a
balanced line In telecommunications and professional audio, a balanced line or balanced signal pair is a circuit consisting of two conductors of the same type, both of which have equal impedances along their lengths and equal impedances to ground and to other ...
(such as twin lead). By symmetry, one can see that the dipole's terminals have an equal but opposite voltage, whereas coax has one conductor grounded. Using coax regardless results in an unbalanced line, in which the currents along the two conductors of the transmission line are no longer equal and opposite. Since you then have a ''net current'' along the transmission line, the transmission line becomes an antenna itself, with unpredictable results (since it depends on the path of the transmission line).Baluns: What They Do And How They Do It (W7EL) http://www.eznec.com/Amateur/Articles/Baluns.pdf This will generally alter the antenna's intended radiation pattern, and change the impedance seen at the transmitter or receiver. A
balun A balun (from "balanced to unbalanced", originally, but now dated from "balancing unit") is an electrical device that allows balanced and unbalanced lines to be interfaced without disturbing the impedance arrangement of either line. A balun ...
is required to use coaxial cable with a dipole antenna. The balun transfers power between the single-ended coax and the balanced antenna, sometimes with an additional change in impedance. A balun can be implemented as a
transformer A transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple circuits. A varying current in any coil of the transformer produces a varying magnetic flux in the transformer's ...
which also allows for an impedance transformation. This is usually wound on a ferrite toroidal core. The toroid core material must be suitable for the frequency of use, and in a transmitting antenna it must be of sufficient size to avoid
saturation Saturation, saturated, unsaturation or unsaturated may refer to: Chemistry * Saturation, a property of organic compounds referring to carbon-carbon bonds **Saturated and unsaturated compounds ** Degree of unsaturation **Saturated fat or fatty aci ...
. Other balun designs are mentioned below.


Current balun

A so-called current balun uses a transformer wound on a toroid or rod of magnetic material such as ferrite. All of the current seen at the input goes into one terminal of the balanced antenna. It forms a balun by choking common-mode current. The material isn't critical for 1:1 because there is no transformer action applied to the desired differential current. A related design involves ''two'' transformers and includes a 1:4 impedance transformation.


Coax balun

A coax balun is a cost-effective method of eliminating feeder radiation, but is limited to a narrow set of operating frequencies. One easy way to make a balun is to use a length of coaxial cable equal to half a wavelength. 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.


Sleeve balun

At VHF frequencies, a sleeve balun can also be built to remove feeder radiation. 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 no connection is made to the pipe. The ''λ''/4 conductor acts as a transformer, converting the zero impedance at the short to the braid into an infinite impedance at the open end. This infinite impedance at the open end of the pipe prevents current flowing into the outer coax formed by the outside of the inner coax shield and the pipe, forcing the current to remain in the inside coax. This balun design is impractical for low frequencies because of the long length of pipe that will be needed.


Common applications


"Rabbit ears" TV antenna

One of the most common applications of the dipole antenna is the ''rabbit ears'' or ''bunny ears''
television antenna A television antenna (TV aerial) is an antenna specifically designed for use with a television receiver (TV) to receive over-the-air broadcast television signals from a television station. Television reception is dependent upon the antenna as ...
, found atop broadcast
television receiver A television set or television receiver, more commonly called the television, TV, TV set, telly, tele, or tube, is a device that combines a tuner, display, and loudspeakers, for the purpose of viewing and hearing television broadcasts, or using ...
s. It is used to receive the VHF terrestrial television bands, consisting in the US of 54 to 88 MHz (
band I Band I is a range of radio frequencies within the very high frequency (VHF) part of the electromagnetic spectrum. The first time there was defined "for simplicity" in Annex 1 of "Final acts of the European Broadcasting Conference in the VHF and U ...
) and 174 to 216 MHz (
band III Band III is the name of the range of radio frequencies within the very high frequency (VHF) part of the electromagnetic spectrum from 174 to 240 megahertz (MHz). It is primarily used for radio and television broadcasting. It is also called high-b ...
), with wavelengths of 5.5 to 1.4 m. Since this frequency range is much wider than a single fixed dipole antenna can cover, it is made with several degrees of adjustment. It is constructed of two telescoping rods that can each be extended out to about 1 m length (one quarter wavelength at 75 MHz). With control over the segments' length, angle with respect to vertical, and compass angle, one has much more flexibility in optimizing reception than available with a rooftop antenna even if equipped with an
antenna rotor An antenna rotator (or antenna rotor) is a device used to change the orientation, within the horizontal plane, of a directional antenna. Most antenna rotators have two parts, the rotator unit and the controller. The controller is normally place ...
.


FM-broadcast-receiving antennas

In contrast to the wide television frequency bands, the FM broadcast band (88-108 MHz) is narrow enough that a dipole antenna can cover it. For fixed use in homes, hi-fi tuners are typically supplied with simple folded dipoles resonant near the center of that band. The feedpoint impedance of a folded dipole, which is quadruple the impedance of a simple dipole, is a good match for 300Ω
twin lead Twin-lead cable is a two-conductor flat cable used as a balanced transmission line to carry radio frequency (RF) signals. It is constructed of two stranded or solid copper or copper-clad steel wires, held a precise distance apart by a plastic ( ...
, so that is usually used for the transmission line to the tuner. A common construction is to make the arms of the folded dipole out of twin lead also, shorted at their ends. This flexible antenna can be conveniently taped or nailed to walls, following the contours of mouldings.


Shortwave antenna

Horizontal wire dipole antennas are popular for use on the HF shortwave bands, both for transmitting and
shortwave listening Shortwave listening, or SWLing, is the hobby of listening to shortwave radio broadcasts located on frequencies between 1700 kHz and 30 MHz. Listeners range from casual users seeking international news and entertainment programming, to hobbyists ...
. They are usually constructed of two lengths of wire joined by a
strain insulator A strain insulator is an electrical insulator that is designed to work in mechanical tension (strain), to withstand the pull of a suspended electrical wire or cable. They are used in overhead electrical wiring, to support radio antennas and ove ...
in the center, which is the feedpoint. The ends can be attached to existing buildings, structures, or trees, taking advantage of their heights. If used for transmitting, it is essential that the ends of the antenna be attached to supports through strain insulators with a sufficiently high flashover voltage, since the antenna's high-voltage antinodes occur there. Being a balanced antenna, they are best fed with a
balun A balun (from "balanced to unbalanced", originally, but now dated from "balancing unit") is an electrical device that allows balanced and unbalanced lines to be interfaced without disturbing the impedance arrangement of either line. A balun ...
between the (coax) transmission line and the feedpoint. These are simple to put up for temporary or field use. But they are also widely used by
radio amateur An amateur radio operator is someone who uses equipment at an amateur radio station to engage in two-way personal communications with other amateur operators on radio frequencies assigned to the amateur radio service. Amateur radio operators hav ...
s and short wave listeners in fixed locations due to their simple (and inexpensive) construction, while still realizing a resonant antenna at frequencies where resonant antenna elements need to be of quite some size. They are an attractive solution for these frequencies when significant directionality is not desired, and the cost of several such resonant antennas for different frequency bands, built at home, may still be much less than a single commercially produced antenna.


Dipole towers

Antennas for MF and LF radio stations are usually constructed as
mast radiator Mast, MAST or MASt may refer to: Engineering * Mast (sailing), a vertical spar on a sailing ship * Flagmast, a pole for flying a flag * Guyed mast, a structure supported by guy-wires * Mooring mast, a structure for docking an airship * Radio mas ...
s, in which the vertical mast itself forms the antenna. Although mast radiators are most commonly monopoles, some are dipoles. The metal structure of the mast is divided at its midpoint into two insulated sections to make a vertical dipole, which is driven at the midpoint.


Dipole arrays

Many types of array antennas are constructed using multiple dipoles, usually half-wave dipoles. The purpose of using multiple dipoles is to increase the directional
gain Gain or GAIN may refer to: Science and technology * Gain (electronics), an electronics and signal processing term * Antenna gain * Gain (laser), the amplification involved in laser emission * Gain (projection screens) * Information gain in de ...
of the antenna over the gain of a single dipole; the radiation of the separate dipoles interferes to enhance power radiated in desired directions. In arrays with multiple dipole
driven element {{Unreferenced, date=April 2016 In a multielement antenna array (such as a Yagi–Uda antenna), the driven element or active element is the element in the antenna (typically a metal rod) which is electrically connected to the receiver or transm ...
s, the
feedline In a radio antenna, the feed line (feedline), or feeder, is the cable or other transmission line that connects the antenna with the radio transmitter or receiver. In a transmitting antenna, it feeds the radio frequency (RF) current from th ...
is split using an electrical network in order to provide power to the elements, with careful attention paid to the relative phase delays due to transmission between the common point and each element. In order to increase antenna gain in horizontal directions (at the expense of radiation towards the sky or towards the ground) one can stack antennas in the vertical direction in a broadside array where the antennas are fed in phase. Doing so with horizontal dipole antennas retains those dipoles' directionality and null in the direction of their elements. However, if each dipole is vertically oriented, in a so-called
collinear antenna array In telecommunications, a collinear antenna array (sometimes colinear antenna array) is an array of dipole or quarter-wave antennas mounted in such a manner that the corresponding elements of each antenna are parallel and collinear; that is, t ...
(see graphic), that null direction becomes vertical and the array acquires an omnidirectional radiation pattern (in the horizontal plane) as is typically desired. Vertical collinear arrays are used in the VHF and UHF frequency bands at which wavelengths the size of the elements are small enough to practically stack several on a mast. They are a higher-gain alternative to quarter-wave ground plane antennas used in fixed base stations for mobile
two-way radio A two-way radio is a radio that can both transmit and receive radio waves (a transceiver), unlike a radio broadcasting, broadcast receiver which only receives content. It is an audio (sound) transceiver, a transmitter and radio receiver, receive ...
s, such as police, fire, and taxi dispatchers. On the other hand, for a rotating antenna (or one used only towards a particular direction) one may desire increased gain and directivity in a particular horizontal direction. If the broadside array discussed above (whether collinear or not) is turned horizontal, then the one obtains a greater gain in the horizontal direction perpendicular to the antennas, at the expense of most other directions. Unfortunately that also means that the direction ''opposite'' the desired direction also has a high gain, whereas high gain is usually desired in one single direction. The power which is wasted in the reverse direction, however, can be redirected, for instance by using a large planar reflector, as is accomplished in the
reflective array antenna In telecommunications and radar, a reflective array antenna is a class of directive antennas in which multiple driven elements are mounted in front of a flat surface designed to reflect the radio waves in a desired direction. They are a type ...
, increasing the gain in the desired direction by another 3 dB An alternative realization of a uni-directional antenna is the ''end-fire array''. In this case the dipoles are again side by side (but not collinear), but fed in progressing phases, arranged so that their waves add coherently in one direction but cancel in the opposite direction. So now, rather than being perpendicular to the array direction as in a broadside array, the directivity is ''in'' the array direction (i.e. the direction of the line connecting their feedpoints) but with one of the opposite directions suppressed.


Yagi antennas

The above described antennas with multiple
driven element {{Unreferenced, date=April 2016 In a multielement antenna array (such as a Yagi–Uda antenna), the driven element or active element is the element in the antenna (typically a metal rod) which is electrically connected to the receiver or transm ...
s require a complex feed system of signal splitting, phasing, distribution to the elements, and impedance matching. A different sort of end-fire array which is much more often used is based on the use of so-called ''
parasitic element In a radio antenna, a passive radiator or parasitic element is a conductive element, typically a metal rod, which is not electrically connected to anything else. Multielement antennas such as the Yagi–Uda antenna typically consist of a " ...
s''. In the popular high-gain
Yagi antenna Yagi may refer to: Places * Yagi, Kyoto, in Japan * Yagi (Kashihara), in Nara Prefecture, Japan * Yagi-nishiguchi Station, in Kashihara, Nara, Japan * Kami-Yagi Station, a JR-West Kabe Line station located in 3-chōme, Yagi, Asaminami-ku, Hiroshima ...
, only one of the dipoles is actually connected electrically, but the others receive and reradiate power supplied by the driven element. This time, the phasing is accomplished by careful choice of the lengths as well as positions of the parasitic elements, in order to concentrate gain in one direction and largely cancel radiation in the opposite direction (as well as all other directions). Although the realized gain is less than a driven array with the same number of elements, the simplicity of the electrical connections makes the Yagi more practical for consumer applications.


Dipole as a reference standard

Antenna
gain Gain or GAIN may refer to: Science and technology * Gain (electronics), an electronics and signal processing term * Antenna gain * Gain (laser), the amplification involved in laser emission * Gain (projection screens) * Information gain in de ...
is frequently measured as decibels relative to a half-wave dipole. One reason is that practical
antenna measurement Antenna measurement techniques refers to the testing of antennas to ensure that the antenna meets specifications or simply to characterize it. Typical parameters of antennas are gain, bandwidth, radiation pattern, beamwidth, polarization, and ...
s need a reference strength to compare the field strength of an antenna under test at a particular distance to. Of course there is no such thing as an isotropic radiator, but the half-wave dipole is well understood and behaved, and can be constructed to be nearly 100% efficient. It is also a fairer comparison, since the gain obtained by the dipole itself is essentially "free," given that almost no antenna design has a smaller directive gain. For a gain measured relative to a dipole, one says the antenna has a
gain Gain or GAIN may refer to: Science and technology * Gain (electronics), an electronics and signal processing term * Antenna gain * Gain (laser), the amplification involved in laser emission * Gain (projection screens) * Information gain in de ...
of "''x'' dBd" (see
decibel The decibel (symbol: dB) is a relative unit of measurement equal to one tenth of a bel (B). It expresses the ratio of two values of a power or root-power quantity on a logarithmic scale. Two signals whose levels differ by one decibel have a po ...
). More often, gains are expressed relative to an
isotropic radiator An isotropic radiator is a theoretical point source of electromagnetic or sound waves which radiates the same intensity of radiation in all directions. It has no preferred direction of radiation. It radiates uniformly in all directions over a ...
, often for advertising reasons as this makes the gain appear higher. In consideration of the known gain of a half-wave dipole, 0 dBd is defined as 2.15 dBi; all gains in dBi are 2.15 higher than gains in dBd.


Hertzian dipole

The ''Hertzian dipole'' or ''elementary doublet'' refers to a theoretical construction, rather than a physical antenna design: It is an idealized tiny segment of conductor carrying a RF current with constant amplitude and direction along its entire (short) length; a real antenna can be modeled as the combination of many Hertzian dipoles laid end-to-end. The Hertzian dipole may be defined as a finite oscillating current (in a specified direction) of Ie^ over a tiny or
infinitesimal In mathematics, an infinitesimal number is a quantity that is closer to zero than any standard real number, but that is not zero. The word ''infinitesimal'' comes from a 17th-century Modern Latin coinage ''infinitesimus'', which originally ref ...
length \delta\ell at a specified position. The solution of the fields from a Hertzian dipole can be used as the basis for analytical or numerical calculation of the radiation from more complex antenna geometries (such as practical dipoles) by forming the superposition of fields from a large number of Hertzian dipoles comprising the current pattern of the actual antenna. As a function of position, taking the elementary current elements multiplied by infinitesimal lengths I\mathord\left(\mathbf\right)\delta\ell~, the resulting field pattern then reduces to an
integral In mathematics, an integral assigns numbers to functions in a way that describes displacement, area, volume, and other concepts that arise by combining infinitesimal data. The process of finding integrals is called integration. Along with ...
over the path of an antenna conductor (modeled as a thin wire). For the following derivation we shall take the current to be in the Z direction centered at the origin where x = y = z = 0, with the sinusoidal time dependence e^ for all quantities being understood. The simplest approach is to use the calculation of the
vector potential In vector calculus, a vector potential is a vector field whose curl is a given vector field. This is analogous to a '' scalar potential'', which is a scalar field whose gradient is a given vector field. Formally, given a vector field v, a ''vec ...
\mathbf\left(\mathbf\right) using the formula for the
retarded potential In electrodynamics, the retarded potentials are the electromagnetic potentials for the electromagnetic field generated by time-varying electric current or charge distributions in the past. The fields propagate at the speed of light ''c'', so t ...
. Although the value of \mathbf is not unique, we shall constrain it according to the Lorenz gauge, and assuming sinusoidal current at radian frequency \omega the retardation of the field is converted just into a phase factor e^, where the wavenumber k = \frac in free space and r is the distance between the point being considered to the origin (where we assumed the current source to be), thus r = \left, \mathbf\. This results in a vector potential \mathbf at position \mathbf due to that current element only, which we find is purely in the Z direction (the direction of the current): :\mathbf\left(\mathbf\right) = I\delta\ell\frace^\hat\mathbf where \mu_0 is the
permeability of free space The vacuum magnetic permeability (variously ''vacuum permeability'', ''permeability of free space'', ''permeability of vacuum''), also known as the magnetic constant, is the magnetic permeability in a classical vacuum. It is a physical constant, ...
. Then using :\mu\mathbf = \mathbf = \nabla\times\mathbf we can solve for the magnetic field \mathbf, and from that (dependent on us having chosen the Lorenz gauge) the electric field \mathbf using :\mathbf = \frac In
spherical coordinates In mathematics, a spherical coordinate system is a coordinate system for three-dimensional space where the position of a point is specified by three numbers: the ''radial distance'' of that point from a fixed origin, its ''polar angle'' meas ...
we find that the magnetic field \mathbf has only a component in the \phi direction: :H_\phi = i\frac\left(\frac - \frac\right)e^\sin while the electric field has components both in the \theta and \mathbf directions: :\begin E_\theta &= i\frac\left(\frac - \frac - \frac\right)e^\sin \\ pt E_r &= \frac\left(\frac - \frac\right)e^\cos \end where \zeta_0 = \sqrt is the
impedance of free space The impedance of free space, , is a physical constant relating the magnitudes of the electric and magnetic fields of electromagnetic radiation travelling through free space. That is, , where is the electric field strength and is the magnetic field ...
. This solution includes near field terms which are very strong near the source but which are ''not'' radiated. As seen in the accompanying animation, the \mathbf and \mathbf fields very close to the source are almost 90° out of phase, thus contributing very little to the
Poynting vector In physics, the Poynting vector (or Umov–Poynting vector) represents the directional energy flux (the energy transfer per unit area per unit time) or '' power flow'' of an electromagnetic field. The SI unit of the Poynting vector is the watt ...
by which radiated flux is computed. The near field solution for an antenna element (from the integral using this formula over the length of that element) is the field that can be used to compute the mutual impedance between it and another nearby element. For computation of the
far field The near field and far field are regions of the electromagnetic (EM) field around an object, such as a transmitting antenna, or the result of radiation scattering off an object. Non-radiative ''near-field'' behaviors dominate close to the ant ...
radiation pattern, the above equations are simplified as only the \frac terms remain significant: :\begin H_\phi &= i\frace^\sin \\ pt E_\theta &= i\frace^\sin \end. The far field pattern is thus seen to consist of a transverse electromagnetic (TEM) wave, with electric and magnetic fields at right angles to each other and at right angles to the direction of propagation (the direction of \mathbf, as we assumed the source to be at the origin). The electric polarization, in the \theta direction, is coplanar with the source current (in the Z direction), while the magnetic field is at right angles to that, in the \phi direction. It can be seen from these equations, and also in the animation, that the fields at these distances are exactly ''in phase''. Both fields fall according to \frac, with the power thus falling according to \frac as dictated by the
inverse square law In science, an inverse-square law is any scientific law stating that a specified physical quantity is inversely proportional to the square of the distance from the source of that physical quantity. The fundamental cause for this can be understoo ...
.


Radiation resistance

If one knows the far field radiation pattern due to a given antenna current, then it is possible to compute the
radiation resistance Radiation resistance, \ R_\mathsf\ or \ R_\mathsf\ , is proportional to the part of an antenna's feedpoint electrical resistance that is caused by power loss from the emission of radio waves from the antenna. Radiation resistance is an ''effect ...
directly. For the above fields due to the Hertzian dipole, we can compute the power flux according to the
Poynting vector In physics, the Poynting vector (or Umov–Poynting vector) represents the directional energy flux (the energy transfer per unit area per unit time) or '' power flow'' of an electromagnetic field. The SI unit of the Poynting vector is the watt ...
, resulting in a power (as averaged over one cycle) of: :\langle\mathbf\rangle = \frac\mathfrak\left(\mathbf\times\mathbf^*\right). Although not required, it is simplest to do the following exercise at a large r where the far field expressions for \mathbf and \mathbf apply. Consider a large sphere surrounding the source with a radius r. We find the power per unit area crossing the surface of that sphere to be in the \hat\mathbf direction according to: :\left\langle\mathbf_\mathsf\right\rangle = \frac\left(\frac\right)^2\sin^2\theta Integration of this flux over the complete sphere results in: :\begin P_\text &= \int_0^\!\!\int_0^\langle\mathbf_\mathsfr^2\sindd\theta \\ &= \frac\left(\left, I\k\delta\ell\right)^2 = \frac\left(\left, I\\frac\right)^2 \end where \lambda = 2\pi/k is the free space wavelength corresponding to the radian frequency \omega. By definition, the radiation resistance R_\text times the average of the square of the current \frac\left, I\^2 is the net power radiated due to that current, so equating the above to \frac\left, I\^2 R_\text we find: :R_\text = \frac\zeta_0\left(\frac\right)^2. This method can be used to compute the radiation resistance for any antenna whose far field radiation pattern has been found in terms of a specific antenna current. If ohmic losses in the conductors are neglected, the radiation resistance (considered relative to the feedpoint) is identical to the resistive (real) component of the feedpoint impedance. Unfortunately this exercise tells us nothing about the reactive (imaginary) component of feedpoint impedance, whose calculation is considered
below Below may refer to: *Earth * Ground (disambiguation) *Soil *Floor * Bottom (disambiguation) *Less than *Temperatures below freezing *Hell or underworld People with the surname *Ernst von Below (1863–1955), German World War I general *Fred Below ...
.


Directive gain

Using the above expression for the radiated flux given by the Poynting vector, it is also possible to compute the directive gain of the Hertzian dipole. Dividing the total power computed above by 4\pi r^2 we can find the flux averaged over all directions P_\text as :P_\text = \frac = \frack^2\left, I\^2\left(\delta\ell\right)^2. Dividing the flux radiated in a ''particular'' direction by P_\text we obtain the
directive gain In electromagnetics, directivity is a parameter of an antenna or optical system which measures the degree to which the radiation emitted is concentrated in a single direction. It is the ratio of the radiation intensity in a given direction fro ...
\operatorname\mathsf\left(\theta\right): :\operatorname\mathsf\left(\theta\right) = \frac = \frac\sin^2\theta The commonly quoted antenna "gain", meaning the peak value of the gain pattern (radiation pattern), is found to be 1.5 to 1.76 dBi, lower than practically any other antenna configuration.


Comparison with the short dipole

The Hertzian dipole is ''similar to'' but differs from the short dipole, discussed above. In both cases the conductor is very short compared to a wavelength, so the standing wave pattern present on a half-wave dipole (for instance) is absent. However, with the Hertzian dipole we specified that the current along that conductor is ''constant'' over its short length. This makes the Hertzian dipole useful for analysis of more complex antenna configurations, where every infinitesimal section of that ''real'' antenna's conductor can be modelled as a Hertzian dipole with the current found to be flowing in that real antenna. However a short conductor fed with a RF voltage will ''not'' have a uniform current even along that short range. Rather, a short dipole in real life has a current equal to the feedpoint current at the feedpoint but falling linearly to zero over the length of that short conductor. By placing a ''capacitive hat'', such as a metallic ball, at the end of the conductor, it is possible for its self capacitance to absorb the current from the conductor and better approximate the constant current assumed for the Hertzian dipole. But again, the Hertzian dipole is meant only as a theoretical construct for antenna analysis. The short dipole, with a feedpoint current of I_0, has an ''average'' current over each conductor of only \fracI_0. The above field equations for the Hertzian dipole of length \delta\ell would then predict the ''actual'' fields for a short dipole using that effective current I = \fracI_0. This would result in a power measured in the far field of ''one quarter'' that given by the above equation for the Poynting vector \langle\mathbf_\mathsf\rangle if we had assumed an element current of I_0. Consequently, it can be seen that the radiation resistance computed for the short dipole is one quarter of that computed above for the Hertzian dipole. But their radiation patterns (and gains) are identical.


Detailed calculation of dipole feedpoint impedance

The impedance seen at the feedpoint of a dipole of various lengths has been plotted above, in terms of the real (resistive) component ''R''dipole and the imaginary ( reactive) component ''jX''dipole of that impedance. For the case of an antenna with perfect conductors (no ohmic loss), ''R''dipole is identical to the
radiation resistance Radiation resistance, \ R_\mathsf\ or \ R_\mathsf\ , is proportional to the part of an antenna's feedpoint electrical resistance that is caused by power loss from the emission of radio waves from the antenna. Radiation resistance is an ''effect ...
, which can more easily be computed from the total power in the far-field radiation pattern for a given applied current as we showed for the short dipole. The calculation of ''X''dipole is more difficult.


Induced EMF method

Using the ''induced EMF method'' closed form expressions are obtained for both components of the feedpoint impedance; such results are plotted above. The solution depends on an assumption for the form of the current distribution along the antenna conductors. For wavelength to element diameter ratios greater than about 60, the current distribution along each antenna element of length is very well approximated as having the form of the sine function at points along the antenna , with the current reaching zero at the elements' ends, where as follows: :\ I(z) = A \sin\Bigl(\ k \left( \tfracL - \left, z \ \right)\ \Bigr)\ , where is the
wavenumber In the physical sciences, the wavenumber (also wave number or repetency) is the ''spatial frequency'' of a wave, measured in cycles per unit distance (ordinary wavenumber) or radians per unit distance (angular wavenumber). It is analogous to temp ...
given by and the amplitude is set to match a specified driving point current at In cases where an approximately sinusoidal current distribution can be assumed, this method solves for the driving point impedance in closed form using the cosine and sine integral functions and . For a dipole of total length , the resistive and reactive components of the driving point impedance can be expressed as: :\begin R_\text = \frac \Biggl\\ , \\ X_\text = \frac \Biggl\\ , \end where is the radius of the conductors, is again the wavenumber as defined above, is the
impedance of free space The impedance of free space, , is a physical constant relating the magnitudes of the electric and magnetic fields of electromagnetic radiation travelling through free space. That is, , where is the electric field strength and is the magnetic field ...
: and \ \gamma_e = 0.57721566... \ is
Euler's constant Euler's constant (sometimes also called the Euler–Mascheroni constant) is a mathematical constant usually denoted by the lowercase Greek letter gamma (). It is defined as the limiting difference between the harmonic series and the natural l ...
.


Integral methods

The induced EMF method is dependent on the assumption of a sinusoidal current distribution, delivering an accuracy better than about 10% as long as the wavelength to element diameter ratio is greater than about 60. However, for yet larger conductors numerical solutions are required which solve for the conductor's current distribution (rather than ''assuming'' a sinusoidal pattern). This can be based on approximating solutions for either ''Pocklington's integrodifferential equation'' or the ''Hallén integral equation''. These approaches also have greater generality, not being limited to linear conductors. Numerical solution of either is performed using the ''moment method solution'' which requires expansion of that current into a set of
basis functions In mathematics, a basis function is an element of a particular basis for a function space. Every function in the function space can be represented as a linear combination of basis functions, just as every vector in a vector space can be repres ...
; one simple (but not the best) choice, for instance, is to break up the conductor into ''N'' segments with a constant current assumed along each. After setting an appropriate weighting function the cost may be minimized through the inversion of a ''N''×''N'' matrix. Determination of each matrix element requires at least one double integration involving the weighting functions, which may become computationally intensive. These are simplified if the weighting functions are simply delta functions, which corresponds to fitting the boundary conditions for the current along the conductor at only ''N'' discrete points. Then the ''N''×''N'' matrix must be inverted, which is also computationally intensive as ''N'' increases. In one simple example, Balanis (2011) performs this computation to find the antenna impedance with different ''N'' using Pocklington's method, and finds that with ''N'' > 60 the solutions approach their limiting values to within a few percent.


See also

*
AM broadcasting AM broadcasting is radio broadcasting using amplitude modulation (AM) transmissions. It was the first method developed for making audio radio transmissions, and is still used worldwide, primarily for medium wave (also known as "AM band") transmi ...
*
Amateur radio Amateur radio, also known as ham radio, is the use of the radio frequency spectrum for purposes of non-commercial exchange of messages, wireless experimentation, self-training, private recreation, radiosport, contesting, and emergency communica ...
*
Balun A balun (from "balanced to unbalanced", originally, but now dated from "balancing unit") is an electrical device that allows balanced and unbalanced lines to be interfaced without disturbing the impedance arrangement of either line. A balun ...
* Coaxial antenna * Dipole field strength in free space *
Driven element {{Unreferenced, date=April 2016 In a multielement antenna array (such as a Yagi–Uda antenna), the driven element or active element is the element in the antenna (typically a metal rod) which is electrically connected to the receiver or transm ...
*
Electronic symbol An electronic symbol is a pictogram used to represent various electrical and electronic devices or functions, such as wires, batteries, resistors, and transistors, in a schematic diagram of an electrical or electronic circuit. These symbols ar ...
*
FM broadcasting FM broadcasting is a method of radio broadcasting using frequency modulation (FM). Invented in 1933 by American engineer Edwin Armstrong, wide-band FM is used worldwide to provide high fidelity sound over broadcast radio. FM broadcasting is capa ...
*
Isotropic antenna An isotropic radiator is a theoretical point source of electromagnetic or sound waves which radiates the same intensity of radiation in all directions. It has no preferred direction of radiation. It radiates uniformly in all directions over a ...
*
Omnidirectional antenna In radio communication, an omnidirectional antenna is a class of antenna which radiates equal radio power in all directions perpendicular to an axis (azimuthal directions), with power varying with angle to the axis (elevation angle), declining ...
*
Shortwave listening Shortwave listening, or SWLing, is the hobby of listening to shortwave radio broadcasts located on frequencies between 1700 kHz and 30 MHz. Listeners range from casual users seeking international news and entertainment programming, to hobbyists ...
*
T-antenna A T-antenna, T-aerial, flat-top antenna, or (capacitively) top-loaded antenna is a monopole radio antenna with transverse capacitive loading wires attached to its top. T-antennas are typically used in the VLF, LF, MF, and shortwave bands, and ...
*
Whip antenna A whip antenna is an antenna consisting of a straight flexible wire or rod. The bottom end of the whip is connected to the radio receiver or transmitter. A whip antenna is a form of monopole antenna. The antenna is designed to be flexible so ...


Notes


References


Elementary, short, and half-wave dipoles

* * *
Electromagnetic Waves and Antennas
Electromagnetic Waves and Antennas, Sophocles J. Orfanidis.

Wire Antenna Resources including off center fed dipole (OCFD), dipole calculators and construction sites
Wayback Machine
*https://web.archive.org/web/20070926195106/http://www.nt.hs-bremen.de/peik/asc/asc_antenna_slides.pdf

* *

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)


External links


AC6V's Homebrew Antennas LinksYour First HF Dipole
- simple yet complete tutorial from eham.net
Dipole articles
- s series of pages about the dipole in its various forms {{Authority control Antennas (radio) Heinrich Hertz Radio frequency antenna types Radio technology Articles containing video clips