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 for the Hertzian dipole, a fictitious entity. Being shorter than a resonant antenna (half a wavelength long) its feedpoint impedance includes a large
capacitive reactance requiring a
loading coil 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{align} H_\mathrm{\phi} \quad &= \quad j\ \frac{\ I_\mathrm{h}\ \ell\ k\ }{4\pi\ r}\ e^{\ j\ \left( \omega t\ -\ k r \right)}\ \sin(\ \theta\ ) \\ E_\mathrm{\theta} \quad &= \quad \zeta_\mathrm{o}\ H_\mathrm{\phi} \quad = \quad j\ \frac{\ \zeta_\mathrm{o}\ I_\mathrm{h}\ \ell\ k\ }{4\pi\ r}\ e^{\ j\ \left( \omega t\ -\ k r \right)}\ \sin(\ \theta\ ) ~. \end{align} where the radiator consists of a current of \ I_\mathrm{h}\ e^{\ j \omega t}\ over a short length and \ j^2 \equiv -1\ in electronics replaces the customary mathematical symbol for the square root of . is the angular (radian) frequency and is the wavenumber is the
impedance of free space {{nobr|(\ \zeta_\mathrm{o} \approx 376.7\ \mathsf{\Omega}\ ),}} 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 nearest end of the arm. In the case of a short dipole, that is essentially a linear drop from \ I_\mathrm{o}\ 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_\mathrm{h} = \tfrac{\ 1\ }{ 2 }\ I_\mathrm{o} ~. With that substitution, the above equations closely approximate the fields generated by a short dipole fed by current \ I_\mathrm{o} ~. 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, '
, which is given by \ \tfrac{\ 1\ }{ 2 } \mathbf{E} \times \mathbf{H}^* ~. Because and ' are at right angles and in phase, there is no imaginary part and the cross product is equal to \ \tfrac{\ 1\ }{ 2 }\ E_\mathrm{\theta}\ H_\mathrm{\phi}^*\ ; the phase factors (the exponentials) cancel out, leaving: :\begin{align} S &= \frac{\ 1\ }{ 2 }\ E_\mathrm{\theta}\ H_\mathrm{\phi}^* &= \frac{\ 1\ }{ 2 }\ \frac{\ \zeta_\mathrm{o}\ I_\mathrm{h}^2\ \ell^2\ k^2\ }{(4\pi r)^2}\ \sin^2\!(\ \theta\ ) &= \frac{\ \zeta_\mathrm{o}\ }{ 32 }\ I_\mathrm{o}^2\ \left(\frac{\ \ell\ }{\lambda}\right)^2\ \frac{ 1 }{\ r^2 }\ \sin^2\!(\ \theta\ ) ~. \end{align} We have now expressed the flux in terms of the feedpoint current 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 to obtain the total radiated power. :P_\text{total} = \frac{\ \pi\ }{ 12 }\ \zeta_\mathrm{o}\ I_\mathrm{o}^2\ \left( \frac{\ell}{\lambda} \right)^2 ~. From that, it is possible to infer the
radiation resistance, equal to the resistive (real) part of the feedpoint impedance, neglecting a component due to ohmic losses (presumed smaller). By setting to the power supplied at the feedpoint \ \tfrac{\ 1\ }{ 2 }\ I_\mathrm{o}^2\ R_\mathrm{radiation}\ we find: :\ R_\mathrm{radiation} = \frac{\ \pi\ }{6}\ \zeta_\mathrm{o}\ \left(\frac{\ell}{\lambda}\right)^2 \approx \left(\frac{\ell}{\lambda}\right)^2 (197\ \Omega) ~. Again, these approximations become quite accurate 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 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 \ \ell\ are in fact resonant at any integer multiple of a half-wavelength: :\ \ell = n \times \frac{\ \lambda\ }{2}\ where is an integer, \ \lambda = \frac{\ c\ }{ f }\ is the wavelength, and is the reduced speed of radio waves in the radiating conductor ( the
speed of light). For a center-fed dipole, however, there is a great dissimilarity between 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 \ \ell \approx \lambda\ . 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 -wave dipole antenna has a much lower but not purely resistive feedpoint impedance, which requires a
matching network 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 antennas designed as half-wave dipoles at 7 MHz can also be used as -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 on the antenna. Since the standing wave is mainly storing energy, not transporting power, the current is not in phase with the voltage but 90° out of phase. The transmission line applies an oscillating voltage \ V_\text{i}\cos \omega t\ from the
transmitter between the two antenna elements, driving the sinusoidal oscillation. The feed voltage step has been increased for visibility; typical dipoles have a high enough
Q factor that the feed voltage is much smaller in relation to the standing wave. Since the antenna is fed at its resonant frequency, the input voltage is in phase with the current (blue bar), so the antenna presents a pure resistance to the feedline. The energy from the driving current provides the energy radiated as radio waves. In a receiving antenna the phase of the voltage at the transmission line would be reversed, since the receiver absorbs energy from the antenna. A half-wave dipole antenna consists of two quarter-wavelength conductors placed end to end for a total length of approximately The current distribution is that of a
standing wave, 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(\omega t) \cos(k z)\ , where and runs from to . In the far field, this produces a radiation pattern whose electric field is given by Instead of altering thickness or spacing, one can add a third parallel wire to increase the feedpoint 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. More extra parallel wires can be added: Any number of extra parallel wires can be joined onto the antenna, with the feedpoint impedance given by : \ R_\mathsf{rad} \approx n^2\ \times\ 73\mathsf{\ \Omega\ ,} where \ n\ is the number of parallel halfwave-long wires laid side-by-side in the antenna, and connected at their ends. It is also possible to modify the so-called flattened-loop design, and get nearly as good performance, by making each of the parallel wires too short by the same amount, but connecting a single capacitive loading wire (going off in nearly any direction, most often dangling) on each of the antenna ends. The loading wire length is equal to the single missing length of one of the parallel wires.
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 antennas which include one or more dipole elements in their design as
driven elements, 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 antennas. radio telescope. The 8 meters (30 feet) long by 1.8 meters (6 feet) diameter galvanized steel wire dipoles have an operating frequency range of 8–33 MHz. • 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 and
shortwave bands for applications such as
over-the-horizon radar and
radio telescopes. • A
halo antenna 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 only a little power wasted toward the zenith, compared to a straight horizontal dipole. In practice, it is categorized either as a bent dipole or as a loop antenna, depending on author preference. • A
turnstile antenna 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 that is rather close to
isotropic, with horizontal polarization in the plane of the elements and
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 is a
turnstile antenna 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 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 at low to intermediate elevation angles and have nearly
omnidirectional 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
antenna is a dipole antenna fed indirectly, through a carefully chosen length of 300 Ω or 450 Ω
twin lead, which acts as an impedance
matching network to connect (through 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 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 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 the same as half of a dipole antenna. The ground (or
ground plane) is considered to be a conductive surface that 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{\ 73\ +\ j\ 43\ }{2} = 36\ +\ j\ 21\ \mathsf{\Omega} ~. Another way of seeing this is that a true dipole receiving a current has voltages on its terminals of and , for an impedance across the terminals of , whereas the comparable vertical antenna has the current but an applied voltage of only . Since the fields above ground are the same as for the dipole, but only half the power is applied, the gain is doubled to 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 (such as a simple choke balun) is then recommended. ==Dipole characteristics==