A dipole antenna, created by Heinrich Rudolph Hertz around 1886, is an antenna that can be made by a simple wire, with a center-fed driven element for transmitting or receiving radio frequency energy. These antennas are the simplest practical antennas from a theoretical point of view; the current amplitude on such an antenna decreases uniformly from maximum at the center to zero at the ends.
Note that this dipole cannot be physically constructed because the current needs somewhere to come from and somewhere to go to. In reality, this small length of conductor will be just one of the multiple segments into which we must divide a real antenna, in order to calculate its properties. The interest of this imaginary elementary antenna is that we can easily calculate the far electrical field of the electromagnetic wave radiated by each elementary doublet. We give just the result:
The far electric field of the electromagnetic wave is coplanar with the conductor and perpendicular with the line joining the dipole to the point where the field is evaluated. If the dipole is placed in the center of a sphere in the axis south-north, the electric field would be parallel to geographic meridians and the magnetic field of the electromagnetic wave would be parallel to geographic parallels.
The above formulas are valid for the far field of the antenna (), and are the only contribution to the radiated field. The formulas in the near field have additional terms that reduce with r2 and r3. These are,
where . The energy associated with the term of the near field flows back and forward out and into the antenna.
A short dipole is a physically feasible dipole formed by two conductors with a total length very small compared with the wavelength . The two conducting wires are fed at the centre of the dipole. We assume the hypothesis that the current is maximal at the centre (where the dipole is fed) and that it decreases linearly to be zero at the ends of the wires. Note that the direction of the current is the same in both the dipole branches - to the right in both or to the left in both. The far field of the electromagnetic wave radiated by this dipole is:
Emission is maximal in the plane perpendicular to the dipole and zero in the direction of wires, that is, the current direction.
The emission diagram is circular section torus shaped (left image) with zero inner diameter. In the right image doublet is vertical in the torus centre.
Knowing this electric field, we can compute the total emitted power and then compute the resistive part of the series impedance of this dipole due to the radiated field, known as the radiation resistance:
where Z0 is the impedance of free space. Using a common approximation of Z0 = 120π ohms, we get:
The surface power carried by an electromagnetic wave is:
The surface power radiated by an isotropic antenna feed with the same power is:
Substituting values for the case of a short dipole, final result is:
dBi simply means decibels gain, relative to an isotropic antenna.
Typically a dipole antenna is formed by two quarter wavelength conductors or elements placed back to back for a total length of . A standing wave on an element of a length ~ yields the greatest voltage differential, as one end of the element is at a node while the other is at an antinode of the wave. The larger the differential voltage, the greater the current flow between the elements.
Assuming a sinusoidal distribution, the current impressed by this voltage differential is given by:
For the far-field case, the formula for the electric field of a radiating electromagnetic wave is somewhat more complex:
But the fraction is not very different from .
The resulting emission diagram is a slightly flattened torus.
The image on the left shows the section of the emission pattern. We have drawn, in dotted lines, the emission pattern of a short dipole. We can see that the two patterns are very similar. The image at right shows the perspective view of the same emission pattern.
This time it is not possible to compute analytically the total power emitted by the antenna (the last formula does not allow), though a simple numerical integration or series expansion leads to the more precise, actual value of the half-wave resistance:
This leads to the gain of a dipole antenna, :
The resistance, however, is not enough to characterize the dipole impedance, as there is also an imaginary part——it is better to measure the impedance.
In the image below, the real and imaginary parts of a dipole's impedance are drawn for lengths going from to , accompanied by a chart comparing the gains of dipole antennas of other lengths (note that gains are not in dBi but in natural number):
|Gain of dipole antennas|
|length L in||Gain||Gain(dB)|
The quarter wave or unipole antenna is a single element antenna fed at one end, that behaves as a dipole antenna. It is formed by a conductor in length. It is fed in the lower end, which is near a conductive surface which works as a reflector (see Effect of ground). The current in the reflected image has the same direction and phase that the current in the real antenna. The set quarter-wave plus image forms a half-wave dipole that radiates only in the upper half of space.
In this upper side of space the emitted field has the same amplitude of the field radiated by a half-wave dipole fed with the same current. Therefore, the total emitted power is one-half the emitted power of a half-wave dipole fed with the same current. As the current is the same, the radiation resistance (real part of series impedance) will be one-half of the series impedance of a half-wave dipole. As the reactive part is also divided by 2, the impedance of a quarter wave antenna is ohms. Since the fields above ground are the same as for the dipole, but only half the power is applied, the gain is twice (3dB over) that for a half-wave dipole (), that is 5.14 dBi.
The earth can be used as ground plane. However, the earth is not a good conductor. It is rather a dielectric. The reflected antenna image is good when seen at grazing angles, that is, far from the antenna, but not when seen near the antenna. Far from the antenna and near the ground, electromagnetic fields and radiation patterns are the same as for a half-wave dipole. The impedance is not the same as with a good conductor ground plane. Conductivity of earth surface can be improved with an expensive copper wire mesh.
When ground is not available, as in a vehicle, other metallic surfaces can serve as a ground plane, for example the roof of the vehicle. In other situations, radial wires placed at the foot of the quarter-wave wire can simulate a ground plane. For VHF bands, the radiating and groundplane elements can be realised as rigid rods or tubes.
Dipoles that are much smaller than the wavelength of the signal are called Hertzian, short, or infinitesimal dipoles. These have a very low radiation resistance and a high reactance, making them inefficient, but they are often the only available antennas at very long wavelengths. Dipoles whose length is half the wavelength of the signal are called half-wave dipoles, and are more efficient. In general radio engineering, the term dipole usually means a half-wave dipole (center-fed).
A half-wave dipole is cut to length according to the formula [ft], where l is the length in feet and f is the center frequency in MHz. This is because the impedance of the dipole is resistive pure at about this length. The metric formula is [m], where l is the length in meters. The length of the dipole antenna is about 95% of half a wavelength at the speed of light in free space.
The magic numbers above are derived from a one Hz wavelength which is the distance that light radio travels in one second. For English that is 186,282 miles times 5280 feet per mile. To convert to metric multiply the previous total by 12 inches per foot and then, by definition, multiply that by 2.54 cm per inch. Divide this number by 100 to convert this length to meters. Then divide the result by one million to account for MHz rather than hertz. This will give a number which must be divided by two for a dipole antenna. To correct for resistance and impedance multiply the dipole wavelength by about 95% to account for the difference in the velocity of wave propagation in wire as opposed to the same wave in free space. If the wire velocity is known, that value should be used to get the magic numbers of 468 feet or 142.65 metric. All that is left is to divide by the desired frequency as measured in MHz to obtain the length of the antenna element.
Dipoles have a toroidal (doughnut-shaped) reception and radiation pattern where the axis of the toroid centers about the dipole. The theoretical maximum gain of a Hertzian dipole is 10 log 1.5 or 1.76 dBi. The maximum theoretical gain of a λ/2-dipole is 10 log 1.64 or 2.15 dBi.
Ideally, a half-wave (λ/2) dipole should be fed with a balanced line matching the theoretical 73 ohm impedance of the antenna. A folded dipole uses a 300 ohm balanced feeder line.
Many people have had success in feeding a dipole directly with a coaxial cable feed rather than a ladder-line. However, coax is not symmetrical and thus not a balanced feeder. It is unbalanced, because the outer shield is connected to earth potential at the other end.  When a balanced antenna such as a dipole is fed with an unbalanced feeder, common mode currents can cause the coax line to radiate in addition to the antenna itself, and the radiation pattern may be asymmetrically distorted.  This can be remedied with the use of a balun.
The most common dipole antenna is the type used with televisions, often colloquially referred to as "rabbit ears" or "bunny ears." While in most applications the dipole elements are arranged along the same line, rabbit ears are adjustable in length and angle. Larger dipoles are sometimes hung in a V shape with the center near the radio equipment on the ground or the ends on the ground with the center supported. Shorter dipoles can be hung vertically. Some have extra elements to get better reception such as loops (especially for UHF transmissions), which can be turnable around a vertical axis, or a dial, which modifies the electrical properties of the antenna at each dial position.
Another common place one can see dipoles is as antennas for the FM band - these are folded dipoles. The tips of the antenna are folded back until they almost meet at the feedpoint, such that the antenna comprises one entire wavelength. This arrangement has a greater bandwidth than a standard half-wave dipole. If the conductor has a constant radius and cross-section, at resonance the input impedance is four times that of a half-wave dipole.
Dipoles for longer wavelengths are made from solid or stranded wire. Portable dipole antennas are made from wire that can be rolled up when not in use. Ropes with weights on the ends can be thrown over supports such as tree branches and then used to hoist up the antenna. The center and the connecting cable can be hoisted up with the ends on the ground or the ends hoisted up between two supports in a V shape. While permanent antennas can be trimmed to the proper length, it is helpful if portable antennas are adjustable to allow for local conditions when moved. One easy way is to fold the ends of the elements to form loops and use adjustable clamps. The loops can then be used as attachment points.
It is important to fit a good insulator at the ends of the dipole, as failure to do so can lead to a flashover if the dipole is used with a transmitter. One cheap insulator is the plastic carrier that holds a pack of beer cans together. This beer can insulator is an example of how a household object can be used in place of an expensive object sold for use as an item of radio equipment. Other objects that can be used as insulators include buttons from old clothing.
The whip antenna is probably the most common and simplest-looking antenna. These are monopoles, and the most common and practical is the quarter-wave monopole which could be considered as half of a dipole using a ground plane as the image of the other half. The commonly referred-to end-fed dipole is actually just a half-wave monopole whip antenna.
Dipoles are generally more efficient than whip antennas (quarter-wave monopoles). The total radiated power and the radiation resistance are twice that of a quarter-wave monopole. Thus, if a whip antenna were used with an infinite perfectly conducting ground plane, then it would be as efficient in half-space as a dipole in free space an infinite distance from any conductive surfaces such as the earth's surface.
US Military personnel occasionally use a doublet antenna, especially during dismounted unconventional warfare. A radio operator may choose to bring several doublet antennas for different frequencies, such as an antenna cut to length for the set MEDEVAC (medical evacuation) frequency, NCS (net control station) frequency, and tactical frequency (the frequency used by troops in the field). This approach may not be acceptable depending on the mission. Note that a doublet antenna will not work with the standard SINCGARS radio when using FH (frequency hop) but is effective for SC (single channel). A doublet antenna is more practical for radios not intended for FH.
Dipoles can be stacked end to end in phased arrays to make collinear antenna arrays, which exhibit more gain in certain directions—the toroidal radiation pattern is flattened out, giving maximum gain at right angles to the axis of the colinear array.
A Slim Jim or J-pole is a form of end-fed dipole connected to a quarter-wave stub matching section.
This type of antenna is a special case where each wire is exactly one-quarter of the wavelength, for a total of a half wavelength. The radiation resistance is about 73 ohms if wire diameter is ignored, making it easily matched to a coaxial transmission line. The directivity is a constant 1.64, or 2.15 dB. Actual gain will be a little less due to ohmic losses.
If the dipole is not driven at the centre then the feed point resistance will be higher. If the feed point is distance x from one end of a half wave (λ/2) dipole, the resistance will be described by the following equation.
If taken to the extreme then the feed point resistance of a λ/2 long rod is infinite, but it is possible to use a λ/2 pole as an aerial; the right way to drive it is to connect it to one terminal of a parallel LC resonant circuit. The other side of the circuit must be connected to the braid of a coaxial cable lead and the core of the coaxial cable can be connected part way up the coil from the RF ground side. An alternative means of feeding this system is to use a second coil which is magnetically coupled to the coil attached to the aerial.
A folded dipole is a half-wave dipole with an additional wire connecting its two ends. If the additional wire has the same diameter and cross-section as the dipole, two nearly identical radiating currents are generated. The resulting far-field emission pattern is nearly identical to the one for the single-wire dipole described above; however, at resonance its input (feedpoint) impedance Rdf is four times the radiation resistance of a single-wire dipole. This is because for a fixed amount of power, the total radiating current I0 is equal to twice the current in each wire and thus equal to twice the current at the feed point. Equating the average radiated power to the average power delivered at the feedpoint, we may write
It follows that
The folded dipole is therefore well matched to 300-Ohm balanced transmission lines.
The Hertzian dipole is a theoretical short dipole (significantly smaller than the wavelength) with a uniform current along its length. A true Hertzian dipole cannot physically exist, since the assumed current distribution implies an infinite accumulation of charge at its ends.
The radiation resistance is given by:
where Z0 is the impedance of free space. This is precisely four times the radiation resistance of the real short dipole with the linearly tapered current distribution.
The radiation resistance is typically a fraction of an ohm, making the infinitesimal dipole an inefficient radiator. The directivity D, which is the theoretical gain of the antenna assuming no ohmic losses (not real-world), is a constant of 1.5, which corresponds to 1.76 dB. Actual gain will be much less due to the ohmic losses and the loss inherent in connecting a transmission line to the antenna, which is very hard to do efficiently considering the incredibly low radiation resistance. The maximum effective aperture is:
A surprising result is that even though the Hertzian dipole is minute, its effective aperture is comparable to antennas many times its size.
Antenna gain is sometimes measured as "x dB above a dipole", which means that the antenna in question is being compared to a dipole, and has x dB more gain (has more directivity) than the dipole tuned to the same operating frequency. In this case one says the antenna has a gain of "x dBd" (see decibel). More often, gains are expressed relative to an isotropic radiator, which is an imaginary aerial that radiates equally in all directions. In this case one uses dBi instead of dBd (see decibel). As it is impossible to build an isotropic radiator, gain measurements expressed relative to a dipole are more practical when a reference dipole aerial is used for experimental measurements. 0 dBd is often considered equal to 2.15 dBi.
A dipole antenna cut from an infinitely large sheet of metal, with sufficient thickness, is complementary to the slot antenna, both giving the same radiation pattern.
When a dipole is used both to transmit and to receive, the characteristics of the feedline become much more important. Specifically, the antenna must be balanced with the feedline. Failure to do this causes the feedline, in addition to the antenna itself, to radiate. RF can be induced into other electronic equipment near the radiating feedline, causing RF interference. Furthermore, the antenna is not as efficient as it could be because it is radiating closer to the ground and its radiation (and reception) pattern may be distorted asymmetrically. At higher frequencies, where the length of the dipole becomes significantly shorter than the diameter of the feeder coax, this becomes a more significant problem. One solution to this problem is to use a balun.
Several type of baluns are commonly used to transmit on a dipole: current baluns and coax baluns.
A coax balun is a cost effective method to eliminate feeder radiation, but is limited to a narrow set of operating frequencies.
At VHF frequencies, a sleeve balun can also be built to remove feeder radiation.
Elementary, short and half-wave dipoles:
The dipole antenna is one of the most important and commonly used type of radio antenna. It is widely used on its own. It is also used in many other radio antenna designs where it forms the radiating or driven element for the antenna.
The dipole antenna is made of two poles into which radio frequency current flows. This current and the voltage causes an electromagnetic or radio signal to be radiated. Being more specific, a dipole is usually an antenna that uses a resonant length of conductor. This conductor is cut so it can be connected to the what is sending the signal. For resonance, the length of the conductor is an odd number of half wavelengths long. In most cases a single half-wavelength is used.
The length of a dipole is very important for selecting the frequency that the antenna will work at. The antenna is an electrical half wavelength, or multiple of half wavelengths. This is not the same length as the wavelength for a signal travelling through air. There are a number of reasons for this. This means that an antenna will be a small amount shorter than the length calculated for a wave moving in air.
For a half wave dipole, the length for a wave moving in air is calculated. This number is multiplied by a factor "A". A is normally between 0.96 and 0.98. It is affected by the ratio of the length of the antenna to the thickness of the wire or tube used poles.