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Dipole antenna

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A simple half-wave dipole antenna that a shortwave listener might build

A dipole antenna, invented by Heinrich Rudolph Hertz around 1886, is an antenna 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.

Dipole characteristics

Frequency versus length

Dipoles that are much smaller than the wavelength of the signal are called Hertzian dipoles. These have a 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.

A half-wave dipole is cut to length according to the formula l = 468 / f {\displaystyle l=468/f} , where l is the length in feet and f is the center frequency in MHz . The metric formula is l = 143 / f {\displaystyle l=143/f} , where l is the length in meters. The length of the dipole antenna is about 80% of half a wavelength at the speed of light in free space. This is because the velocity of propagation of electromagnetic waves in wire is slower than that in free space.

Radiation pattern and gain

A dipole's radiation pattern

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.

Feeder line

Ideally, a 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.

Common applications of dipole antennas

Set-top TV antenna

The most common dipole antenna is the "rabbit ears" type used with televisions. While theoretically the dipole elements should be 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.

Folded dipole

Another common place one can see dipoles is as antennas for the FM band - these are folded dipoles. The tips of the antenna are folded back until they almost meet at the feedpoint, such that the antenna comprises one entire wavelength. The main advantage of this arrangement is an improved bandwidth over a standard half-wave dipole.

Shortwave antenna

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.

Whip antenna

The whip antenna is probably the most common and simplest-looking antenna, and is actually half of a dipole using a ground plane as the image of the other half.

Dipoles vs. Marconi whip antennas

Dipoles can be more efficient than whip antennas. If a whip antenna were used with an infinite perfectly conducting ground plane, then it would be as efficient as a dipole that was an infinite distance from any conductive surfaces such as the earth's surface.

Groundplane antenna

The groundplane antenna takes the form of a driven vertical element 1/4 wave long in the center of a grounded plane 1/2 wave in diameter. The end of the vertical element nearest the ground plane is connected to the radio, and the far end is in hanging in free space. The ground plane can take the form of the natural Earth surface, or a network of wires and ground rods, or a solid metal sheet, or four wires arranged as two crossed dipoles and centrally connected to ground.

Dipole towers

Large constructed half-wavelength dipole towers include the Warsaw radio mast and Blaw-Knox Towers.

Collinear antenna systems based on dipoles

Dipoles can be stacked end to end in phased arrays to make collinear antennas, 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 collinear array.

Slim Jim or J-pole

A Slim Jim or J-pole is a form of end-fed dipole connected to a quarter-wave transmission line used as a matching section.

Dipole types

Ideal half-wavelength dipole

This type of antenna is a special case where each wire is exactly one-quarter of the wavelength, for a total of a half wavelength. The terminal impedance is about 73 ohms if wire diameter is ignored.

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 the resistance will be described by the following equation.

R x = 75 / sin 2 ⁡ ( 2 π x / λ ) {\displaystyle R_{x}=75/\sin ^{2}(2\pi x/\lambda )}

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.

Folded dipole

A folded dipole is a dipole where an additional wire (λ/2) links the two ends of the (λ/2) half wave dipole. The folded dipole works in the same way as a normal dipole, but the radiation resistance is about 300 ohms rather than the 75 ohms which is expected for a normal dipole. The increase in radiation resistance allows the antenna to be driven from a 300 ohm balanced line.

Infinitesimal dipole

The length of this antenna is significantly smaller than the wavelength:

l < λ 50 {\displaystyle l<{\frac {\lambda }{50}}}

The radiation resistance is given by:

R r = 80 π 2 ( l λ ) 2 {\displaystyle R_{r}=80\pi ^{2}\left({\frac {l}{\lambda }}\right)^{2}}

The radiation resistance is typically a fraction of an ohm, making the infinitesimal dipole an inefficient radiator. In the far field, the maximum directive gain is 1.5. The maximum effective aperture is:

A e = 3 λ 2 8 π {\displaystyle A_{e}={\frac {3\lambda ^{2}}{8\pi }}}

A surprising result is that even though the infinitesimal dipole is minute, its effective aperture is comparable to antennas many times its size!

Dipole as a reference standard

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. More often, gains are expressed relative to an isotropic radiator, which is an imaginary aerial that radiates equally in all directions. 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.

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.

Dipole with baluns

File:Dipolefeedrad.jpg
Coax acting as a radiator instead of the antenna

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.

Current balun

File:Dipolewidebandbalun.jpg
Dipole with a current balun

A current balun is a bit more expensive but has the characteristic of being more broadband.

Coax balun

File:Dipolehalfwavebalun.jpg
Here is a dipole using a coax balun.

A coax balun is a cost effective method to eliminate feeder radiation, but is limited to a narrow set of operating frequencies.

Sleeve balun

File:Dipolesleevebalun.jpg
Here is a dipole using a sleeve balun.

At VHF frequencies, a sleeve balun can also be built to remove feeder radiation.

See also

References

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