Dipole antenna

German physicist Heinrich Hertz first demonstrated the existence of radio waves in 1887 using what we now know as a dipole antenna (with capacitative end-loading).

[7](p 3) 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.

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, S, which is given by

the phase factors (the exponentials) cancel out, leaving: We have now expressed the flux in terms of the feedpoint current Io and the ratio of the short dipole's length ℓ to the wavelength of radiation λ.

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).

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.

[13] 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.

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 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!)

[8][9] 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.

For the same reason, antennas with thicker conductors have a wider operating bandwidth over which they attain a practical standing wave ratio which is degraded by any remaining reactance.

Ideally, a half-wave dipole should be fed using a balanced transmission line matching its typical 65–70 Ω input impedance.

Twin lead with a similar impedance is available but seldom used and does not match the balanced antenna terminals of most radio and television receivers.

[19][full citation needed] Most FM broadcast band tuners and older analog televisions include balanced 300 Ω antenna input terminals.

Many types of coaxial cable (or coax) have a characteristic impedance of 75 Ω, which would otherwise be a good match for a half-wave dipole.

[20][24] A coax balun is a cost-effective method of eliminating feeder radiation but is limited to a narrow set of operating frequencies.

For fixed use in homes, hi-fi tuners are typically supplied with simple folded dipoles resonant near the center of that band.

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 radios, such as police, fire, and taxi dispatchers.

In the popular high-gain Yagi antenna, only one of the dipoles is actually connected electrically, but the others receive and reradiate power supplied by the driven element.

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.

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 resulting field pattern then reduces to an integral over the path of an antenna conductor (modeled as a thin wire).

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.

Using the induced EMF method closed form expressions are obtained for both components of the feedpoint impedance; such results are plotted above.

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  Si(x)  and  Ci(x) .

[e] 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.

Determination of each matrix element requires at least one double integration involving the weighting functions, which may become computationally intensive.

UHF half-wave dipole
Dipole antenna used by the radar altimeter in an airplane
Animated diagram of a half-wave dipole antenna receiving a radio wave. The antenna consists of two metal rods connected to a receiver R . The electric field ( E , green arrows ) of the incoming wave pushes the electrons in the rods back and forth, charging the ends alternately positive (+) and negative (−) . Since the length of the antenna is one half the wavelength of the wave, the oscillating field induces standing waves of voltage ( V , represented by the red band ) and current in the rods. The oscillating currents ( black arrows ) flow down the transmission line and through the receiver (represented by the resistance R ).
Diagram of a short dipole antenna
Diagram of a short dipole antenna
Radiation pattern of the short dipole (dashed line) compared to the half-wave dipole (solid line)
Animation of a transmitting half wave dipole showing the voltage ( red , ) and current ( blue , ) due to the standing wave 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 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.
Cage dipole antennas in the Ukrainian UTR-2 radio telescope. The 8 m by 1.8 m diameter galvanized steel wire dipoles have a bandwidth of 8–33 MHz.
A 1 / 4 λ monopole antenna and its ground image together form a 1 / 2 λ dipole that radiates only in the upper half of space.
Resistive (black) and reactive (blue) parts of the dipole feedpoint impedance versus total length in wavelengths, assuming a conductor diameter of 0.001 wavelengths
Feedpoint impedance of (near-) half-wave dipoles versus electrical length in wavelengths. Black: radiation resistance ; blue: reactance for four different values of conductor diameter.
Length reduction factor for a half-wave dipole to achieve electrical resonance (purely resistive feedpoint impedance). Calculated using the induced EMF method , an approximation that breaks down at larger conductor diameters (dashed portion of graph).
"Rabbit-ears" VHF television antenna (the small loop is a separate UHF antenna).
Collinear folded dipole array
A reflective array antenna for radar consisting of numerous dipoles fed in-phase (thus realizing a broadside array ) in front of a large reflector (horizontal wires) to make it uni-directional
Hertzian dipole of tiny length with current and field sensed at a distance in the direction
Animated diagram showing how the E and H fields in the x y plane depend on both time and distance
Electric field lines and magnetic field components at right angles composing the electromagnetic wave radiated by the current element