Dipole antenna facts for kids

A schematic of a half-wave dipole antenna that a shortwave listener might build.

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.

Basic facts dipole 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.

Length of a dipole antenna

A dipole antenna ("rabbit ears") on top of a television

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.

Uses of dipole antennas

Dipole antennas are used for many different things. These include television antennas (commonly referred to as "rabbit ears"), shortwave radio antennas and whip antennas.

Images for kids

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  UHF half-wave dipole

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  Dipole antenna used by the radar altimeter in an airplane

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  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 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).

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  Animation of a transmitting half wave dipole showing the voltage V(x) (red,     ) and current I(x) (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 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.

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  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.

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  Real (black) and imaginary (blue) parts of the dipole feedpoint impedance versus total length in wavelengths, assuming a conductor diameter of 0.001 wavelengths

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  Feedpoint impedance of (near-) half-wave dipoles versus electrical length in wavelengths. Black: radiation resistance; blue: reactance for 4 different values of conductor diameter.

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  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).

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  "Rabbit-ears" VHF television antenna (the small loop is a separate UHF antenna).

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  Collinear folded dipole array

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  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.

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  Hertzian dipole of tiny length \delta\ell, with current I, and field sensed at a distance r in the \theta direction.

See also

In Spanish: Dipolo (antena) para niños