Dipole antenna

From Academic Kids

A dipole antenna is an antenna with two driven elements. These antennas are the simplest antennas from a theoretical point of view, though the whip antenna (with only one driven element—it is driven against earth) is probably the most common and simplest-looking antenna.

The two driven elements in a dipole antenna are arranged end to end, along a line. The two elements are separate, and fed at the center. The length of the two elements is the same, and their overall length determines the antenna's wavelength. The two elements are fed the same signal 180 degrees out of phase.

The most common dipole antenna is the kind used with televisions called "rabbit ears". 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.

While the common rabbit ears are made of tubular metal segments, most dipoles for larger 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.

Dipoles are more efficient than whip antennas. Dipoles have a toroidal radiation pattern where the axis of the toroid centers about the dipole.

Antenna gain is commonly 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.

A dipole antenna cut from an infinitely large sheet of metal, with some thickness, is complementary to the slot antenna the cut formed, both giving the same radiation pattern.

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.

A Slim Jim is a form of apparently end-fed dipole—one element is doubled back to form a feed/match for the other. End-feeding is an advantage in some situations.

Infinitesimal Dipole

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

<math>l < \frac{\lambda}{50} <math>

The radiation resistance is given by:

<math>R_r = 80 \pi^2 \left ( \frac{l}{\lambda} \right )^2 <math>

The radiation resistance is typically fractions 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:

<math>A_e = \frac{3 \lambda ^2 }{8 \pi} <math>

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

Ideal Half Wavelength Dipole

This type of antenna is a special case where each wire is exactly one-quarter of the wavelength. The terminal impedence is about 73 Ohms if wire diameter is ignored.ja:ダイポールアンテナ


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