Antenna types

This article provides a summary description of many of the different antenna types used for radio receiving or transmitting systems.

The list below summarizes the several parts of this article; the bold-face links lead into the named sections and subsections.

[2] The elongated segments of dipole antenna are most often made of either thin, strong wire or hollow metal tubing.

However they can also be made out of "cages" of wire: Several segments of fairly close spaced electrically parallel wires, used to simulate the electrical behavior of a much wider metal tube, most particularly lower "Ohmic" resistance and wider bandwidth, without as much trouble from wind loading.

Almost all of the radiation from a dipole comes roughly from the half of its total length closest to its center, around the usual feedpoint where the two arms meet; approximately the last third of each of the dipole arms only radiates a minuscule amount of the outgoing signal, so for the purpose of emitting radio waves, the shape of each outer end is not important.

This shape-indifference allows otherwise prohibitively long dipoles to have their far ends bent sideways, folded over, or zig-zagged, in order to shorten the antenna to fit inside an available space.

[9][2] The only serious constraint on end folding is electrical safety: The dangerous high-voltage antenna tips (remarkably high, even for modestly low power transmission) must be out of harm's way, including anywhere a dangling wire might reach if blown by a strong wind.

[2] For the most part, fold shapes are freely improvised by the person raising the antenna; various possible end folds are not listed in this article as a separate design, and should be considered a normal, electrically inconsequential, convenience modification for every type of linear antenna.

[3][10] They are sometimes classed together with dipoles (see above) in the broader category of linear antennas, or more plainly straight wire antennas,[citation needed] since their radiating section is normally a straight (linear) wire or section of metal tubing.

[b] Their exclusive interaction with the magnetic field makes them relatively insensitive to electrical spark noise within about ⁠1/ 6 ⁠ wavelength of the antenna.

The fact that they can be efficiently tuned to accept only a very narrow frequency range (similar to a preselector) helps alleviate much of the trouble caused by the pervasive static always encountered on the mediumwaves and lower shortwaves where small loops are most popular.

The null direction of small loops can also be exploited to exclude unwanted signals from an interfering station or noise source.

Broadside arrays consist of multiple, parallel, identical driven elements, usually dipoles, fed in phase, radiating a beam perpendicular to the plane containing the simple antennas, analogous to a firing line of musketeers,[citation needed] all simultaneously shooting in the same direction, perpendicularly out from the line of shooters.

The same phasing technique works in reverse, with signals transmitted from the several antennas combining to form a wave front departing mostly in one direction.

Phase change can electrically steer the radiation receive and transmit direction without physically moving the antennas.

Endfire arrays have their driven elements fed out-of-phase, with the phase difference corresponding to the distance between them; they radiate within the plane that the consitituent parallel antennas all lie in.

[3][13][4](pp 283–371) Continuing with the musketeer[citation needed] analogy, an endfire array works similarly to a column of shooters, one behind the other; three, for example: One lying on the ground, the next kneeling behind the first, and the last standing at their backs, all aiming in the same direction they are lined up in, those in back firing over the heads of the musketeers in front of them.

Parasitic arrays are a specific type of endfire array that consist of multiple antennas, usually dipoles, with one driven element and the rest parasitic elements, which draw power from the radiated beam and then re‑radiate that signal power along the line of the antenna rods.

They can also be compared to a column of a team of especially skillful badminton players, with the server standing at or near the back, and each team-mate in front taking a swing at the shuttlecock as it passes by, to further it along with better aim.

The guiding structure is often dish-shaped or funnel-shaped, and quite large compared to a wavelength, with an opening, or aperture, to emit the radio waves in only one direction.