Antenna types

In radio systems, many different antenna types are used whose properties are especially crafted for particular applications. Most often, the greatest effect is due to the size (wavelength) of the radio waves the antenna is to intercept or produce; one competing second effect is differences in optimization for receiving and for transmitting; another competing influence is the number and bandwidth of the frequenc(y/ies) that any single antenna must intercept or emit.

Antennas can be classified in various ways, and various writers organize the different aspects of antennas with different priorities, depending on whether their text is most focused on specific frequency bands; or antenna size, construction, and placement feasibility; or explicating principles of radio theory and engineering that underlie, guide, and constrain antenna design. The list below groups together antennas under a commonly used set of operating principles, that follows the classification and sub-classifications used in most typical antenna engineering textbooks.^{[1][2][3][4](p4)} The following list is a summary of the several sections in this article:

Simple antennas

"Simple" antennas include dipoles, monopoles, and loops: These three types of antenna are typically (but not necessarily) used on frequencies where they are self-resonant^[a] These are used as building-blocks for the more complicated antenna types that follow. "Simple" antennas are usually further subdivided into

linear antennas

"Straight-line" antennas or "straight-wire" antennas are on rare occasions called "electric" antennas, since they exclusively couple to the electric part of the electromagnetic radio waves that they emit and absorb

dipole

Two-armed antennas, like "rabbit ears"

monopole

Single-armed antennas, like a single "telescoping" antenna

both of which are often built large enough to be self-resonant. Other linear antenna types are whip antennas and random wire antennas, which are usually not sized to be (naturally) resonant.

loop antennas

Are made of segments of wire or metal bent into a circle or polygon – any two-dimensional figure closed in on itself – and on rare occasions are generically called "magnetic" antennas, since they exclusively interact with the magnetic portion of the radio waves passing through them.

large loops

"Large" loops are loop antennas whose perimeter is at least one full wavelength at their design frequency

halo antennas

"Halos" are resonant loops with half-wavelength perimeters

small loops

"Small" loop antennas loop antennas that are designed for use at frequencies where their perimeter is smaller than a half-wave

Composite antennas

Composite antennas are made by combining one or more simple antenna(s) either with other simple antenna(s) or with some kind of a reflecting surface formed into a screen, or curtain, or curved dish. The two subtypes of array and aperture antennas are otherwise not especially closely related; they are often listed separately.

array antennas

Array antennas are made out of combinations of several simple antennas that function as a single antenna

aperture antennas

Aperture antennas are made of an outer, surrounding reflective surface whose shape concentrates waves striking it onto a small inner simple antenna; the inner antenna can be either resonant or non-resonant.

traveling wave

A traveling wave antenna is a notably non-resonant antenna type: Current and voltage waves are induced by RF radiation traveling through them, in opposite directions, along the antenna element(s); but by design, only the half delivered to the feedpoint end are collected; at the end opposite the feedpoint, the waves raised by radio signals arriving from the direction of the feedpoint are completely absorbed by a carefully chosen terminating resistor. Absorption prevents those waves from reflecting back towards the feedpoint, hence prevents reception of waves from that direction. The termination on the antenna makes it uni-directional, similar to an aperture antenna, but using using less assembled material.

In addition to the inevitable

"other"

antenna type, for an "everything else" category for a few peculiar antennas that don't conveniently fit into any one category, there is a unique "fake" type of antenna called an

isotropic antenna

A truly "isotropic" antenna is a convenient fiction that is useful as "worst possible" performance comparison-case for real antenna directivity. Real antennas that are very nearly isotropic are useful for emergency backup antennas and for test equipment for other antennas, since their signal strength (almost) does not vary with different orientations.

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