This article provides a summary description of many of the different antenna types used for radio receiving or transmitting systems. Different types of antennas are made with properties especially optimized for particular uses, and the electrical design of antennas serves as a way to group them:
This section lists the article's main sections and subsections in the order that they occur. Each group of antennas fit together due to some commonly used electrical operating principle: In at least one regard, the grouped antennas all work in the same way.
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 classification and sub-classifications below follow those typically used in most antenna engineering textbooks. [1] [2] [3] [4] (p 4)
The list below is a summary the several parts of this article, and the bold-face links lead into the named sections and subsections. Links within the linked sections themselves lead further on, to other Wikipedia articles on that antenna type.
The category of simple antennas consists of dipoles, monopoles, and loop antennas. Nearly all can be made with a single segment of wire (ignoring the break made in the wire for the feedline connection).[ citation needed ]
Dipoles and monopoles called linear antennas (or straight wire antennas) since their radiating parts lie along a single straight line. On rare occasions they are called electric antennas since they engage with the electric part of RF radiation, in contrast to loops, which correspondingly are magnetic.
The dipole consists of two conductors, usually metal rods or wires, usually arranged symmetrically, end-to-end, with one side of the balanced feedline from the transmitter or receiver attached to each, and usually elevated as high as feasible above the ground. [3] [c] Some varieties of dipoles differ only in having off-center feedpoints or feedpoints at their ends, others vary the alignment or shape of the dipole arms. [6] Although dipoles are used alone as omnidirectional antennas, they are also a building block of many other more complicated directional antennas.
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 radiation 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. (The use of fold here is not the same as "fold" in "folded dipole" or "folded unipole".) This apparent mangling has very little affect on the antenna's radiation. [9] [2]
The only serious constraint is 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. [9]
A monopole antenna is a half-dipole (see above); it consists of a single conductor such as a metal rod, usually mounted over electrically conductive ground, or an artificial conducting surface (called a ground plane , ground system , or a counterpoise ). [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 tubing; rarely, both dipoles and monopoles are called electric antennas,[ citation needed ] since they interact with the electric field of a radio wave, to contrast them against all sizes of loops, which are correspondingly magnetic antennas. [b]
One side of the feedline from the receiver or transmitter is connected to the radiating arm of the antenna, and the other side to ground or the artificial ground plane. The radio waves from the monopole reflected off the ground plane appear as if they came from a fictitious image antenna seemingly below the ground plane, with the monopole and its phantom image effectively forming a dipole. Hence, the monopole antenna has a radiation pattern identical to the top half of the pattern of a similar dipole antenna, and a radiation resistance a bit less than half of a dipole. Since all of the equivalent dipole's radiation is concentrated in a half-space, the antenna has twice the gain (+3 dB) of a similar dipole, neglecting power lost in the ground plane. [2]
Loop antennas consist of a loop (or coil) of wire. Loop antennas interact directly with the magnetic field of the radio wave, rather than its electric field as linear antennas do; for that reason they are on rare occasions categorized as magnetic antennas, but that generic name is confusingly similar to the term magnetic loop normally used to describe small loops. [b] Their exclusive interaction with the magnetic field makes them relatively insensitive to electrical spark noise within about 1/ 6 wavelength of the antenna. [3] [11] [2] There are essentially two broad categories of loop antennas: large loops (or full-wave loops) and small loops. The halo is the only loop antenna does not exclusively fit in either the large loop or small loop category.
Full-wave loops have the highest radiation resistance, and hence the highest efficiency of all antennas: Their radiation resistances are a few hundreds of Ohms, whereas dipoles and monopoles are tens of Ohms, and small loops and short whip antennas are a few ohms, or even fractions of an Ohm. [2]
Small loop antennas have very low radiation resistance – typically much smaller than the loss resistance of the wire they are made of, making them inefficient for transmitting. Their directionality and low radiation efficiency is drastically different from full-wave loops. In the expected case that the loop perimeter is smaller than a half-wavelength, if the loop needs to be resonant it must be electrically modified in some way to resonate it artificially – usually by attaching a shunt capacitor across the feedpoint.
Despite their drawbacks, small loops are widely used as receiving antennas, especially at frequencies below 10~20 MHz, where their inefficiency is not an issue and their small size makes them a useful solution to the excessive sizes even of quarter-wave antennas. 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. Small loops are called "magnetic loops"; they are also called "tuned loops" since small loops typically must be modified by adding capacitance to make them resonate on some frequency lower than any that they would "naturally" resonate on.
The nulls in the radiation pattern of small receiving loops and ferrite core antennas are bi-directional, and are much sharper than the directions of maximum power of either loop or of linear antennas, and even most beam antennas; the null directionality of small loops is comparable to the maximal directionality of large dish antennas (aperture antennas, see below).[ citation needed ] For accurately locating a signal source, this makes the small receiving loop's null direction much more precise than the direction of the strongest signal, and the small loop / ferrite core type antennas are widely used for radio direction finding (RDF).
The null direction of small loops can also be exploited to exclude unwanted signals from an interfering station or noise source. [3] [11] [2] Several construction techniques are used to ensure that small receiving loops' null directions are "sharp", including making the perimeter 1/ 10 wavelength, (or at most 1 /4 wavelength). Small transmitting loops' perimeters are instead made as large as possible, up to 1 /3 wave, or even 1 /2, if feasible, in order to improve their generally poor efficiency; however, doing so blurs or erases small transmitting loops' directional nulls, unlike the precise nulls of smaller receiving loops.
Composite antennas are made up of combinations of several simple antennas configured to function as a single antenna,[ citation needed ] similar to how a compound optical lens combines multiple simple lenses. Likewise, for antennas that combine one or more simple antennas with a curved metal surface or flat reflecting screen, the metal dish or curtain functions for radio waves similar to a mirror in optical systems, hence those antennas are analogous to reflecting telescopes and Kleig lights.
Composite antennas are most often designed to bolster directivity beyond that of simple antennas. But a different performance-enhancement is to broaden an antenna's usable frequency range. [v] Just by wiring together several simple antennas at a shared feedpoint, the resulting combined antenna can be made broadbanded or widebanded or multibanded – that is, made to operate well either on several distinct frequencies, or over a single wider frequency span.
Array antennas are composites of multiple simple antennas, either linear, or loops, or combinations of each. The multiple parallel-aligned simple antennas work together as a single compound antenna. The constituent simple antennas can be dipoles, monopoles, or loops, or mixed loops and dipoles. There are three or four types, called broadside arrays, endfire arrays, and parasitic arrays, among others.
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 shooting away from their line in the same direction.
This subsection could also be named "phased arrays", a type of composite directional antenna where the various component simple antennas are laid out a large fraction of a wavelength apart, with each antenna's feedline's phase individually shifted, so that the signal from a radio wave moving in some selected direction across the layout of the several simple antennas arrives simultaneously at the receiver, and hence constructively re-enforce; conversely, waves arriving from other directions will interfere destructively to either suppress or eliminate signals from the unwanted directions. 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. Within limits, how narrowly a particular direction may be selected improves with a greater number of antennas, and / or with antennas spaced more widely apart.
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 others' heads.
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 re‑radiate the beam they intercept along the line of the antenna rods. It is parasitic arrays that are the closest RF analogs of compound optical lenses made from combinations of simple lenses. 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.
An aperture antenna consists of a small dipole or loop feed antenna embedded inside a larger, three-dimensional surrounding structure that guides the radio waves from the feed antenna in a particular direction, and vice versa. 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. Since the outer antenna structure is itself not resonant, it can be used for a wide range of frequencies, by replacing or retuning the inner feed antenna, which often is resonant.
Unlike the antennas discussed so-far, traveling-wave antennas are not resonant so they have inherently broad bandwidth. [3] [4] (pp 549–602) They are typically wire antennas that are multiple wavelengths long, through which the voltage and current waves travel in a single pass, in one direction, as opposed to resonant antennas in which waves instead bounce back-and-forth, and form standing waves.
In order to make traveling-wave antennas receive in a single direction, they are normally terminated by a resistor at one end, with the resistor's resistance matched to the antenna wire's characteristic impedance. Matching the impedance of the termination to the antenna wire maximizes the resistor's absorption of the waves traveling towards it along the antenna wire, hence almost no signals from unwanted directions are reflected backwards toward the feedpoint. Since the resistor absorbs the intercepted waves traveling towards its end of the antenna, the antenna feedpoint opposite the terminating resister only receives waves traveling in a direction away from the resistor and toward the feedpoint. When used for receiving the resistive termination removes more than half of the noise coming in from all directions, while preserving all signal power from the desired direction.
The longer a traveling wave antenna is (in wavelengths) the more narrow its receive direction becomes, approaching or exceeding the performance of compound beam antennas. The great lengths typical of traveling wave antennas makes them unsteerable, hence a fixed antenna must be erected for every desired direction.
If used for transmitting, the resistor makes traveling-wave antennas inefficient, since the resistor absorbs any radio wave after the wave has made a single pass through the antenna wire, as opposed to a resonant antenna in which radio waves cycle back-and-forth several times, giving the signal multiple opportunities to radate. [aa] However, because they are made non-resonant by the terminating resistor, traveling-wave antennas can easily be fed power regardless of frequency – unlike resonant antennas without transmatches, which are limited to frequencies very near their resonances. Because they have no practical restrictions on frequency, traveling-wave antennas may still be favored for transmitting if it is legally and electrically possible to raise the transmit power enough to compensate for the considerable amount of power wasted as heat in the terminating end resistor.
The following are some antenna types that don't comfortably fit into any of the simplified types listed above. Note that although it might well seem like a joke to describe antennas that are laid down on the ground (or even buried in it!) instead of put up high in the air, they actually do work, although with limits.
An isotropic antenna (isotropic radiator) is not a real antenna: It is a hypothetical, completely directionless antenna that radiates equal signal power in all vertical, horizontal, and transverse directions. An old-fashioned incandescent light bulb is often used as an example of a nearly isotropic radiator (of heat and light). Paradoxically, every antenna of any type, shorter than ~ 1 /10 wave in its longest dimension is approximately isotropic, but no real antenna can ever be exactly isotropic.
An antenna that is exactly isotropic is only a mathematical model, used as the base of comparison to calculate either the directivity or gain of real antennas. No real antenna can produce a perfectly isotropic radiation pattern, but the isotropic radiation pattern serves as a "worst possible case" reference for comparing the degree to which other antennas, regardless of type, can project some extra radiation in a preferred direction.
All simple antennas approach closer and closer to being isotropic, as the waves they are transmitting or receiving increase in length beyond several times the antennas' longest side.[ citation needed ]Nearly isotropic antennas can be made by combining several small antennas. Nearly isotropic antennas are used for field strength measurements and as standard reference antennas for testing other antennas, since their alignment is a non-issue: Their signal strength measures exactly the same for almost any orientation. They are used as emergency antennas on satellites, since they work even if the satellite is tilted out of alignment with its communication station.
Isotropic antennas, which don't actually exist, should not be confused with omnidirectional antennas , which are real and fairly common.
An isotropic antenna radiates equal power in all three dimensions, while an omnidirectional antenna radiates equal power in all horizontal directions, but little or none vertically. An omnidirectional antenna's radiated power varies with elevation angle: Maximum in the horizontal, and diminishing as the azimuth rises to align with the antenna's vertical axis. Several types of antennas do not radiate at all in the exactly vertical direction, even despite wavelength increasing; compare that preservation of the null response in the vertical direction to the idealized isotropic antenna, which would radiate equally in every direction.
In electrical engineering, electrical length is a dimensionless parameter equal to the physical length of an electrical conductor such as a cable or wire, divided by the wavelength of alternating current at a given frequency traveling through the conductor. In other words, it is the length of the conductor measured in wavelengths. It can alternately be expressed as an angle, in radians or degrees, equal to the phase shift the alternating current experiences traveling through the conductor.
In radio engineering, an antenna or aerial is an electronic device that converts an alternating electric current into radio waves (transmitting), or radio waves into an electric current (receiving). It is the interface between radio waves propagating through space and electric currents moving in metal conductors, used with a transmitter or receiver. In transmission, a radio transmitter supplies an electric current to the antenna's terminals, and the antenna radiates the energy from the current as electromagnetic waves. In reception, an antenna intercepts some of the power of a radio wave in order to produce an electric current at its terminals, that is applied to a receiver to be amplified. Antennas are essential components of all radio equipment.
Effective radiated power (ERP), synonymous with equivalent radiated power, is an IEEE standardized definition of directional radio frequency (RF) power, such as that emitted by a radio transmitter. It is the total power in watts that would have to be radiated by a half-wave dipole antenna to give the same radiation intensity as the actual source antenna at a distant receiver located in the direction of the antenna's strongest beam. ERP measures the combination of the power emitted by the transmitter and the ability of the antenna to direct that power in a given direction. It is equal to the input power to the antenna multiplied by the gain of the antenna. It is used in electronics and telecommunications, particularly in broadcasting to quantify the apparent power of a broadcasting station experienced by listeners in its reception area.
A directional antenna or beam antenna is an antenna which radiates or receives greater radio wave power in specific directions. Directional antennas can radiate radio waves in beams, when greater concentration of radiation in a certain direction is desired, or in receiving antennas receive radio waves from one specific direction only. This can increase the power transmitted to receivers in that direction, or reduce interference from unwanted sources. This contrasts with omnidirectional antennas such as dipole antennas which radiate radio waves over a wide angle, or receive from a wide angle.
In radio communication, an omnidirectional antenna is a class of antenna which radiates equal radio power in all directions perpendicular to an axis, with power varying with angle to the axis, declining to zero on the axis. When graphed in three dimensions (see graph) this radiation pattern is often described as doughnut-shaped. This is different from an isotropic antenna, which radiates equal power in all directions, having a spherical radiation pattern. Omnidirectional antennas oriented vertically are widely used for nondirectional antennas on the surface of the Earth because they radiate equally in all horizontal directions, while the power radiated drops off with elevation angle so little radio energy is aimed into the sky or down toward the earth and wasted.
Radiation resistance is that part of an antenna's feedpoint electrical resistance caused by the emission of radio waves from the antenna. A radio transmitter applies a radio frequency alternating current to an antenna, which radiates the energy of the current as radio waves. Because the antenna is absorbing the energy it is radiating from the transmitter, the antenna's input terminals present a resistance to the current from the transmitter.
A helical antenna is an antenna consisting of one or more conducting wires wound in the form of a helix. A helical antenna made of one helical wire, the most common type, is called monofilar, while antennas with two or four wires in a helix are called bifilar, or quadrifilar, respectively.
In radio and telecommunications a dipole antenna or doublet is one of the two simplest and most widely-used types of antenna; the other is the monopole. The dipole is any one of a class of antennas producing a radiation pattern approximating that of an elementary electric dipole with a radiating structure supporting a line current so energized that the current has only one node at each far end. A dipole antenna commonly consists of two identical conductive elements such as metal wires or rods. The driving current from the transmitter is applied, or for receiving antennas the output signal to the receiver is taken, between the two halves of the antenna. Each side of the feedline to the transmitter or receiver is connected to one of the conductors. This contrasts with a monopole antenna, which consists of a single rod or conductor with one side of the feedline connected to it, and the other side connected to some type of ground. A common example of a dipole is the rabbit ears television antenna found on broadcast television sets. All dipoles are electrically equivalent to two monopoles mounted end-to-end and fed with opposite phases, with the ground plane between them made virtual by the opposing monopole.
A whip antenna is an antenna consisting of a straight flexible wire or rod. The bottom end of the whip is connected to the radio receiver or transmitter. A whip antenna is a form of monopole antenna. The antenna is designed to be flexible so that it does not break easily, and the name is derived from the whip-like motion that it exhibits when disturbed. Whip antennas for portable radios are often made of a series of interlocking telescoping metal tubes, so they can be retracted when not in use. Longer whips, made for mounting on vehicles and structures, are made of a flexible fiberglass rod around a wire core and can be up to 11 m long.
The Beverage antenna or "wave antenna" is a long-wire receiving antenna mainly used in the low frequency and medium frequency radio bands, invented by Harold H. Beverage in 1921. It is used by amateur radio operators, shortwave listeners, longwave radio DXers and for military applications.
A mast radiator is a radio mast or tower in which the metal structure itself is energized and functions as an antenna. This design, first used widely in the 1930s, is commonly used for transmitting antennas operating at low frequencies, in the LF and MF bands, in particular those used for AM radio broadcasting stations. The conductive steel mast is electrically connected to the transmitter. Its base is usually mounted on a nonconductive support to insulate it from the ground. A mast radiator is a form of monopole antenna.
A ‘T’-antenna, ‘T’-aerial, or flat-top antenna is a monopole radio antenna consisting of one or more horizontal wires suspended between two supporting radio masts or buildings and insulated from them at the ends. A vertical wire is connected to the center of the horizontal wires and hangs down close to the ground, connected to the transmitter or receiver. The shape of the antenna resembles the letter "T", hence the name. The transmitter power is applied, or the receiver is connected, between the bottom of the vertical wire and a ground connection.
A loop antenna is a radio antenna consisting of a loop or coil of wire, tubing, or other electrical conductor, that for transmitting is usually fed by a balanced power source or for receiving feeds a balanced load. Within this physical description there are two distinct types:
A monopole antenna is a class of radio antenna consisting of a straight rod-shaped conductor, often mounted perpendicularly over some type of conductive surface, called a ground plane. The driving signal from the transmitter is applied, or for receiving antennas the output signal to the receiver is taken, between the lower end of the monopole and the ground plane. One side of the antenna feedline is attached to the lower end of the monopole, and the other side is attached to the ground plane, which is often the Earth. This contrasts with a dipole antenna which consists of two identical rod conductors, with the signal from the transmitter applied between the two halves of the antenna.
A radio transmitter or receiver is connected to an antenna which emits or receives the radio waves. The antenna feed system or antenna feed is the cable or conductor, and other associated equipment, which connects the transmitter or receiver with the antenna and makes the two devices compatible. In a radio transmitter, the transmitter generates an alternating current of radio frequency, and the feed system feeds the current to the antenna, which converts the power in the current to radio waves. In a radio receiver, the incoming radio waves excite tiny alternating currents in the antenna, and the feed system delivers this current to the receiver, which processes the signal.
The folded unipole antenna is a type of monopole mast radiator antenna used as a transmitting antenna mainly in the medium wave band for AM radio broadcasting stations. It consists of a vertical metal rod or mast mounted over and connected at its base to a grounding system consisting of buried wires. The mast is surrounded by a "skirt" of vertical wires electrically attached at or near the top of the mast. The skirt wires are connected by a metal ring near the mast base, and the feedline feeding power from the transmitter is connected between the ring and the ground.
An umbrella antenna is a capacitively top-loaded wire monopole antenna, consisting in most cases of a mast fed at the ground end, to which a number of radial wires are connected at the top, sloping downwards. One side of the feedline supplying power from the transmitter is connected to the mast, and the other side to a ground (Earthing) system of radial wires buried in the earth under the antenna. They are used as transmitting antennas below 1 MHz, in the MF, LF and particularly the VLF bands, at frequencies sufficiently low that it is impractical or infeasible to build a full size quarter-wave monopole antenna. The outer end of each radial wire, sloping down from the top of the antenna, is connected by an insulator to a supporting rope or cable anchored to the ground; the radial wires can also support the mast as guy wires. The radial wires make the antenna look like the wire frame of a giant umbrella hence the name.
A turnstile antenna, or crossed-dipole antenna, is a radio antenna consisting of a set of two identical dipole antennas mounted at right angles to each other and fed in phase quadrature; the two currents applied to the dipoles are 90° out of phase. The name reflects the notion the antenna looks like a turnstile when mounted horizontally. The antenna can be used in two possible modes. In normal mode the antenna radiates horizontally polarized radio waves perpendicular to its axis. In axial mode the antenna radiates circularly polarized radiation along its axis.
A shortwave broadband antenna is a radio antenna that can be used for transmission of any shortwave radio band from among the greater part of the shortwave radio spectrum, without requiring any band-by-band adjustment of the antenna. Generally speaking, there is no difficulty in building an adequate receiving antenna; the challenge is designing an antenna which can be used for transmission without an adjustable impedance matching network.
A halo antenna, or halo, is a center-fed 1 /2 wavelength dipole antenna, which has been bent into a circle, with a break directly opposite the feed point. The dipole's ends are close, but do not touch, and the ends on either side of the gap may be flared out to form a larger air gap capacitor, whose spacing is used to fine-adjust the antenna's resonant frequency. Most often halos are mounted horizontally, resulting in the antenna's radiation being horizontally polarized and very nearly omnidirectional.
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