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:
In addition to the inevitable
antenna type, which is 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
The category of simple antennas consists of dipoles, monopoles, and loop antennas, all of which can be made with a single section of wire (with some exceptions).[ 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] [lower-alpha 2] The most common type, the half-wave dipole, consists of two resonant elements just under a quarter wavelength long. This antenna radiates maximally in directions perpendicular to the antenna's axis, giving it a small directive gain of 2.15 dBi. Although half-wave dipoles are used alone as omnidirectional antennas, they are also a building block of many other more complicated directional antennas.
A monopole antenna is a half-dipole (see above); it consists of a single conductor such as a metal rod, usually mounted over the ground or an artificial conducting surface (a so-called ground plane ). [3] [6] They are sometimes classed together with dipoles (see above) in the broader category of linear antennas, or more plainly straight wire antennas, since their radiating section is normally a straight (linear) wire or aluminum tubing; rarely, both dipoles and monopoles are called electric antennas, since they interact with the electric field of a radio wave, to contrast them against all sizes of loops, which are correspondingly magnetic antennas. [lower-alpha 5]
One side of the feedline from the receiver or transmitter is connected to the conductor, 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 efficiency a bit less than half of the dipole. Since all of the equivalent dipole's radiation is concentrated in a half-space, the antenna has twice (3 dB increase of) the gain of a similar dipole, neglecting power lost in the ground plane.
The most common form is the quarter-wave monopole which is one-quarter of a wavelength long and has a gain of 5.12 dBi when mounted over a ground plane. Single monopoles have an omnidirectional radiation pattern, so they are used for broad coverage of an area, and have vertical polarization. To reduce signal absorption by the Earth, ground waves used for broadcasting at low frequencies must be vertically polarized, so large vertical monopole antennas are used for broadcasting in the MF, LF, and VLF bands. Small monopoles ("whips") are used as nondirectional antennas on portable radios in the HF, VHF, and UHF bands.
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 used to describe small loops. [lower-alpha 5] Their exclusive interaction with the magnetic field makes them relatively insensitive to electrical spark noise within about 1/ 6 wavelength of the antenna, [3] [7] [2] and more tolerant of being mounted close to the ground, since for most soils the earth is closer to being somewhat radio-transparent to the magnetic part of mediumwaves and longer shortwaves, than is the case for their electric part. There are essentially two broad categories of loop antennas: large loops (or full-wave loops) and small loops. Only one design, a halo antenna, that is normally called a loop does not clearly fit exclusively into either the large or small loop categories.
Full-wave loops have the highest radiation resistance, and hence the highest efficiency of all antennas: Their radiation resistances are several 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]
The approximately-omnidirectional pattern of halos resembles small loops; their radiation efficiency lies inbetween the extreme high efficiency of large loops and the generally poor efficiency of small loops. Halos are self-resonant like full-wave loops, but have no useable higher harmonics. In some regards they represent the extreme upper size limit of small transmitting loops. [3] [2] [4] (pp231–275)
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, and if the loop perimeter is smaller than a half-wavelength, if resonance is necessary the loop must be modified in some way to make it resonant. Despite their drawbacks, small loops are widely used as receiving antennas, especially at lower frequencies, 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. 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] [7] [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 feasibly possible, up to 1 /3 wave, or even 1 /2, if possible, in order to improve their generally poor efficiency; however, doing so blurs or erases small transmitting loops' directional nulls.
Composite antennas are made up of combinations of several simple antennas combined together to function as a single antenna,[ citation needed ] similar to how a compound optical lens combines multiple simple lenses, or a combination of one or more simple antennas with a curved or reflecting screens, whose function for radio waves is similar to a mirror for light waves.
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; they are the RF analogs of compound optical lenses made from combinations of simple lenses. The constituent simple antennas can be dipoles, monopoles, or loops, or mixed loops and dipoles. Broadside arrays consist of multiple identical driven elements, usually dipoles, fed in phase, radiating a beam perpendicular to the plane containing the simple antennas. Endfire arrays are fed out-of-phase, with the phase difference corresponding to the distance between them; they radiate parallel to the plane that the consitituent antennas all lie in. [3] [9] [4] (pp283–371)Parasitic arrays consist of multiple antennas, usually dipoles, with one driven element and the rest parasitic elements, which radiate a beam along the line of the antennas.
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 normally is resonant.
Unlike the antennas discussed so-far, traveling-wave antennas are not resonant so they have inherently broad bandwidth. [3] [4] (pp549–602) They are typically wire antennas that are multiple wavelengths long, through which the voltage and current waves travel in one direction, instead of bouncing back and forth to form standing waves as in resonant antennas. All except the helical antenna have linear polarization. Unidirectional traveling-wave antennas are terminated by a resistor at one end, with the resistor's "Ohmage" equal to the antenna's characteristic impedance, in order to maximally absorb the waves traveling toward that end, with almost no unwanted signals reflected back to the feedpoint. The terminating resistor absorbs the waves traveling along the wire towards the resistor, which causes the antenna to only receive waves traveling in the opposite direction, away from the resistor and toward the feedpoint at the opposite end. This makes traveling-wave antennas inefficient transmitting antennas, but when used for receiving removes more than half of the incident radio noise while preserving the desired signal.
An isotropic antenna (isotropic radiator) is a hypothetical antenna that radiates equal signal power in all directions. An old-fashioned incandescent light bulb is often described 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 the directionality 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 radiation in some 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, as standard reference antennas for testing other antennas, and as emergency antennas on satellites, since they work even if the satellite has lost its orientation to its communication station.
Isotropic antennas, which don't actually exist, should not be confused with omnidirectional antennas , which are 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 direction rises to be parallel to the antenna's vertical axis of symmetry. Several antenna types do not radiate at all in the exactly vertical direction, even as wavelength increases, as opposed to an isotropic antenna, which radiates equally in every direction.
The shape and length of a "random" wire is determined by the available space, locations and number of possible elevated attachment points, and how far the total available length of wire can reach. It is not laid out in a single straight line in a planned direction, and is generally is not trimmed to any particular (resonant) length. A random wire antenna typically has a complicated radiation pattern, with several lobes at varying angles to each wire segment, in different directions for each, which depend on frequency and segment length.
Random wire antennas are sometimes arbitrarily included as a sub-category of folded monopole antennas, if their lengths are a quarter-wave or less, or folded end-fed dipoles if a half-wave or more, up to one or two wavelengths or less. When a random wire is laid out with at least one extended segment oriented in a straight line, one to several wavelengths long, it operates roughly similar to a Beverage antenna, although since it presumably has no resistive termination, it will receive in two opposite directions, aligned with its longest segment, rather than being one-directional like a Beverage.
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 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.
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. Omnidirectional antennas are widely used for radio broadcasting antennas, and in mobile devices that use radio such as cell phones, FM radios, walkie-talkies, wireless computer networks, cordless phones, GPS, as well as for base stations that communicate with mobile radios, such as police and taxi dispatchers and aircraft communications.
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 excites with a radio frequency alternating current an antenna, which radiates the exciting energy 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.
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.
The J-pole antenna, more properly known as the J antenna, is a vertical omnidirectional transmitting antenna used in the shortwave frequency bands. It was invented by Hans Beggerow in 1909 for use in Zeppelin airships. Trailed behind the airship, it consisted of a single one half wavelength long wire radiator, in series with a quarter-wave parallel transmission line tuning stub that matches the antenna impedance to the feedline. By 1936 this antenna began to be used for land-based transmitters with the radiating element and the matching section mounted vertically, giving it the shape of the letter "J", and by 1943 it was named the J antenna. When the radiating half-wave section is mounted horizontally, at right-angles to the quarter-wave matching stub, the variation is usually called a Zepp antenna.
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:
Near vertical incidence skywave, or NVIS, is a skywave radio-wave propagation path that provides usable signals in the medium distances range — usually 0–650 km. It is used for military and paramilitary communications, broadcasting, especially in the tropics, and by radio amateurs for nearby contacts circumventing line-of-sight barriers. The radio waves travel near-vertically upwards into the ionosphere, where they are refracted back down and can be received within a circular region up to 650 km from the transmitter. If the frequency is too high, refraction is insufficient to return the signal to earth and if it is too low, absorption in the ionospheric D layer may reduce the signal strength.
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.
In RF engineering, radial has three distinct meanings, both referring to lines which radiate from a radio antenna, but neither meaning is related to the other.
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 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.
In electronics and radio communication, a counterpoise is a network of suspended horizontal wires or cables, used as a substitute for an earth (ground) connection in a radio antenna system. It is used with radio transmitters or receivers when a normal earth ground cannot be used because of high soil resistance or when an antenna is mounted above ground level, for example, on a building. It usually consists of a single wire or network of horizontal wires, parallel to the ground, suspended above the ground under the antenna, connected to the receiver or transmitter's "ground" wire. The counterpoise functions as one plate of a large capacitor, with the conductive layers of the earth acting as the other plate.
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 their crossections may be broadened to form an air capacitor, whose spacing is used to adjust the antenna's resonant frequency. Most often mounted horizontally, this antenna's radiation is then approximately omnidirectional and horizontally polarized.
An antenna array is a set of multiple connected antennas which work together as a single antenna, to transmit or receive radio waves. The individual antennas are usually connected to a single receiver or transmitter by feedlines that feed the power to the elements in a specific phase relationship. The radio waves radiated by each individual antenna combine and superpose, adding together to enhance the power radiated in desired directions, and cancelling to reduce the power radiated in other directions. Similarly, when used for receiving, the separate radio frequency currents from the individual antennas combine in the receiver with the correct phase relationship to enhance signals received from the desired directions and cancel signals from undesired directions. More sophisticated array antennas may have multiple transmitter or receiver modules, each connected to a separate antenna element or group of elements.
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