Antenna types

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. This section lists the article's sections and subsections in groups, with each group of antennas fitting together under some commonly used electrical operating principle: In at least one regard, the grouped antennas all work the same way. 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 relevant subsections. Links within the linked sections themselves lead further on, to other wikipedia articles on that antenna type.

Simple antennas

There are three types of "simple" antennas: dipoles, monopoles, and loops. The three simple antenna types are all typically (but not necessarily) used on frequencies where they self-resonate.^[a] "Simple" antennas are also used as building-blocks for the more complicated antenna types, such as composite antennas. Simple antennas are usually further subdivided into

Linear antennas

"Straight-wire" or "straight-line" 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 dipoles and monopoles are often built large enough to be self-resonant, usually each arm is a quarter-wave long, although small whip antennas and unplanned random wire types specifically are not sized for natural resonance

Loop antennas

Loops are ring-like antennas made out of segments of wire or metal bent into a circle or polygon – any regular or irregular two-dimensional figure that closes in on itself. On rare occasions all loops are generically called "magnetic" antennas,^[b] 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 slightly over one full wavelength at their design frequency; they are naturally resonant on all frequencies that are whole number multiples of that design frequency

Halo antennas

"Halos" are loops with a small gap cut in them, and are naturally resonant for the wavelength that's double their perimeter length

Small loops

"Small" loop antennas are loops of wire or tubing designed for use as antennas at frequencies where their perimeter is smaller than a half-wave; they are not naturally resonant on any frequency they are used on, and must be resonated artificially, usually by attaching a capacitor across their feedpoint.

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. Usually only one of the component antennas is resonant on the design frequency, and usually the feedline connects to that resonant component.

Broadbanded composite antennas

Antennas can be made to be "broadband" or "wideband" in several different ways. Perhaps the most common method of broadbanding is to combine two or more different antennas, connected at a single shared feedpoint, with each separate component readily accepting transmit power on a different frequency. The combined antenna then covers more frequencies than a simple antenna can.

Array antennas

Array antennas are made out of combinations of several simple antennas that function as a single antenna; most compact but highly directional / "high gain" / beam antennas are some type of an array antenna

Aperture antennas

Aperture antennas are made of an outer, surrounding reflective surface many wavelengths wide, whose shape concentrates waves striking the surface onto a small, inner, simple antenna; the inner antenna can be either resonant or non-resonant and of any type

The two subtypes of composites, array and aperture antennas, are otherwise not especially closely related, and are often separately listed as distinct types.

Traveling wave antennas

Traveling wave antennas are notably one of the few types of antennas that are normally not self resonant: Electrical waves induced by received radio waves travel through the antenna wire in the direction of the arriving RF signals. Only electrical waves traveling toward the feedpoint are collected; waves traveling away from the feedpoint are grounded through a terminating resistor at the opposite end. The resistive termination makes the antenna receive in only one direction, similar to an aperture antenna but much simpler to build. In order to make them even more directional, they are made several wavelengths long, hence unsteerable. Absorption in the terminating resistor makes them inefficient radiators, but still sometimes used for transmitting since they work on any frequency.

"Other" antennas

Inevitably some antennas won't conveniently fit into any one basic type, so the last section on real antennas is an "everything else" category for a few peculiar antennas that don't fit cleanly into any of the categories or subcategories used in this article; for example, random wire antennas and antennas that are laid down on the ground instead of raised up in the air.

Isotropic antenna

The last section is for a unique type of "fake" antenna, called an isotropic antenna or isotropic radiator. It is a convenient fiction used as "worst possible case" to compare the directivity performance of real antennas against. Although no real antenna can be exactly isotropic, a few antennas are built to be as near to isotropic as possible; they are used for emergency backup antennas and for test equipment for other antennas: Because received and transmitted signal strength is (almost) the same in every direction, they work without any need for them to be any better than very crudely oriented, if at all.

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.

•  
  "Rabbit ears" dipole variant for VHF television reception
•  
  Two-element turnstile antenna for reception of weather satellite data, 137 MHz. Has circular polarization.

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] 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.

Doublet

It is not necessary for an antenna to be resonant to transmit well, rather resonance is preferred to easily feed power to it;^[7][8][6] when a dipole antenna is intentionally built to operate on a below-resonant frequency (by using a transmatch) radio amateurs sometimes call the antenna a "doublet". (The term is not strictly distinguished; many use it as a synonym for "dipole".^[6]) Often "doublets" are carefully sized to avoid resonance, in order to make impedance matching less challenging.

Folded dipole

A typical folded dipole is two half-wave dipoles mounted parallel to each other, a few inches apart, with the far ends connected. Only one of the dipoles is fed, and the second dipole connects straight through the center where the first has the usual feedpoint. The two-wire version is often described as a "squashed loop antenna", since the total length of wire is one wavelength, and efficiency / radiation resistance of the folded dipole is very high: 4× that of a single dipole,^{[citation needed]} analogous to the high efficiency of large loops. Any number of similar parallel wires may be added, with the efficiency rising as the square of the number of parallel wires; hence a three-wire folded dipole would be 9× more efficient.

Inverted-'V' antenna

When the two arms of a dipole are individually straight, but bent towards each other in a 'V' shape, at an angle visibly less than 180°, the dipole is called a 'V' antenna, and when the dipole's far ends are staked closer to the ground than the center, it is called an inverted-'V' (rarely, a 'Λ', Lambda antenna). The inverted-'V' is popular since it provides some of the good electrical performance of a dipole, but only requires erecting one high mounting point, whereas an ordinary dipole requires at least two, often three. Due to ground reflections the inverted-'V' tends to be mostly omnidirectional, but depending on the center angle, slightly directional toward the opening of the 'V'.^[8][6][d]

Sloper

A sloper or sloper dipole is a half-wave wire slanting down from a single elevated mounting point. It is usually fed at its center with the feedline sloping away perpendicularly from the slanting wire, towards a stake in the ground near the base of the mast. The elaborate perpendicular slope of the feedwire is to prevent it from interacting with the antenna. The sloper's far end is attached by a cord to a short pole or to a ground stake. It is popular because it requires only a single mast, and with a good ground system below it, has a nearly omnidirectional pattern.^[2][6]

Windom

Its more proper name is off-center-fed dipole, since the original "Windom" antenna was slightly different, but the common reuse of the old name is well understood. The modern Windom is a dipole which is fed approximately one third of the distance from one of its ends, but otherwise erected like an ordinary dipole, including most dipole variations (such as inverted-'V' and sloper dipoles). The strategically chosen feed location has a fairly high impedance, but fortuitously has roughly the same high impedance on most (but not all) of its harmonics (for example, not the 6th harmonic). Details vary with slight changes in the feedpoint location. The modern version is popular because it has all of the advantages of an ordinary dipole, but functions well on almost twice as many shortwave frequencies as one that is identical in size, but center-fed. The price for the extra working frequencies is the impedance matching needed to accommodate the high feed impedance.^[2][e] Because the feedpoint is located near the high-voltage end of the antenna, its feedline tends to capacitively couple to the dipole and pick up unbalanced currents, which then causes radiation from the feedline if not blocked. Modern designs, e.g. a "Carolina Windom", exploit the radiating feedline current and only block it near where the cable reaches the ground, so that radiation from the vertical feedline fills in the gaps that form in the horizontal dipole's radiation pattern on higher resonances.^[2]

End-fed dipole

A dipole can be fed from very near its end (needing only about ⁠1/20⁠th of the dipole as an end-stub) but the impedances are exceedingly high – a few thousands of ohms, depending on fine details of the average height of the antenna and thickness of the wire. The chosen location has a high impedance, but roughly similar high impedance for all the harmonics. Impedance matching for end fed antennas tends to use very high ratio impedance transformers in combination with high-impedance feedline, which then connects to a remotely controlled (or automatic) antenna tuner, so the needed reduction of impedance down to the 50 ohm transmitter is a combination of multiple steps. The benefit of these extensive measures, is that the antenna can be operated on every harmonic (no exceptions, unlike a "Windom") and hence used nearby exactly twice as many frequencies as a conventional center-fed dipole (only odd harmonics feasible). There are other layout benefits, since the placement of the highest radiating part of the antenna wire, near the center, is not limited by any need to reach it with a feedline, and the end-feedpoint is at a location which does not radiate, hence its placement is only limited by safety for high voltages.^[2]

Turnstile

Two dipole antennas mounted at right angles, fed with a phase difference of 90°. This antenna is unusual in that it radiates in all directions (no nulls in the radiation pattern), with horizontal polarization in directions coplanar with the elements, circular polarization normal to that plane, and elliptical polarization in other directions. Used for receiving signals from satellites, as circular polarization is transmitted by many satellites.

Patch (microstrip)

A type of antenna with elements consisting of metal sheets mounted over a ground plane. Similar to dipole with gain of 6–9 dBi. Integrated into surfaces such as aircraft bodies. Their easy fabrication using PCB techniques have made them popular in modern wireless devices. Often combined into arrays.

Biconical antenna

A dipole with cone-shaped arms, with the feedpoint where their tips meet; they are sometimes called "fat dipoles" or "double bowling pins". They show broader bandwidth than ordinary dipoles, up to three octaves above their base frequency.^[f] The monopole version is called a discone antenna.

Bow-tie antenna

A "bow-tie" is a flattened version of a biconical antenna, with similar broad-band advantages. Also called butterfly antennas, they are dipoles with arms shaped like triangles or arrow-heads (⧓ ⨝ ⪥); the antenna feedpoint is where the tips of the triangles meet. The triangles can either be a metal sheet with solid metal centers (⧓), or two wires with their far ends connected (⨝) outlining the shape of a bow-tie, or with unconnected ends in an "X" shape (⪥).^[6][f]

•  
  Quarter-wave whip antenna on an FM radio for 88–108 MHz
•  
  Rubber ducky antenna on 446 MHz UHF walkie-talkie with rubber cover removed.
•  
  VHF ground plane antenna
•  
  Mast radiator antenna of medium wave AM radio station, Germany
•  
  'T' antenna of amateur radio station, 80 ft high, used at 1.5 MHz.
•  
  Folded unipole antenna with a solid metal mast surrounded by six skirt wires, held away by insulated standoffs

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][9] 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 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^{[citation needed]} 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]

Quarter-wave monopole

The most common monopole is a vertical, ⁠1/ 4 ⁠ wave tall, which is the minimum size for it to self-resonate; a less often used, but better performing size is ⁠5/ 8 ⁠ wave.^[g] A one-quarter wave monopole has a gain of 5.12 dBi when mounted over a good ground plane. A single monopole's radiation pattern is omnidirectional, so they are used for broad coverage of an area, and when mounted vertically, to have vertical polarization, since the ground waves used for broadcasting at frequencies below 2 MHz must be vertically polarized to reduce signal absorption by the Earth.^[2] Large vertical monopole antennas are used for broadcasting in the lower half of the HF band, and all of the MF, LF, and VLF bands. Small monopoles ("whips") are used as compact, but low-gain antennas on portable radios in the HF, VHF, and UHF bands.

Whip

Type of antenna used on mobile and portable radios in the VHF and UHF bands such as FM "boom boxes", consists of a flexible rod, often made of telescoping segments. In the HF band, "whip" typically refers to an antenna or a terminal antenna segment that is too short to resonate naturally;^{[citation needed]} when a whip is long enough to self-resonate (a quarter wavelength or more), it is instead usually just called by the generic name "monopole".^{[citation needed]}

"Rubber ducky"

More formal technical name is normal-mode helix. Most common antenna used on portable two-way radios and cordless phones due to its compactness. Consists of an electrically short wire helix. The helical shape adds inductance to cancel the capacitive reactance of the short radiator, making it resonant. Like all electrically short antennas it is nearly isotropic^{[citation needed]} – has very low gain, if any. Not to be confused with the similar shaped, but much larger axial mode helix (see below).^[h]

Ground plane

A whip antenna with several rods extending horizontally from base of the whip in a star-shaped pattern, similar to an upside-down radiate crown, that form the artificial, elevated ground plane that gives the antenna its name. The ground plane rods attach to the ground wire of the feedline, the other wire feeds the whip. Since the whip is mounted above ground, the horizontal rods form an elevated ground plane just below the whip to reflect its radiation away from the earth and increase its gain.^[2] Used for elevated base station antennas for land mobile radio systems such as police, ambulance, and taxi dispatchers.

Mast radiator

A radio tower in which the tower structure itself serves as the antenna. Common form of transmitting antenna for AM radio stations and other MF and LF transmitters. At its base the tower is usually, but not necessarily, mounted on a ceramic insulator to isolate it from the ground.

Folded monopole

A folded monopole antenna is the monopole version of a folded dipole: It is an ordinary quarter-wave monopole with a second wire run parallel to the first, a few inches apart, with the top ends of the two wires connected. The second wire connects directly to the ground system instead of connecting to feedpoint as the first wire does. Adding the second wire raises the efficiency of the monopole by 4×,^{[citation needed]} and correspondingly raises the feedpoint impedance, giving the added benefit of making impedance matching to standard coaxial cable somewhat easier. Similar to a folded dipole, one can add a third wire to get 9× the efficiency, and so on.^[2] Although the name is similar to the folded unipole, the two antennas are electrically different: The folded monopole is a much simpler antenna.

Discone antenna

The discone is a monopole version of a biconical antenna. It is omnidirectional, vertically polarized, and has a gain similar to a dipole. It is equally efficient as a monopole and is exceptionally wideband, offering a frequency range ratio of up to approximately 10:1 .

Folded unipole

A modified mast antenna, usually grounded at its base, augmented by one or several parallel wires called "skirt wires" that attach to the mast part-way up the antenna. The skirt wires can attach at any height between part-way up and the top of the mast. One or more of the skirt wires is fed with the signal, similar to a gamma match. The number and relative thickness of the mast and the skirt wires adjusts the feedpoint impedance.^[i] It is much more elaborate and not electrically the same as the similar-sounding folded monopole.

Half sloper

A half-sloper is a quarter-wave wire slanting down from a single elevated mounting point. It is fed at its top mounting point, with the low, far end attached by a cord to a short pole or to a ground stake. It is a monopole version of a sloper dipole (see above); like the sloper dipole it is popular because it requires only a single mast. Also like a sloper dipole it has a nearly omnidirectional pattern if used with a good ground system, but can function with a single counterpoise wire lying on the soil under the sloping wire, attached at the bottom of the support mast to the ground wire of the feed cable. Because its strongest currents (near the top-end feedpoint) are high up, it tends to have a stronger signal toward the horizon (better low angle gain) than a monopole fed near its base.^[2] It is somewhat like a monopole version of an inverted 'V' dipole.

'T' antenna

Consists of a long horizontal wire suspended between two towers with insulators, with a vertical wire hanging down from it, forming the shape of the letter 'T'. The dangling wire attaches to a feedline to the receiver or transmitter on one of the feed wires; the other feedline wire connects to a mandatory low resistance ground. The dangling vertical wire is the radiating part of the antenna; normally it is shorter than the quarter wavelength required for resonance. For the 'T' antenna, the dangling wire attaches in the exact center of the top wire. Used on MF and the lower HF bands. Since at these frequencies the vertical wire is electrically short – much shorter than a quarter wavelength – the horizontal wire serves as a "capacitance hat" to increase the current in the vertical radiator, improving the efficiency and gain.^[2][j]

Inverted 'L'

Similar in construction to a 'T' antenna described above, but with the dangling vertical wire attached to one end of the horizontal wire instead of the center. The altered connection point gives the antenna the shape of the Greek letter 'Γ'. Unlike the 'T' antenna, both the vertical and horizontal wires radiate, with their respective radiation being vertically and horizontally polarized, and their combined radiation diagonally polarized, usually at a steep angle. Although all parts of the antenna radiate, the strongest radiation comes from the vertical wire, so the horizontal wire serves both as a "capacitance hat" and as a weak radiator.^[2][k]

Inverted 'F'

Effectively a shunt-fed inverted-L, with the feed point attached to the horizontal wire, making the antenna shape like the letter 'F' tilted to the right by 90°, so it has the shape of the Hangul letter ㄲ, or the line-drawing character ╓. The unusual feedpoint with its adjustable location along the horizontal section gives the inverted 'F' the good feedpoint matching of a unipole, and the compact size of an inverted-L. The antenna is grounded at the base and fed at some intermediate point, and the position of that feed point determines the antenna impedance, so the feedpoint impedance can be matched to the feedline without needing a separate transmatch.

Umbrella

An elaborated and enlarged version of a 'T' antenna; it is a very large wire transmitting antenna used on VLF bands for VLF time signals or long-range submarine communications. Relative to the even larger wavelengths it is used for, it is paradoxically an ultra-short antenna, hence has an extremely narrow bandwidth. It consists of a central radiating tower with multiple wires attached at the top, extending out radially from the mast and insulated at the ends, resembling a metal umbrella frame. Like other ultra-short antennas it has an extremely high capacitive reactance and minimal radiation resistance, which require a large loading coil and low-resistance counterpoise system.

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][10][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, whereas soil is either opaque to, or reflective of the waves' electrical 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 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]

Large loops

Large loops have a perimeter of one full wavelength, or larger. When they are one, two, or three wavelengths, or any whole-number multiple of a wavelength, they are naturally resonant and act somewhat similarly to the full-wave or multi-wave dipole. When it is necessary to distinguish them from small loops, they are called "full-wave" loops.^[l][3][10]

Half-loop

the upper half of a vertical full-wavelength loop antenna mounted on the ground (not to be confused with the visually similar but electrically different half-square antenna described below, under array antennas^[m]). The full loop is cut at two opposite points along its perimeter, and the lower half is omitted; the upper half mounted on the ground at the cut points, sticking up from the ground like a satchel handle. It is shaped like the Greek letter Π or an upside-down capital letter U, and is the loop antenna analog of a ground-mounted monopole antenna. Similar to how a vertical monopole uses its ground system to produce a "phantom" image of the rest of a dipole, the missing lower half of the half-loop is replaced by its ground-plane image. If shaped like a half of a square, a half-loop can operate either as a loop antenna or on its first harmonic as a dipole antenna whose ends have been bent over and grounded.^[11][n]

Halo antennas

Loops one half-wavelength in perimeter, that have a small gap cut in the loop, are called "halos". They are naturally resonant on one frequency, and are intermediate in size and function between small and large loops. They are often described as a half-wavelength dipole that has been folded into a circle, rather than being a "true loop" antenna.^{[3][10][2][4](pp 231–275)}

The approximately-omnidirectional pattern of halos resembles small loops; their radiation efficiency lies between 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 usable 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 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. 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.

Small receiving loops

Small receiving loops are sized at ⁠ 1 /4⁠~⁠1/ 10 ⁠ wave perimeters, sometimes with many turns of wire around the same supporting frame. Small loops are widely used as compact direction finding antennas, since their "null" direction is exceptionally precise, and their small size makes them much more compact as hand-carried equipment than dipole-based directional antennas.^[3][10][2]

Ferrite loop antennas

Also called "loopsticks", they consist of a wire coiled around a cylindrical ferrite core (the "stick"). The ferrite increases the coil's inductance by hundreds to thousands of times, and likewise magnifies its effective signal-capturing area. The improvement makes them even more compact than (ordinary) small loops made without ferrite, and yet receive RF just as well (or better). Loopsticks' radiation pattern is identical to a dipole antenna, with a maximum in all directions perpendicular to the ferrite rod. These are used as the receiving antenna in most portable and desktop consumer AM radios made for the medium wave broadcast band, and for lower frequencies.^[o]

Small transmitting loops

Small transmitting loops are loop antennas whose perimeters are smaller than a half-wave, that have been specifically optimized for transmitting. Their much smaller size than dipole antennas (only ~10% as wide) sometimes makes them a viable choice when space is limited, despite their lower efficiency. Small transmitting loops are made larger in size than most small receiving loops, with perimeters near ⁠ 1 /3⁠~⁠ 1 /4⁠wave,^[p] in order to improve on their generally poor efficiency. For that same reason, their parts are carefully joined by brazing or welding to reduce losses from contact resistance. Because of their larger size, small transmitting loops lack the sharp nulls of small receiving loops, so they are not as useful for direction finding, and also are more bulky (roughly double the size) so would not be as convenient as the more accurate small loops for hand-held use in radio searches.^[2]

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][10][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.

Composite antennas are made up of combinations of several simple antennas combined to function as a single antenna,^{[citation needed]} similar to how a compound optical lens combines multiple simple lenses. Likewise, for antennas that combine (a) simple antenna(s) 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.

Other types of composite antennas are most often designed to enhance simple antennas' directionality, but a different performance-enhancement is to make a broadband antenna. Combining multiple simple antennas at a common feedpoint, the composite antenna can be made broadbanded or widebanded or multibanded – that is made to operate well on several distinct frequencies, or over a single wider frequency span. (Different methods of broadbanding put a terminating resistor on antennas.)

Fan dipole

Also called a multi-dipole – a common broadband and / or wideband dipole variant that superficially resembles the bow-tie antenna, but is electrically different. It is a composite of pairs of dipole arms; both arms of one of the dipoles are equal-length, but each dipole pair is a different length from every other pair. The several dipole arms extend away ( ⚞⚟ ⪫⪪ ⫸⫷ ) from the common central connection point of the combined antenna.^[q]

Fan monople

A fan monopole, or multi-monopole is a half of a fan dipole: It combines several different-sized monopole antennas, all sharing the same feedpoint, with each sized to transmit well on a different band or sub-band. Its design for wideband or broadband behavior is essentially identical to the fan dipole.^[q]

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  VHF collinear array of folded dipoles
•  
  A full-wave two-element quad antenna used by an amateur radio station
•  
  Yagi–Uda television antenna for analog channels 2–4, 47–68 MHz
•  
  Log-periodic dipole array covering 140–470 MHz
•  
  Sector antennas (white bars) on cell phone tower. Collinear arrays of dipoles, these radiate a flat, fan-shaped beam.
•  
  Reflective array UHF TV antenna, with bowtie dipoles to cover the UHF 470–890 MHz band
•  
  US Air Force PAVE PAWS phased array radar antenna for ballistic missile detection, Alaska. The two circular arrays are each composed of 2677 crossed dipole antennas.
•  
  Curtain array shortwave transmitting antenna, Austria. Wire dipoles suspended between towers.
•  
  Four-bay batwing television broadcasting antenna, Germany

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.

• 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.
• 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][12][4](pp283–371)}
• Parasitic arrays 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. Parasitic arrays are the RF analogs of compound optical lenses made from combinations of simple lenses.

Vertical collinear

A broadside array that consists of several dipoles fed in phase, with their axēs stacked atop each other, in a single vertical line. It is a high-gain omnidirectional antenna, meaning more of the power is radiated in horizontal directions and less wasted radiating up into the sky or down onto the ground. Gain of 8–10 dBi. Used as base station antennas for land mobile radio systems such as police, fire, ambulance, and taxi dispatchers, and sector antennas for cellular base stations.

Yagi–Uda

Also called a "Yagi", is a parasitic array that is one of the most common directional antennas at UHF, VHF, and upper HF frequencies. Consists of multiple half-wave dipole elements aligned with their axēs parallel, in the same plane, with a single resonant-length driven element wired to the feedline, usually the next-to-last, next-to-longest element in the array.^[r] The multiple other elements are parasitic, which reflect and direct the radiated signal into a single direction, hence a beam antenna.^[s] The simple antennas used to make a Yagi-Uda can either all be linear or bent linear antennas, or all loops (a quad antenna) or (rarely) a mixed combination of loops and straight-wire antennas.

Yagi–Udas are used for rooftop television antennas, point-to-point communication links, and long distance shortwave communication using skywave ("skip") reflection from the ionosphere. They typically have gains between 10 and 20 dBi depending on the number of director elements used, but their bandwidths are very narrow.^[t]

Moxon antenna

Also called a Moxon rectangle; it is a rectangular-shaped, folded version of a two-element Yagi-Uda, hence a minimal parasitic array.^[13]

Quad

Although "quad" can refer to a single quadrilateral-shaped loop, the term usually refers to two or more loops stacked side by side as a parasitic array; at first glance, quads resemble a box kite frame. Only one of the loops in the quad is connected to the feedline, and that loop functions as the driver for the antenna and is the original source for the radiated signal. The other loops are parasitic elements that act as reflectors or directors, focusing the radiated waves in a narrower, single direction and thereby increasing the gain. Quad antennas are Yagi-Uda antennas made from loops instead of dipoles or monopoles, and are likewise used as a directional antennas on the HF bands for shortwave communication. They are sometimes preferred for longer wavelengths because (if square) they are half as wide as a Yagi built from dipoles and have slightly better directivity.^[3][10][2]

Log-periodic dipole array

An endfire array of multiple dipole elements along a boom with gradually decreasing lengths, back to front, all connected to the transmission line with alternating polarity. It is a directional antenna with a wide bandwidth, which makes it ideal for use as a rooftop television antenna, although its gain is much less than a Yagi of comparable size. Sometimes called a "fishbone" antenna because of it looks like the ribs of a fish.^[r] For long wavelengths in the lower HF band the array may be made of ground-mounted monopoles instead of dipoles.

Adcock antenna

A doubled endfire array of four parallel dipole (or monopole) antennas, all equal size and equidistant, aligned vertically at four corners of a square. All four dipoles are driven, but with opposing phases for adjacent dipole elements, and identical phases for elements at opposite corners. The spacing and phasing of the dipole elements makes the combination of the arrayed elements moderately directional.

Curtain array

A curtain array is any one of several designs for large, directional, long-distance, broadside transmitting array antennas used at HF by shortwave broadcasting stations. It consists of a vertical rectangular array of identical dipoles suspended in a parallel row in front of a flat reflector screen (the "curtain"). The screen or curtain consists of a second row of vertical parallel wires, all supported between two metal towers. It is aligned to efficiently radiate a horizontal beam of vertically polarized radio waves into the sky just barely above the horizon; once the signal reaches the ionosphere past the horizon the beam is refracted (or "bounced") off the F layer back towards Earth, to reach equally far beyond and over the horizon, perhaps to be reflected off the ground for another "hop". There are several designs for curtain arrays, among them Sterba curtains, bobtail curtains, and HRS antennas; the half-square antenna (below) is a minimal curtain array, with only two radiating elements and no reflecting screen.

Reflective array

Multiple dipoles in a two-dimensional broadside array mounted in front of a flat reflecting screen, usually called a "curtain". Used for radar and UHF television transmitting and receiving antennas.

Phased array

Is a high gain antenna used at UHF and microwave frequencies which is electronically steerable by phasing from being an endfire array to a broadside array, and every direction inbetween. It consists of multiple dipoles in a two-dimensional array, each fed through an electronic phase shifter, with the phase shifters controlled by a computer control system. The beam can be instantly pointed in any direction over a wide angle in front of the antenna. Used for military radar and jamming systems.

Half-square

A broadside array made of two inverted quarter-wave vertical monopoles, whose bases are not connected to the ground, and whose tops are each monopole's nominal feedpoint. The tops are connected by a wire one half-wavelength long, which serves as both a counterpoise wire and a crossover phasing feedline. The verticals are the radiators and function as a minimal two-element curtain array, similar to a bobtail curtain. The structure is shaped like the Greek letter Π (not to be confused with the similar-looking half-Loop antenna described above).^[m] Unlike a half-loop, neither monopole element has any DC connection to the ground beneath it (although there typically is considerable RF capacitive coupling, which can be exploited to shorten the verticals). The top-to-top half-wave connecting wire serves as a phasing line that keeps radiation from the two antennas in-phase; even if the system feedpoint is connected elsewhere. Since a quarter-wave monopole's current is highest nearest its feedpoint, the nominal top-feed puts the maximum radiating current up high, at the top of each monopole. Because they are top-fed, inverted monopoles produce a strong signal lower to the horizon than an ordinary bottom-fed monopole, whose maximum radiation must angle-up from the base, to pass above surrounding obstructions.^[11]

Batwing

Also called a superturnstile, is a specialized broadside array antenna used for VHF television broadcasting. It is a hybrid flattened biconical and turnstile antenna, that consists of perpendicular pairs of dipoles with radiators resembling bat wings. The batwing shape is a flattened biconic ("butterfly antenna") that gives wide bandwidth which whole-channel TV transmission needs. Stacking the batwings vertically on a mast concentrates more of the combined antennas' radiation in the horizontal direction, and with matched pairs at right angles, each pair fills-in the nulls of its counterpart, making their combined radiation pattern more nearly omnidirectional.^[f]

Microstrip

A small-sized microwave antenna printed on a circuit board (PCB). Because of the short wavelengths it handles, the small antenna can still be shaped to achieve large gains in compact space, as an array of patch antennas on a substrate fed by microstrip feedlines. Often the antennas printed on a PCB are composites of multiple different small antennas, each shaped to have complementary performance advantages supplementing the others'. Further, components' beamwidth and polarization can be made actively reconfigurable by switching and phasing circuitry printed on the same board. Ease of fabrication by modern PCB manufacturing techniques have made them popular in modern wireless devices.

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  NASA Cassegrain, Extremely high gain ~70 dBi.
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  Corner reflector UHF TV antenna with "bowtie" dipole driven element at its center.
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  Microwave horn antenna bandwidth 0.8–18 GHz.
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  X band marine radar slot antenna on ship, 8–12 GHz.
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  Dielectric lens antenna used in millimeter wave radio telescope.

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.

Corner reflector

A directive antenna with moderate gain of about 8 dBi often used at UHF frequencies. Consists of a dipole mounted in front of two reflective metal screens joined at an angle, usually 90°. Used as a rooftop UHF television antenna and for point-to-point data links.

Parabolic

The most widely used high gain antenna at microwave frequencies and above. Consists of a dish-shaped metal parabolic reflector with a feed antenna at the focus. It can have some of the highest gains of any antenna type, up to 60 dBi, but the dish must be large compared to a wavelength. Used for radar antennas, point-to-point data links, satellite communication, and radio telescopes.

Horn

A horn antenna has a flaring metal horn attached to a waveguide. It is a simple antenna with moderate gain of 15 to 25 dBi, used for applications such as radar guns, radiometers, and as feed antennas for parabolic dishes.

Slot

Consists of a waveguide with one or more slots cut in it to emit the microwaves. Linear slot antennas emit narrow fan-shaped beams. Used as UHF broadcast antennas and marine radar antennas.

Lens

A lens antenna is made from a layer of dielectric, or a metal screen, or multiple waveguide structure of varying thickness, mounted in front of a feed antenna. The waveguide / screen / dialectric refracts the radio waves, focusing them on the feed antenna, similar to a focusing lens placed in front of a flashlight.

Dielectric resonator

The "resonator" part consists of small ball or puck-shaped piece of dielectric material, placed at the opening of a waveguide, where the material is excited by waves fed into the other end of the guide. If well-designed, the resonating material efficiently re-radiates the absorbed waves. Used at millimeter wave frequencies (c. 10~100 GHz).

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  Animation showing a Beverage antenna .
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  Quadrant antenna, similar to rhombic, at an Austrian shortwave broadcast station. Radiates horizontal beam at 5-9 MHz, 100 kW
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  Array of four axial-mode helical antennas used for satellite tracking, France

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 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. 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.

Beverage

Simplest unidirectional traveling-wave antenna. Consists of a straight wire one to several wavelengths long, suspended near the ground, connected to the receiver at one end and terminated at the other end by a resistor equal to its characteristic impedance (typically 400~800 Ω). Its radiation pattern has a main lobe at a shallow angle in the sky off the terminated end. It is used for reception of skywaves reflected off the ionosphere in long distance "skip" shortwave communication.

Rhombic

Consists of four equal wire sections shaped like a rhombus (〈〉). It is fed by a balanced feedline at one of the acute corners, and the two sides are connected to a resistor equal to the characteristic impedance of the antenna at the other acute corner. It has a main lobe in a horizontal direction off the terminated end of the rhombus. Used for skywave communication on shortwave bands in circumstances where it is practical to increase transmit power enough to compensate for power dissipated in the terminating resistor.

Leaky wave

Leaky wave antennas are used for microwave frequencies where microwave signals are normally passed through waveguides rather than solid wires. They are made by cutting slots, or "apertures", in a waveguide or coaxial cable, that allows the signal to radiate out along the length of the slot (hence "leaking" waves).

Axial mode helix

Consists of a wire in the shape of a helix mounted above or in front of a reflecting screen ( ⸠ꕊ ) whose total coiled length is on the order of at least one wavelength. It radiates circularly polarized waves in a beam out of the open end of the helix, with a typical gain of 15 dBi. It is used at VHF and UHF frequencies where antenna sizes are feasible. Often used for satellite communication, which uses circular polarization because it is insensitive to the relative rotation on the beam axis.^[u] Not to be confused with a "rubber ducky" antenna (normal mode helix), which is much smaller.^[h]

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 limitations.

Resistively terminated antennas

One method for making a broadband antenna is to place a resistive termination on the antenna. The resistive termination is used to inhibit resonance and consequently reduce bothersome reactance found on resonant antennas on their non-resonant and near their antiresonant frequencies; this allows adequate performance on any frequency, but at the cost of wasting some transmit power in the resistor. Some examples are the terminated coaxial cage monopole (TC²M),^[14] the tilted terminated folded dipole (T²FD), and the similar Robinson-Barnes antenna (essentially a T²FD with a second radiating wire parallel to the first).^[15][v]

Earth antennas

Earth antennas are made of wires actually buried under the soil, hence also called buried antennas. Most amateur use is limited to non-directional MF and LF receiving antennas, but transmitting ground dipoles^[w] are used for military communication with submarines. In order to work, the wire must be near enough to the soil surface for the radio waves to penetrate and reach it; mediumwaves and longwaves are much better at penetrating soil, and those are the frequencies where buried antennas are most used, although still rare.^[16][x]

Random wire antenna

A random wire is the typical informal antenna erected for receiving shortwave and AM radio.^[y] It consists of a random length of wire either strung outdoors between elevated supports, or indoors across a ceiling, running in an erratic zigzag pattern along walls or between supports.^[z] One end of the wire is usually connected directly to the back of the receiver. Moxon (1993) calls it "the odd bit of wire".^{[13][page needed]}

Snake antenna

Random wire antennas laid out on the surface of the ground are called "snake antennas", which are not clearly distinguished as any particular type.^{[citation needed]}

B.O.G. antenna

A "Beverage on the ground" – often called a "B.O.G." or bog antenna – is a "snake antenna" laid in a straight line that has its end opposite the feedpoint grounded. It is a travelling wave antenna, and technically is an extreme instance of a low-hanging Beverage antenna.^{[citation needed]}

An isotropic antenna (isotropic radiator) is not a real antenna: It is a hypothetical, completely directionless antenna that radiates equal signal power in every vertical and horizontal direction. 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 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 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 has fallen 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.