Antenna types

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:

• Most often, the greatest design constraint is the size (wavelength) of the radio waves the antenna is to intercept or emit.
• A competing second influence is optimization criteria for either receiving and for transmitting, which have practical differences for shortwaves and longer wavelengths.
• A competing third criterion is the number and bandwidth of the frequenc(y/ies) that a single antenna intercepts or emits.
• A fourth design goal is to make the antenna directional: To project or intercept radio waves from only one vertical and / or horizontal direction as exclusively as possible.

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.

Simple antennas

There are three types of "simple antennas": dipoles, monopoles, and loops.^{[citation needed]} 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, which is analogous to using multiple simple optical lenses to make a single compound lens. Simple antennas are usually further subdivided into

Linear antennas ("electric" antennas)

"Straight-wire" or "straight-line" antennas are on rare occasions called "electric antennas",^{[citation needed]} since they exclusively couple to the electric part of the electromagnetic radio waves that they emit and absorb. (The term "linear" is not strict: End-sections of a linear antenna far from its center can be bent away from a straight line with only slight or unnoticeable loss in performance.)

Dipole

Two-armed antennas, like "rabbit ears". For resonance, each arm is slightly under a quarter-wave base to end, which makes the whole antenna nearly a half-wave end to end.

Monopole

Single-armed antennas, like a single "telescoping" antenna. At the lowest resonant frequency that arm is slightly under a quarter-wave long.

Both dipoles and monopoles are often built large enough to be self-resonant; usually each arm is a quarter-wave long. However a few types of linear antennas are specifically made too small to resonate – short whip antennas,^{[disputed – discuss]} and unplanned random wire antennas, for example.

Loop antennas ("magnetic" antennas)^[b]

Loops are ring-like antennas made out of segments of wire or metal tubing 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, that naturally resonate at the frequency where their perimeter length is a half-wavelength.

Small loops

"Small" loop antennas are loops of wire or metal 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 in that typical case, the feedline connects only to the 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 that the arriving RF signals are travelling. 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 a "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 the 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. 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.

• 
  "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] Although dipoles are used alone as omnidirectional antennas, they are also a building block of many other more complicated directional antennas.

Half-wave dipole

The most common type of dipole consists of two resonant elements, each just under a quarter wavelength long, hence a total length of about a half-wave. This antenna radiates maximally in directions perpendicular to the antenna's axis, giving it a small directive gain of 2.15 dBi (2.15 dBi means that in the direction of maximum radiation, signal strength is 1.64× the signal from a directionless "isotropic" antenna).

Doublet

"Doublet" is a name radio amateurs sometimes use for a dipole antenna that is used on a frequency below the antenna's lowest self-resonance. It is not necessary for an antenna to be resonant to transmit well, rather resonance is preferred to easily feed power to it; using a transmatch may make feeding power to an antenna on its nonresonant frequencies possible.^[7][8][6] Some "doublets" are carefully sized to avoid resonance, in order to make impedance matching less challenging. (The term is "doublet" is not strictly distinguished; many use it as a synonym for "dipole".^[6])

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' ('Λ') 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 counter-sloping perpendicularly, away from the slanting antenna wire, towards a stake in the ground near the base of the mast.^[e] The sloper's far end is attached by a cord to a short pole or fastened by an insulated cord to a ground anchor. 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]

Modern Windom

Its more proper name is off-center-fed dipole, which some writers prefer, since the obsolete, original "Windom" type antenna was somewhat different; however, 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 offset feed location has a fairly high impedance, but fortuitously has roughly the same high impedance on most of its harmonics.^[f] The Windom antenna is popular because it has all of the advantages of an ordinary dipole, but functions well on almost twice as many shortwave frequencies as an identical sized center-fed dipole. The price for the extra working frequencies is the needed to match a feed impedance 5–7 times higher than the standard 50 Ohm transmitter impedance.^[2][g][h]

End-fed dipole

A dipole can be fed from very near its end (needing to be only about ⁠1/20⁠th of the dipole length from the actual end) but the end impedances are exceedingly high – a few thousands of ohms, depending on the average height of the antenna and thickness of its wire. The end location has an inconveniently high impedance, but it is roughly the same high impedance for all the harmonics, and accommodation for any one harmonic will be near to right for all the other harmonics. The benefit of the extensive measures needed for matching to the high impedance^[i] is that the antenna can then function well on every harmonic (no exceptions, unlike a "Windom"), and hence used for transmitting on exactly twice as many frequencies as a same-size center-fed dipole (only odd harmonics feasible).^[2][j]

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 or reception 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 used by most satellites for both transmit and receive, and since it can emit and receive signals in all directions, can operate from a simple, fixed mount, without needing to be aimed or steered towards the target satellite.

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. The monopole version is called a discone antenna.^[k]

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][k]

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]

• 
  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 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]

Quarter-wave monopole

The most common monopole is a vertical, ⁠1/ 4 ⁠ wave tall, which is the minimum size for it to self-resonate.^[l] 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, they have vertical polarization. Vertically polarized outgoing radiation is important for long-distance transmissions in the mediumwaves and lower: The ground waves that carry radio signals at frequencies below about 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 a flexible 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"

It's 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 that resembles a narrow, inch to half-inch long coiled wire spring, such as one might find in a retractable ballpoint pen. 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), nor to be confused with loop-type antennas.^[m]

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. The name of the antenna describes its shape: A metal disk above a metal cone. The cone points upwards and is made of solid metal, wire mesh, or a skirt of about a dozen sloping wires that outline a cone. The cone measures near one quarter-wave long along the side from tip to bottom rim, at the antenna's lowest frequency. There is a smaller, flat metal disk mounted horizontally, slightly above the tip of the cone; sometimes the solid disc is replaced by a radiate crown of metal rods, similar to the base of a ground plane antenna. One of the feed wires connects to the tip of the cone, the other wire to the center of the disk. A discone is exceptionally wideband, offering a frequency range ratio of up to approximately 10:1 , over three octaves above the antenna's lowest frequency, but otherwise only functions just as well as other quarter-wave monopoles: It is omnidirectional, vertically polarized, equally efficient as a monopole, and has gain similar to a ground plane antenna.

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.^[n] 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 an insulated cord to a short pole or to a ground anchor. 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 vertical wire is the radiating part of the antenna, and 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. Normally the height of a 'T' antenna is less than the quarter wavelength required for resonance. It is distinguished from the similar 'L' antenna by the dangling, radiating wire's attachment point: For the 'T' antenna the dangling wire attaches to the exact center of the horizontal 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][o]

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][p]

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 expanded 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, although the antenna is enormous on human-scale it is paradoxically an ultra-short antenna. Being much smaller than a wavelength gives the antenna many troublesome properties: An extremely narrow bandwidth, low radiation resistance, and excessive capacitive feedpoint reactance. It consists of a central radiating tower with multiple wires attached at the top as a "capacitance hat", that extend out radially from the mast and are insulated at their ends; the overhead configuration resembles an open metal umbrella frame, hence the name "umbrella antenna". Like other ultra-short antennas it requires a large loading coil and a meticulously constructed low-resistance counterpoise system to cope with the extremely high reactance and minimal radiation resistance.

Small transmitting loop antenna for high frequencies, 2 m diameter

Separate small receiving loop antenna for AM radio

Ferrite loopstick antenna from an AM broadcast receiver, about 4 in (10 cm) long. The antenna is inductive and, in conjunction with a variable capacitor, forms the tuned circuit at the input stage of the receiver.

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]

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.^[q][3][11]

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,^[r] nor to be confused with the halo antenna, described next). 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.^[12][s]

Halo antennas

They are loop antennas that uniquely sit in-between large and small loops; they are one half-wavelength in perimeter, with a small gap cut into the loop rim. For practical purposes, "halos" are naturally resonant on one frequency. They are intermediate in size and function between small and large loops, and are often described as a half-wavelength dipole that has been bent from a straight line into a round circle, but with its far ends left unconnected.^{[3][11][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 practical higher harmonics. In some regards they represent the extreme upper size limit of small transmitting loops.^{[3][2][4](pp 231–275)}

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. 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.

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][11][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.^[t]

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 /4⁠ wave,^[u] 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][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.

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.^[w]

Log-periodic dipole array

Log-periodic dipole arrays are made broadband by having multiple dipoles share a common feedpoint, and so warrant mention in this section. However, they are separately made directional by the front-to-back interaction between a single resonant dipole in some middle-position and wave reflection by its immediate rearward too-long dipole, and in front of the resonant element the focusing or "directing" action of the several too-short elements; so log-periodic arrays are described in the subsection on endfire arrays, below.

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. Like all monopoles, it requires a ground system to function. Its design for wideband or broadband behavior is essentially identical to the fan dipole.^[w]

• 
  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. 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.

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.

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.

Half-square

A broadside array made of two "upside down" vertical monopoles. Their dangling tips / bases correspond electrically to the tops of ordinary monopoles, and are not connected to the ground. The two top ends that each monopole hangs from electrically corresponds to the base of a normal monopole, and are the monopole's nominal feedpoints (the actual feedpoint for the combined system is often placed elsewhere). The attachment points at the tops are interconnected 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).^[r] 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.^[12]

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.^[k]

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.

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.

Phased array

Is a high gain antenna used at UHF and microwave frequencies which is electronically steerable by phase adjustments, from being an endfire array to a broadside array, and every direction in between. 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.

Adcock antenna

An Adcock antenna is a pair of side-by-side endfire arrays, hence it is also broadside. It is made of four parallel dipole (or monopole) antennas, all equal size and equidistant, vertically aligned 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 combination of spacing and phasing of the dipole elements makes the combination of the arrayed elements moderately directional. Unlike phased arrays, Adcock antennas are typically physically rotated towards a given direction, rather than being steered by changing phase on the feedlines.

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.

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.^[x] For long wavelengths in the lower HF band the array may be made of ground-mounted monopoles instead of dipoles.

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.

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.^[x] The multiple other elements are parasitic, which reflect and direct the radiated signal into a narrower beam, hence the name beam antenna.^[y] 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.^[z]

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.^[14]

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][11][2]

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

• 
  Animation showing a Beverage antenna .
• 
  Quadrant antenna, similar to rhombic, at an Austrian shortwave broadcast station. Radiates horizontal beam at 5–9 MHz, 100 kW
• 
  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 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.

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. Out of the open end of the helix, the antenna radiates a beam of circularly polarized waves, 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.^[ab] Not to be confused with a "rubber ducky" antenna (normal mode helix), which is much smaller.^[m]

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.

Resistively terminated antennas

One method for making a broadband antenna is to place a resistive element somewhere in the antenna. The resistive termination is used to dampen resonance and consequently reduce bothersome reactance found on most antennas at frequencies far from a resonance, and frequencies very near an antiresonance; the resistor's resonance dampening allows adequate operation on any frequency, but at the cost of wasting some transmit power in the resistor. Some examples are the terminated coaxial cage monopole (TC²M),^[15] the tilted terminated folded dipole (T²FD), and the similar Robinson-Barnes antenna (essentially a T²FD with a second radiating wire parallel to the first).^[16][ac]

Earth antennas, buried antennas, and ground antennas

Earth antennas are made of wires actually buried under the soil, hence also called buried antennas; if laid onto the soil instead of buried in it, they are called ground antennas. Most amateur use is limited to non-directional MF and LF receiving antennas, but transmitting ground dipoles^[ad] 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.^[17][ae]

Random wire antenna

Moxon (1993) describes the random-wire antenna as an "odd bit of wire".^{[14][page needed]} It is the typical informal antenna erected for receiving shortwave and AM radio.^[af] 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.^[ag] The near-end of the antenna wire is usually directly connected to the back of the radio.

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 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.