Antenna types

Simple antennas

The category of simple antennas consists of dipoles, monopoles, and loop antennas. Nearly all can be made with a single segment of wire (ignoring any break made in the wire for a feedline connection).

Dipoles and monopoles are called linear antennas (or straight wire antennas) since their radiating parts lie along a single straight line – ignoring convenience bends at the far ends, if any. On rare occasions they are called electric antennas since they engage with the electric part of RF radiation, in contrast to loops, which correspondingly are magnetic.

The term "linear" used to describe the typical shape of these antennas is not a strict criterion for the category: End-sections of a so-called "linear" antenna that are far from its center can be bent away from a straight line with hardly any noticeable electrical consequences; sag is common and comfortably tolerated in the nominally "straight" antennas; finally, in some varieties of "linear" dipoles, the two arms or "poles" can be bent into a "V" shape, even though each individually may be a mostly straight line.

Dipoles

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. 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. Although dipoles are used alone as omnidirectional antennas, they are also a building block of many other more complicated directional antennas. All types of dipoles can be mounted either vertically or horizontally, and the chosen orientation determines their receive / transmit directions and wave polarity.

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. Unless specified otherwise, dipoles are fed at their center-point, a quarter-wave from either end on the lowest design frequency, and without artificial compensation are only usable at odd-numbered multiples of their designed lowest frequency.

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 non-resonant frequencies possible. Some "doublets" are carefully sized to avoid resonance – and especially "antiresonances" – in order to make impedance matching for multiple frequencies less challenging. (The term is "doublet" is not strictly distinguished; many use it as a synonym for "dipole".)

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 / feedpoint impedance of the folded dipole is very high: 4× that of a single dipole, 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 a 9× greater impedance.

'V' antenna

When the two arms of a dipole are individually straight, but bent towards each other in a 'V' shape, at an angle noticeably less than 180°, the dipole is called a 'V' antenna. When the ends of the dipole's arms are about level with the branch point, it is called a flat 'V', or a horizontal 'V'; when its ends are much closer to the ground than their center branch-point, the antenna is called an inverted 'V' ('Λ'). The inverted 'V' is popular since it provides some of the good electrical performance of a dipole, but only requires one high mounting point, whereas an ordinary dipole requires at least two, often three. Due to a mixture of direct radiation and ground reflected radiation, the inverted-'V' tends to be mostly omnidirectional, but depending on the center angle, can slightly favor the direction along the midline between the two arms of the 'V'.

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 cable itself slanting away at a perpendicular counter-slope from the sloping antenna wire, towards a small pole or a ground anchor near the base of the mast. The sloper's far end is attached 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.

A sloper dipole is sometimes called a half-wave sloper, which is an accurate descriptive name, but unfortunately easy to confuse with a half-sloper: One of the names of the half-size monopole version of a sloper dipole, which is only ⁠1/ 2 ⁠ × ⁠1/ 2 ⁠ = a quarter-wave long.

Modern Windom antenna

More formally called an off-center-fed dipole. 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 shows roughly the same high impedance on most of its harmonics. 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.

End-fed dipole

A dipole can be fed from very near its end but at the ends, the 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 extreme 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 is that the antenna can then function well on every harmonic (no exceptions, unlike a "Windom"), and hence can be used for transmitting on exactly twice as many frequencies as a same-size center-fed dipole (only odd harmonics feasible).

Turnstile

A turnstile antenna is made of 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 patch antenna, or strip antenna, or microstrip, is a type of antenna with elements consisting of a shaped sheet of metal mounted over a second sheet that serves as the antenna's ground plane. The upper sheet is the radiating part of the antenna; it often has small, carefully sized and arranged holes cut into it that improve performance. Often, several parallel mounted strip antennas are combined into an array antenna. When used on airplanes the plane's skin serves as the ground plane, but is normally countersunk into the aircraft body so the overlying antenna strip can sit flush with the plane's metallic surface, to keep it aerodynamically smooth. The gap between the upper surface of the antenna and the plane's metal skin is then rimmed by a narrow, flight-worthy insulating material that seals the gap and securely holds the upper strip. Consequently, a surface-mounted strip antenna looks like a patch on the airplane body, hence the name. The performance of a patch antenna is similar to an ordinary dipole of the same length, with gain of 6–9 dBi. Small size for UHF and easy fabrication have made patch antennas popular in modern wireless devices, using parallel metal-plated sections on a printed circuit board's upper and lower surfaces for the radiating strip and its underlying ground plane.

Biconical antenna

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

Bow-tie antenna

A "bow-tie" antenna 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 (▶◀ ᐅᐊ ⪥ ); 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 (ᐅᐊ) outlining the shape of a bow-tie, or with unconnected ends in an "X" shape (⪥).

Dipole and monopole design variability

Designs of linear antennas can be modified by using segments made of a bundled "cage" of wires instead of just a single wire, in order to simulate a single very "fat" wire. Another adaption is to bend otherwise straight segments near their ends, instead of only using completely straight wire, and exploiting the bent, folded, and zig-zagged dipole ends to fit in a tight space. Both of these adaptions to linear antenna designs are considered relevant, but minor distinctions between antennas.

Monopoles

A monopole antenna is a half-dipole (see above); it consists of a single conductor such as a metal rod, necessarily mounted over electrically conductive ground, or an artificial conducting surface (called a ground plane, ground system, or a counterpoise). They are sometimes classed together with dipoles (see above) in the broader category of linear antennas, or more plainly straight wire antennas, since their radiating section is normally a straight (linear) wire or section of metal tubing. Rarely, both dipoles and monopoles are called electric antennas, since they interact with the electric field of a radio wave, to contrast them against all sizes of loops, which are correspondingly magnetic antennas.

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 simulating an effectively complete 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, not counting any power lost in the ground plane.

Quarter-wave monopole

A quarter-wave vertical is the most common monopole; it is a ⁠1/ 4 ⁠ wave tall vertical radiator; its size is the minimum length for a straight wire to self-resonate. 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 in the soil. 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.

Short whip

Small whip monopoles are used as compact, but low-gain antennas on portable radios in the HF, VHF, and UHF bands. It is the type of antenna, typically shorter than a quarter-wave, often seen on mobile and portable radios such as FM "boom boxes". The "whip" consists of a flexible rod, often made of telescoping segments. In the HF band, the term "whip" is often used slightly differently to refer not only to any flexible antenna, but also to the terminal, self-supported, short segment at the top of a full-size vertical antenna. In the UHF and VHF bands a conveniently sized whip is long enough to self-resonate (a quarter wavelength or more); in that case, it is often just called by the generic name "monopole".

"Rubber ducky"

Rubber ducky is a commonly used 'joke name' for a plastic-covered spiral stub antenna; its 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 both electrical length and inductance; the extra, compressed length lowers the excess capacitive reactance of the short radiator, and the inductance cancels at least part of that which remains, ideally making the helix resonant. Like all electrically short antennas it is nearly isotropic – has very low gain, if any.

Not to be confused with the similar shaped, but much larger axial mode helix, nor loop antennas made from multiple stacked turns.

Ground plane

A ground plane antenna is a quarter-wave vertical whip antenna with several rods extending horizontally, or slanting down slightly, from base of the whip to form a portable ground plane that is fixed to the bottom end of the whip part of the antenna (hence the name). The ground plane stays in its position at the whip's base when the whip is moved, and is elevated along with the whip when the whole antenna is mounted at a high point, like the top of a mast. The rods that make up the elevated ground system can be any number from one to a half-dozen (usually three or four) arrayed in a mostly horizontal star-shaped pattern, similar to a downward-facing radiate crown. The radials at the monopole base form an elevated, artificial, electrical ground system 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 counterpoise just below the whip that reflects its radiation away from the soil below, and increases its gain. Often 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 is called a mast radiator, or tower antenna, among other names. Common form of transmitting antenna for AM radio stations and other MF and LF transmitters. At its base the tower is often – but not necessarily – mounted on a ceramic insulator that isolates 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×, 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.

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. At the antenna's lowest operating frequency the cone measures near one quarter-wave long along the cone's sloping side, from tip to bottom rim. 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 folded unipole is an electrically modified mast antenna, usually grounded at its base, that has been 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. It is much more elaborate and not electrically the same as the similar-sounding folded monopole.

Quarter-wave sloper

A quarter-wave sloper or half-sloper is a quarter-wave wire that slants down from a single elevated mounting point, and is half the size of a sloper dipole – the monopole version of a sloper dipole (see above). A quarter-wave sloper is fed at its top mounting point, with the low-hanging, far end attached by an insulated cord to a short pole or a ground anchor. Like the sloper dipole it is popular because it requires only a single tall mast. Also like a sloper dipole it has a nearly omnidirectional pattern when used with a good ground system. 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. It can be thought of as like a monopole version of an inverted 'V' dipole.

'T' antenna

A 'T' antenna consists of a long horizontal wire crossing the gap between the tops of two towers, with a vertical wire attached to the center of the horizontal wire, hanging down from its center; the dangling vertical wire is the radiating part of the antenna. It is typically used for MF and the lower HF bands, where practical antennas are too short to be resonant. The wires outline the shape of the letter 'T', hence the name. The dangling wire may run directly into the back of a radio, but more often a 'T' antenna is fed by a cable attached near where the dangling wire comes to ground. Additionally, the antenna requires a low resistance ground system, normally centered directly below the bottom of the 'T'. The typical height of a 'T' antenna is shorter than the quarter wavelength required for resonance. A 'T' antenna is distinguished from the similar 'L' antenna by the place where the dangling, radiating wire attaches to the horizontal cross wire: For the 'T' antenna the dangling wire attaches to the exact center of the top horizontal wire. Current from the dangling wire feeds into the center of the horizontal section and flows away, left and right, in equal amounts; the balanced, opposing left-and-right currents do not radiate, but the unbalanced vertical current does radiate. Since for typically used frequencies, the vertical wire is much shorter than a quarter wavelength, the horizontal wire acts as a "capacitance hat" that increases the flow of current in the vertical radiator by giving it somewhere further to go, once it reaches the top. The increased current improves the efficiency and gain.

Inverted 'L'

An inverted 'L' is 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'. It can be thought of as an ordinary monopole that has been bent over somewhere in the middle, with its lower antenna segment vertical, as usual, but the "upper" part running horizontally – out sideways instead of up. 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.

Inverted 'F'

An inverted 'F' is 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 antenna

An umbrella antenna is 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. Umbrella antennas are enormous on human-scale but are paradoxically ultra-short antennas, relative to the even larger wavelengths they are built for. An umbrella antenna 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 opened metal umbrella frame, hence the name. Since umbrella antennas are much smaller than a wavelength they have many troublesome properties: Extremely narrow bandwidth, low radiation resistance, and excessive capacitive feedpoint reactance. Like other ultra-short antennas they require both a large loading coil and a meticulously constructed low-resistance counterpoise system to cope with the exceedingly high negative reactance and minimal radiation resistance.

Loop antennas

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 called magnetic antennas, but that generic name is confusingly similar to the term magnetic loop normally used to describe small loops. Their exclusive interaction with the magnetic field makes them relatively insensitive to electrical spark noise within about ⁠1/ 6 ⁠ wavelength of the antenna. 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.

Large loops

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.

Large loops

Large loops have a perimeter slightly larger than one full wavelength on their lowest resonant frequency. 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.

Half-loop

the upper half of a vertical full-wavelength loop antenna mounted on the ground. The full loop is cut at two opposite points along its perimeter, and the lower half is omitted; the upper half wired to an extensive ground system at each of the cut points, sticking up from the earth 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 image in the ground-plane. 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.

Not to be confused with the visually similar but electrically different half-square antenna (see array antennas), nor the halo antenna.

Halo antennas

Halo antennas 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 only used on their lowest natural resonant frequency. Halos 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.

Halos' radiation pattern is mostly in-plane and nearly omnidirectional, about the same as small loops' radiation pattern. Halos' radiation efficiency is intermediate between the high efficiency of large loops and the generally poor efficiency of small loops. Halos are self-resonant like full-wave loops, but are almost never used on their higher (odd multiple) harmonics, because of the nuisance side-lobes that form at otherwise-usable odd-numbered higher resonances. In some regards they represent the extreme upper size limit of small transmitting loops.

Small loops

Small loop antennas' loop perimeters are smaller than a half-wavelength. They 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. If the loop needs to be resonant for transmitting or frequency-selective receiving, 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 pervasive static makes their inefficiency irrelevant, 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" (not to be confused with the more general category magnetic antenna, which includes small 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 as long as their perimeter is kept small enough, their "null" direction is exceptionally precise, and for hand-carried direction-finding equipment, their small size makes them more compact than dipole-based directional antennas.

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.

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, in order to improve on their generally poor efficiency. For that same reason, their parts are carefully joined by brazing or welding (not soldering) 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.

Highly accurate small loop null directions

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). 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.

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

Composite antennas are made up of combinations of several simple antennas configured to function as a single antenna, analogous 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.

Broadbanded composite antenna

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. 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, contiguous span of frequencies.

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.

Fan monopole

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.

Log-periodic dipole array

Log-periodic dipole arrays are broadband because they are made from multiple dipoles share a common feedpoint, and so warrant mention in this section, but are fully outlined in the subsection on endfire arrays, below. Log periodic antennas are made with an aligned series of gradually shorter dipoles for directionality. As a consequence, the antenna transmits well (with varying gain) for the range of frequencies between the resonant frequency of its longest dipole in the back, through to the higher resonant frequency of its shortest dipole, in the front. The effective transmit frequencies have a broader range than the frequencies for which the dipole array is usefully directional.

Array antenna

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

Broadside arrays consist of multiple, parallel or colinear, identical driven elements, usually dipoles. They can either be stacked along line - a vertical line of vertical elements or a horizontal line of horizontal elements – or they can be placed in a row of parallel elements – a vertical stack of horizontal antennas or a horizontal row of vertical antennas. The antennas are all fed in phase and collectively radiate a beam perpendicular to the plane containing the simple antennas, analogous to a firing line of musketeers, all simultaneously shooting in the same direction, perpendicularly out from the line of shooters.

Vertical collinear

A broadside array that consists of several dipoles, fed in phase, either with their axēs stacked atop each other, in a single vertical line, or more rarely lined up end-to-end horizontally. 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 Bruce arrays, 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. Distinct from the similar-looking half-loop, no conductive part of a half-square has any direct connection to the ground beneath it. The top-to-top half-wave connecting wire serves as a phasing line that keeps radiation from the two vertical segments in-phase; the line serves as an effective feedpoint even though the actual feedpoint for the whole antenna system could connect at some more convenient place on the antenna. Half-square antennas radiate from near their apex, where the verticals connect to the horizontal wire, and hence for long-distance communication their signals tend to radiate along a clearer local line-of-sight to the horizon, compared to a conventional bottom-fed monopole whose strongest radiation comes from the segment nearest its base.

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.

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.

Both broadside and endfire

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

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 constituent parallel antennas all lie in.

Continuing with the musketeer 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 heads of the musketeers in front of them. Each analogy musketeer fires his / her shot just as the bullets from the rearward musketeers' shots pass overhead, so that the combined volley of bullets are all bunched in a single aligned group, and all arrive at the target simultaneously.

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

Parasitic arrays

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 draw power from the radiated beam and then re‑radiate that signal power 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. A Yagi-Uda 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. The multiple other elements are parasitic, which reflect and direct the radiated signal into a narrower beam, hence the name beam antenna. 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.

Moxon antenna

Also called a Moxon rectangle; it is a two dipole-element Yagi-Uda, hence a minimal parasitic array, whose dipole ends are bent towards each other and attached by an insulator into an overall rectangular shape. It is usually a driver-reflector dipole pair on the design frequency, but may additionally be used less optimally as a driver-director pair at another frequency. The ends of the separate dipoles are bent inward, towards each other, and connected by insulated cord or robust tubing to form a rectangular shape. Bending and connecting the ends makes the antenna more sturdy under wind load, and somewhat reduces the width, making the directional antenna easier to rotate than a full-width Yagi-Uda.

Quad

Although "quad" can refer to a single quadrilateral-shaped loop, the term usually refers to two or more parallel-plane loops stacked as a parasitic array. Although very often the loops are indeed square-shaped, to minimize diameter and height, the individual loops can actually be non-quadrilateral shapes, despite the customary name. At first glance, square 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.

Aperture antenna

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).

Traveling wave antenna

Unlike the antennas discussed so-far, traveling-wave antennas are not resonant so they have inherently broad bandwidth. 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 to 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. Individual traveling wave antennas are preferably about 1.5–2.5 waves from end-to-end, which requires a large space to place them. In the lower HF band, the difficulty of securing enough room to erect several long antennas for receiving from several directions makes traveling wave antennas infeasible for those without a large property.

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. 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 lozenge or 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 place at the opposite acute corner. The design makes for a good broadband shortwave receiving antenna with a somewhat directional horizontal main lobe, intercepting waves moving in the direction from the terminating resistor at one acute corner, towards the feedpoint at the other acute corner. The larger the rhombus, in wavelengths, the more directional the antenna is. Always a good receiving antenna, it can be used for two-way skywave communication when increasing transmit power enough to compensate for power dissipated in the terminating resistor is practical.

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.

Not to be confused with a "rubber ducky" antenna (normal mode helix), which is much smaller.

Other antenna types

The following are some antenna types that don't comfortably fit into any of the 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 some limitations.

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 mediumwave and longwave receiving antennas, but transmitting ground dipoles 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 although still rare, those are the bands buried antennas are most used on.

Resistively loaded antennas

One method for making a broadband antenna is to place a resistive element in the electrical path through the antenna, sometimes disguised by, and made somewhat more frequency-selective by embedding brass or tin inside an inductor. The extra resistance 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), the tilted terminated folded dipole (T²FD), and the similar Robinson-Barnes antenna (essentially a T²FD with a third wire running parallel to the other two).

Random wire antenna

Moxon (1993) describes the random-wire antenna as an "odd bit of wire". It is the typical informal antenna erected for listening to shortwave signals and for receiving AM broadcasts from distant stations. A random wire antenna consists of an unplanned / as-available length aerial, either strung outdoors between elevated supports, or indoors across a ceiling, running in an erratic zigzag pattern along walls or between supports. 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.

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 might technically be considered an extreme instance of a low-hanging Beverage antenna.

L.O.G. antenna

A "loop on the ground" – often called a "L.O.G." or 'log' antenna – is a loop of wire laid flat on open ground, in an oval or rectangle, with the two ends wired to a feedline. It is somewhat more popular than 'snake' or 'bog' antennas since it requires roughly a quarter as much linear space (although more square space). It has the same advantage that all magnetic antennas have: It is "deaf" to electric spark noise originating nearby – within about one sixth of the receiving wavelength. Because it responds to the magnetic part of the electromagnetic radio waves, and since most soils are much more transparent to magnetic fields than to electric fields, it is expected to show less signal loss from soil absorption than 'snake' or 'bog' antennas, which respond to the electric part.

Isotropic

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, whose electrical length is under ~⁠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.

Nearly isotropic antennas

Real antennas that are nearly isotropic can be made by combining two or three small antennas, with each small element (electrical length shorter than ~⁠1/10⁠ wave) aligned differently from the others.

Nearly isotropic antennas are used as emergency antennas on satellites, since they work even if the satellite is tilted out of alignment with its communication station. They are also sometimes used for field strength meters, or as standard reference receive or transmit antennas for testing other antennas, since small mis-alignments are a non-issue: Their signal strength measures nearly the same for any orientation.

Omnidirectional is not isotropic

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 altitude angle rises to align with the antenna's vertical axis. Several types of antennas – such as vertical, short whips or horizontal small magnetic loops – 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.