In radio systems, many different antenna types are used whose properties are especially crafted for particular applications. Antennas can be classified in various ways. The list below groups together antennas under common operating principles, following the way antennas are classified in many engineering textbooks.[1][2][3](p4)

Dipole

Main article: dipole antenna

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.[2][4] The most common type, the half-wave dipole, consists of two resonant elements just under a quarter wavelength long. This antenna radiates maximally in directions perpendicular to the antenna's axis, giving it a small directive gain of 2.15 dBi. Although half-wave dipoles are used alone as omnidirectional antennas, they are also a building block of many other more complicated directional antennas.

Monopole

Main article: monopole antenna

A monopole antenna is a half-dipole (see above); it consists of a single conductor such as a metal rod, usually mounted over the ground or an artificial conducting surface (a so-called ground plane).[2][5] One side of the feedline from the receiver or transmitter is connected to the conductor, and the other side to ground or the artificial ground plane. The radio waves reflected from the ground plane appear as if they came from a fictitious image antenna seemingly below the ground plane, with the monopole and its phantom image effectively forming a dipole. Hence, the monopole antenna has a radiation pattern identical to the top half of the pattern of a similar dipole antenna, and a radiation efficiency a bit less than half of the dipole. Since all of the equivalent dipole's radiation is concentrated in a half-space, the antenna has twice (3 dB increase of) the gain of a similar dipole, neglecting power lost in the ground plane.

The most common form is the quarter-wave monopole which is one-quarter of a wavelength long and has a gain of 5.12 dBi when mounted over a ground plane. Monopoles have an omnidirectional radiation pattern, so they are used for broad coverage of an area, and have vertical polarization. To reduce signal absorption by the Earth, ground waves used for broadcasting at low frequencies must be vertically polarized, so large vertical monopole antennas are used for broadcasting in the MF, LF, and VLF bands. Small monopoles ("whips") are used as nondirectional antennas on portable radios in the HF, VHF, and UHF bands.

Array

Main article: Antenna array

Array antennas consist of multiple simple antennas working together as a single compound antenna. The simple antennas can be dipoles, monopoles, or loops, or mixed loops and dipoles. Broadside arrays consist of multiple identical driven elements, usually dipoles, fed in phase, radiating a beam perpendicular to the antenna plane. Endfire arrays are fed out-of-phase, with the phase difference corresponding to the distance between them; they radiate within the antenna plane.[2][6][3](pp283–371) Parasitic arrays consist of multiple antennas, usually dipoles, with one driven element and the rest parasitic elements, which radiate a beam along the line of the antennas.

Loop

Main articles: (large) Loop antenna, Halo antenna, and Magnetic loop antenna

Ferrite loopstick antenna from an AM broadcast radio, 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 antenna for transmitting at high frequencies, 2m diameter
Separate loop antenna for AM radio
A two-element quad antenna used by an amateur radio station

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, making them relatively insensitive to electrical noise within about 1/ 6  wavelength of the antenna.[2][9][10] There are essentially two broad categories of loop antennas: large loops (or full-wave loops) and small loops. Only one design, a halo antenna, that is usually called a loop does not exclusively fit into either the large or small loop categories.

Large loops

Full-wave loops have the highest radiation resistance, and hence the highest efficiency of all antennas: Their radiation resistances are several hundreds of Ohms, whereas dipoles and monopoles are tens of Ohms, and small loops are a few ohms, or even fractions of an Ohm.[10]

In between

The approximately-omnidirectional pattern of halos resembles small loops; their radiation efficiency lies inbetween the extremely high efficiency of large loops and the generally poor efficiency of small loops. Like full-wave loops, halos are self-resonant. In some regards they represent the extreme upper size limit of small transmitting loops.[2][10][3](pp231–275)

Small loops

The great disadvantage of any small antenna, including small loop antennas, is a very small radiation resistance – typically much smaller than the loss resistance, making small loops very inefficient for transmitting. However, small loops are very effective receiving antennas, especially at low frequencies, where all feasible antennas are "small" compared to a wavelength.

The nulls in the radiation pattern of small loop / ferrite core antennas are bi-directional, and are much sharper than the directions of maximum power of either loop or of electric 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] This makes the direction of the small loop's null more useful than the direction of the strongest signal for accurately locating a signal source, and the small loop / ferrite core type the smallest-size antenna useful for radio direction finding (RDF). The null direction of small loops can also be exploited to reject unwanted signals from an interfering station or noise source.[2][9][10]

Aperture

An aperture antenna consists of a small dipole or loop feed antenna embedded inside a larger, three-dimensional surrounding structure that guides the radio waves from the feed antenna in a particular direction, and vice versa. The guiding structure is often dish-shaped or funnel-shaped, and quite large compared to a wavelength, with an opening, or aperture, to emit the radio waves in only one direction. Since the outer antenna structure is itself not resonant, it can be used for a wide range of frequencies, by replacing or retuning the inner feed antenna, which normally is resonant.

Traveling wave

Main article: Traveling-wave antenna

Unlike the antennas discussed so-far, traveling-wave antennas are not resonant so they have inherently broad bandwidth.[2][3](pp549–602) They are typically wire antennas that are multiple wavelengths long, through which the voltage and current waves travel in one direction, instead of bouncing back and forth to form standing waves as in resonant antennas. They have linear polarization (except for the helical antenna). Unidirectional traveling-wave antennas are terminated by a resistor at one end equal to the antenna's characteristic resistance, to absorb the waves from one direction. This makes them inefficient as transmitting antennas, but removes half of the incident radio noise when used for receiving.

Isotropic

Main article: isotropic radiator

An isotropic antenna (isotropic radiator) is a hypothetical antenna that radiates equal signal power in all directions.

An antenna that is exactly isotropic is only a mathematical model, used as the base of comparison to calculate the directionality or gain of real antennas. No actual antenna can produce a perfectly isotropic radiation pattern, but the isotropic radiation pattern serves as a "worst case" reference for comparing the degree to which other antennas, regardless of type, can project radiation in one direction.

Nearly isotropic antennas can be made by combining several small antennas; these are used for field strength measurement, as standard reference antennas for testing other antennas, and as emergency antennas on satellites, since they work even when the satellite has lost orientation towards its communication station.

An isotropic antenna should not be confused with an omnidirectional antenna; an isotropic antenna radiates equal power in all three dimensions, while an omnidirectional antenna radiates equal power in all horizontal directions, with the power radiated varying with elevation angle, but decreasing in the direction parallel to the antenna's vertical axis; for several antenna types there is no radiation at all in the exact vertical direction.

Other antenna types

A typical random wire antenna for shortwave reception, strung between two buildings, with an extended segment out to a remote post. Assuming the building is about 20 feet tall, the length of wire seems to be on the order of 100 feet long – too short for a Beverage antenna.
A typical random wire antenna for shortwave reception, strung between two buildings, with an extended segment out to a remote post. Assuming the building is about 20 feet tall, the length of wire seems to be on the order of 100 feet long – too short for a Beverage antenna.

The shape and length of a "random" wire is determined by available space and supports, instead of being laid out in a single straight line in a planned direction, with a trimmed resonant length. A random wire antenna typically has a complicated radiation pattern, with several lobes at varying angles to each wire segment, in different directions for each, which depend on frequency and segment length.

Random wire antennas are often included as a sub-category of folded monopole antennas, if their lengths are a quarter-wave or less, or folded end-fed dipoles if a half-wave or more, up to one or two wavelengths or less. When they are laid out with at least one extended segment oriented in a straight line, one to several wavelengths long, they operate essentially the same as a traveling-wave antenna. Random wire antennas laid out along the ground are called "snake antennas" and are sometimes included among Beverage antennas as an extreme case of that type.

Notes

  1. ^ The multiple wires make the antenna wider-band than a simple two-arm dipole. The multiple wires spreading from the bow-tie feedpoint are connected in matching pairs, each pair a different length, give the dipole a wider range of resonances. The idea is that the feed current naturally flows into whichever piece of wire offers the lowest impedance at the frequency being fed. If the several dipole pairs are near the same length, the antenna will show a wider bandwidth than a single dipole. If the dipole pairs have more widely different lengths, the bow-tie antenna will show multiple distinct resonant frequencies.
    For analogous antennas see batwing antenna and butterfly antenna (flattened version of the biconical antenna).
  2. ^ The folded unipole's skirt wire(s) and mast form an elongated, vertical coil which inductively loads the antenna. Alternatively, the mast and each skirt wire can be viewed as a shorted loading stub. The inductance is set by the attachment point's height and the space between the skirt and the mast.
  3. ^ A parasitic element that is slightly too long for resonance reflects the driven element's signal back towards it, similar to a mirror, is called a "reflector", and is usually the last and longest element in the array. Elements that are slightly shorter than resonant length increase the intensity of radio waves passing through them and are called "directors"; adding more director elements causes the waves radiated from the driven element to be concentrated in a more narrow beam.
  4. ^ There are more elaborate, compounded designs which can ease this limitation.
  5. ^ a b Because of their similar "fishbone" shapes multi-element Yagi-Uda antennas and log-periodic antennas are often confused.
  6. ^ The popular "quad" antenna design is necessarily made from full-wave loops, usually two full-wave loops, so no other distinction is needed.
  7. ^ A half-loop antenna is different from a half-square array antenna, despite the confusingly similar names.
  8. ^ The glaring exceptions are car radios, which require an antenna mounted outside the metal car chassis, which blocks AM band and longer-wave reception.

References

  1. ^ Bevelaqua, Peter J. "Types of antennas". Antenna-Theory.com. Archived from the original on 30 June 2015. Retrieved 28 June 2015. — Peter Bevelaqua's private website.
  2. ^ a b c d e f g h i j k l m n
    Aksoy, Serkan (2008). "Lecture Notes - v.1.3.4" (PDF). Electrical Engineering. Antennas. Gebze, Turkey: Gebze Technical University. Archived from the original (PDF) on 22 February 2016. Retrieved 29 June 2015.
  3. ^ a b c d e
    Balanis, Constantine A. (2005). Antenna Theory: Analysis and Design. Vol. 1 (3rd ed.). John Wiley and Sons. ISBN 047166782X – via Google Books.
  4. ^ It is a basic antenna type upon which more elaborate antennas are based. Bevelaqua, Peter J. "Dipole Antenna". Antenna-Theory.com. Archived from the original on 17 June 2015.
  5. ^ Bevelaqua, Peter J. "Monopole Antenna". Antenna-Theory.com. Archived from the original on 15 June 2015.
  6. ^ Bevelaqua, Peter J. "Antenna Arrays". Antenna-Theory.com. Archived from the original on 25 April 2017.
  7. ^ a b Moxon, Les A. (G6XN) (1993). HF Antennas for All Locations (2 ed.). Radio Society of Great Britain. ISBN 1-872309-15-1.
  8. ^ a b Severns, Rudy (N6LF) (1996). "Using the half-square antenna for low-band DXing". In Straw, R. Dean (N6BV); Roznoy, Rich (KA1OF) (eds.). ARRL Antenna Compendium. Vol. 5. Newington, CT: American Radio Relay League. pp. 35–44. ISBN 0-87259-562-5.
  9. ^ a b c d e f g h
    Bevelaqua, Peter J. "Loop Antennas". Antenna-Theory.com. Archived from the original on 17 June 2015.
  10. ^ a b c d e f g h i
    Silver, H. Ward, ed. (2011). ARRL Antenna Book for Radio Communications (22nd ed.). Newington, CT: American Radio Relay League. Chapter 5, Section 9.6, Section 11.6, Section 16.5, Section 20.6, Chapter 22. ISBN 978-0-87259-680-1.