Directional antenna

Source: Wikipedia, the free encyclopedia.
GDSCC
, California
Patch antenna gain pattern

A directional antenna or beam antenna is an

antenna which radiates or receives greater radio wave power in specific directions. Directional antennas can radiate radio waves in beams, when greater concentration of radiation in a certain direction is desired, or in receiving antennas receive radio waves from one specific direction only. This can increase the power transmitted to receivers in that direction, or reduce interference from unwanted sources. This contrasts with omnidirectional antennas such as dipole antennas
which radiate radio waves over a wide angle, or receive from a wide angle.

The extent to which an antenna's angular distribution of radiated power, its

space missions,[2] these antennas are also in use all over Earth, most successfully in flat, open areas where there are no mountains to disrupt radiowaves.[citation needed
]

In contrast, a low-gain antenna (LGA) is an omnidirectional antenna, with a broad radiowave beam width, that allows the signal to propagate reasonably well even in mountainous regions and is thus more reliable regardless of terrain. Low-gain antennas are often used in spacecraft as a backup to the high-gain antenna, which transmits a much narrower beam and is therefore susceptible to loss of signal.[3]

All practical antennas are at least somewhat directional, although usually only the direction in the plane parallel to the earth is considered, and practical antennas can easily be omnidirectional in one plane. The most common directional antenna types are[citation needed]

These antenna types, or combinations of several single-frequency versions of one type or (rarely) a combination of two different types, are frequently sold commercially as residential

frequencies, tower arrays
are used in most cases as directional antennas.

Principle of operation

When transmitting, a high-gain antenna allows more of the transmitted power to be sent in the direction of the receiver, increasing the received signal strength. When receiving, a high gain antenna captures more of the signal, again increasing signal strength. Due to reciprocity, these two effects are equal—an antenna that makes a transmitted signal 100 times stronger (compared to an isotropic radiator) will also capture 100 times as much energy as the isotropic antenna when used as a receiving antenna. As a consequence of their directivity, directional antennas also send less (and receive less) signal from directions other than the main beam. This property may avoid interference from other out-of-beam transmitters, and always reduces antenna noise. (Noise comes from every direction, but a desired signal will only come from one approximate direction, so the narrower the antenna's beam, the better the crucial signal-to-noise ratio.)

There are many ways to make a high-gain antenna; the most common are

Yagi-Uda antennas, and phased arrays of smaller antennas of any kind. Horn antennas can also be constructed with high gain, but are less commonly seen. Still other configurations are possible—the Arecibo Observatory
used a combination of a line feed with an enormous spherical reflector (as opposed to a more usual parabolic reflector), to achieve extremely high gains at specific frequencies.

Antenna gain

Watt transmitter look like a 100 Watt transmitter, then the beam can cover at most 1/100 of the sky (otherwise the total amount of energy radiated in all directions would sum to more than the transmitter power, which is not possible). In turn this implies that high-gain antennas must be physically large, since according to the diffraction limit
, the narrower the beam desired, the larger the antenna must be (measured in wavelengths).

Antenna gain can also be measured in dBd, which is gain in

Yagi-type
aerials this more or less equates to the gain one would expect from the aerial under test minus all its directors and reflector. It is important not to confuse dBi and dBd; the two differ by 2.15 dB, with the dBi figure being higher, since a dipole has 2.15 dB of gain with respect to an isotropic antenna.

Gain is also dependent on the number of elements and the tuning of those elements. Antennas can be tuned to be resonant over a wider spread of frequencies but, all other things being equal, this will mean the gain of the aerial is lower than one tuned for a single frequency or a group of frequencies. For example, in the case of wideband TV antennas the fall off in gain is particularly large at the bottom of the TV transmitting band. In the UK this bottom third of the TV band is known as group A.[5]

Other factors may also affect gain such as aperture (the area the antenna collects signal from, almost entirely related to the size of the antenna but for small antennas can be increased by adding a ferrite rod), and efficiency (again, affected by size, but also resistivity of the materials used and impedance matching). These factors are easy to improve without adjusting other features of the antennas or coincidentally improved by the same factors that increase directivity, and so are typically not emphasized.

Applications

High gain antennas are typically the largest component of deep space probes, and the highest gain radio antennas are physically enormous structures, such as the

Deep Space Network
uses 35 m dishes at about 1 cm wavelengths. This combination gives the antenna gain of about 100,000,000 (or 80 dB, as normally measured), making the transmitter appear about 100 million times stronger, and a receiver about 100 million times more sensitive, provided the target is within the beam. This beam can cover at most one hundred millionth (10−8) of the sky, so very accurate pointing is required.

Use of high gain and

NP-Hard problem.[6]

Gallery

  • An early example (1922) of a directional AM radio transmitter using a long wire antenna, built for WOR, then in Newark, New Jersey and targeting both New York City and Philadelphia in addition to Newark.
    An early example (1922) of a directional AM radio transmitter using a long wire antenna, built for WOR, then in Newark, New Jersey and targeting both New York City and Philadelphia in addition to Newark.
  • Karl Jansky and his rotating directional radio antenna (1932) in Holmdel, New Jersey, which was the world's first radio telescope, discovering radio emissions from the Milky Way.
    Holmdel, New Jersey, which was the world's first radio telescope, discovering radio emissions from the Milky Way
    .
  • Grote Reber's homemade antenna in Wheaton, Illinois (1937), world's second radio telescope and first parabolic radio telescope
    Grote Reber's homemade antenna in Wheaton, Illinois (1937), world's second radio telescope and first parabolic radio telescope
  • Holmdel Horn Antenna in Holmdel, New Jersey (1960s). Built to support the Echo satellite communication program,[7] it was later used in experiments that revealed the Cosmic microwave background permeating the universe.[8]
    Echo satellite communication program,[7] it was later used in experiments that revealed the Cosmic microwave background permeating the universe.[8]
  • Parabolic antenna – the 70 m antenna at Goldstone Deep Space Communications Complex in the Mojave Desert, California
    Parabolic antenna – the 70 m antenna at Goldstone Deep Space Communications Complex in the Mojave Desert, California
  • Voyager 2 spacecraft. The HGA (a parabolic antenna) is the large bowl-shaped object.
    Voyager 2 spacecraft. The HGA (a parabolic antenna) is the large bowl-shaped object.
  • A giant phased-array radar in Alaska
    A giant
    phased-array
    radar in Alaska
  • A Yagi-Uda antenna. From left to right, the elements mounted on the boom are called the reflector, driven element, and director. The reflector is easily identified as being a bit longer (5% or more) than all the other elements, and the director(s) a bit shorter (5% or more).
    A
    Yagi-Uda antenna
    . From left to right, the elements mounted on the boom are called the reflector, driven element, and director. The reflector is easily identified as being a bit longer (5% or more) than all the other elements, and the director(s) a bit shorter (5% or more).

See also

References

  1. ISBN 978-981-15-5281-6. Extract of page 535
  2. ISBN 978-1-4381-1018-9. Extract of page 364
  3. ^ "Low-gain antenna". Oxford Reference (oxfordreference.com).
  4. ^ "Low gain aerial acceptance angle". Row ridge TX. aerialsandtv.com.
  5. ^ For comparison of groups of aerials to a wideband aerial of the same size / model, see "Gain graph". aerialsandtv.com.
  6. S2CID 2285688
    .
  7. doi:10.1002/j.1538-7305.1961.tb01639.x
    .
  8. ^ "Horn antenna". Astronomy and astrophysics. History. U.S. National Park Service. 2001-11-05. Archived from the original on 2008-05-12. Retrieved 2008-05-23.

External links

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