In
array antenna
An antenna array (or array antenna) is a set of multiple connected antennas which work together as a single antenna, to transmit or receive radio waves. The individual antennas (called ''elements'') are usually connected to a single receiver o ...
systems, array gain is the measure of the improvement in
signal-to-noise ratio
Signal-to-noise ratio (SNR or S/N) is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. SNR is defined as the ratio of signal power to the noise power, often expressed in de ...
(SNR) achieved by the array. It is calculated as the SNR of the array output signal divided by the SNR of the array input signal. Intuitively, the array gain is realized by the fact that the signal is
coherently added from ''N'' array elements, while the noise is incoherently added from those same elements. If the noise is presumed to be uncorrelated the array gain is ≤ ''N'', the number of array elements, and the array gain reduces to the inverse of the square of the
2-norm of the array weight vector, under the assumption that the weight vector is normalized such that its sum is unity, so that
:
For a uniformly weighted array (un-tapered such that all elements contribute equally), the array gain is equal to ''N''.
Array gain is not the same thing as "gain," "power gain," "directive gain," or "directivity," but if the noise environment around the array is isotropic and the array input signal is from an isotropic radiator, then array gain is equal to gain defined in the usual way from the array beam pattern. The terms "power gain" and "directive gain" are deprecated by IEEE.
Applications
In
radio astronomy
Radio astronomy is a subfield of astronomy that studies celestial objects at radio frequencies. The first detection of radio waves from an astronomical object was in 1933, when Karl Jansky at Bell Telephone Laboratories reported radiation comin ...
it is difficult to achieve a good signal-to-noise ratio because of background noise from modern communications. Even for strong astronomical radio emission it is typical for SNR levels to be below 0
decibel
The decibel (symbol: dB) is a relative unit of measurement equal to one tenth of a bel (B). It expresses the ratio of two values of a power or root-power quantity on a logarithmic scale. Two signals whose levels differ by one decibel have a ...
s. To counter this problem exposure of the antenna to the source over large periods of time are needed just as in visible sky viewing. Array gain is achieved using multiple, even dozens of radio receivers to collect as much signal as possible.
Robots equipped with antenna arrays have better communication using array gain to eradicate dead spots and reduce interferences.
[Hambling, David. "Robots fall into line with the help of sparse array antenna." New Scientist 207 (2010): 22. OmniFile Full Text Mega. Web. 24 Apr. 2011.]
See also
*
Diversity gain Diversity gain is the increase in signal-to-interference ratio due to some diversity scheme, or how much the transmission power can be reduced when a diversity scheme is introduced, without a performance loss. Diversity gain is usually expressed in ...
References
Antennas (radio)
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