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Definition of dBi - RF Cafe Forums
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| malvinas2
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Post subject: Definition of dBi
Posted: Fri Aug 26, 2005 3:23 am
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You can find some tutorials about antennas on Kirts
Website:
decibels relative to isotropic
radiator
The expression dBi is used to define
the gain of an antenna system relative to an isotropic
radiator at radio frequencies. The symbol is an
abbreviation for "decibels relative to isotropic."
The dBi specification is based on the decibel,
a logarithmic measure of relative power. Suppose
an antenna A produces an electromagnetic field of
intensity IA microwatts per square meter (IAµW/m2)
in its favored direction at a point located some
distance away. Also, suppose an isotropic antenna
Q produces an electromagnetic field of intensity
IQ µW/m2 at the same distance. Then the gain G of
antenna A, in dBi, is:
G = 10 log10 (IA /
IQ)
A dipole antenna has a gain of 2.15 dBi.
An isotropic radiator has a gain of 0 dBi.
Note: In real life an isotropic radiator doesn't
exist, it's just an idea.
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Graham |
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Post subject:
Posted: Sun Sep 04, 2005 7:40 pm
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| Colonel |
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Joined: Fri Sep 02, 2005
7:25 pm
Posts: 34
Location: Hampshire UK
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Expanding on that a bit, the notion of a mathematiaclly
"pure" isotropic radiation point source has, for
the antenna manufacturers the agreeable effect of
allowing them to add 2.15dB to their gain figures.
In practice, you can only measure the gain of
the antenna relative to a known reference antenna
- this being a dipole which can be made accurately
and repeatably. The test environment either eliminates
or accounts for ground or other surfaces contributing
(reflection gain). Then you add the 2.15 dB to turn
dBd into dBi
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Posted 11/12/2012
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