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decibels relative to isotropic
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 /
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.