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Below are all of the forum threads, including all
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Post subject: Definition of dBi
Posted: Fri Aug 26, 2005 3:23 am
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.
Posted: Sun Sep 04, 2005 7:40 pm
Joined: Fri Sep 02, 2005 7:25 pm
Location: Hampshire UK
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