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Antenna Gain Measured in dBi & dBd.

DBi is a unit measuring the gain of an antenna.
The reference level or dBi is the strength of the signal that would be transmitted by a non-directional isotropic antenna i.e.radiates equally in all directions.
This antenna exists as a mathematical concept used only as a known reference to measure antenna gain per dBi.
In electronics, the term "gain" is often repeated but misunderstood.
Gain implies increase e.g 20 dBi but without respect to where the increase originated.

An antenna transmits and receives radio waves. Antenna gain is used to indicate the increase in power of one antenna (transmitting or receiving) as compared to another antenna.
Gain is actually a ratio of power levels and is stated in decibels dBi.

The dipole or basic antenna concentrates it signals in two directions. The isotropic antenna doesn't favor any particular direction so its dBi gain equals 0.
The dipole has a 2.15 dBi measurement gain over an isotropic radiator. Therefore a 6 dBi antenna gain over an isotropic radiator computes to a 3.9 dBi gain over a dipole.
For every 3 dBi of improvement added to your antenna, results in a noticeable effect on the receiving station. Any dBi gain less than 3 dBi leads to an undetectable dBi improvement, however don't discount improvements under 3 dBi. Sometimes a .5 dBi means hearing or not hearing a station!

dBd compares the gain of an antenna to the gain of a reference dipole antenna (defined as 2.15 dBi gain).
To convert dBi to dBd:
Gain in dBd = gain in dBi - 2.15 dB
Gain in dBi = gain in dBd + 2.15 dB