Antenna measurements normally involve comparisons expressed in decibels defined as: dB=10*Log10(P/Pref). P is the value we�re measuring, and Pref is some reference.
Note that dB are logarithmic with respect to the measured value. A convenient rule of thumb is that each three-dB increase requires the power be doubled.
References in antenna work are usually either an isotropic radiator or a dipole. To keep straight which comparison is intended, an �i� (as in dBi) is used to denote an isotropic radiator and a �d� (as in dBd) is used to denote a dipole.
An isotropic radiator is an antenna that radiates equally in all directions. While it can�t actually be built, it�s a convenient mathematical �benchmark� against which to compare real antennas.
Since a dipole is a practical antenna, comparisons with it become context-dependent. For example, a half-wavelength dipole in free space has a gain of 2.1 dBi (0.0 dBd=2.1 dBi). Over actual earth, however, the dipole�s gain depends on its height and the earth�s conductivity and permitivity.
DBi comparisons are usually more meaningful unless the intent is to actually compare the performance of a particular antenna with a dipole installed in a similar manner.
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