The dBd 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 (µW/m2) in its favored direction at a point located a defined distance away. Also, suppose a half-wave dipole antenna D produces an electromagnetic field of intensity ID µW/m2 in its favored direction at the same distance. Then the gain G of antenna A, in dBd, is:
G = 10 log10 (IA / ID)
A dipole antenna has a gain of 0 dBd. An isotropic radiator has a gain of -2.15 dBd.