What is dBd?

Antennas RF Basics 
The antenna that I am planning on using has a gain of 5 dBd. What is dBd? Is it the same as dB?
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Editorial Team - everything RF

Nov 10, 2018

Antenna gain is measured in decibels. When we calculate the gain of an antenna and compare it to an isotropic antenna the unit of the gain is dBi (i stands for Isotropic antenna). An Isotropic antenna has a 0 dB gain.

dBd refers to the antenna gain with respect to a reference dipole antenna. A reference dipole antenna is defined to have 2.15 dBi of gain. So converting between dBi and dBd is as simple as adding or subtracting 2.15 according to these formulas:

dBi = dBd + 2.15

dBd = dBi - 2.15