 # Antenna Gain dBi vs. dBd

## How does antenna gain stated in dBi compare to gain stated in dBd for the same antenna?

A. dBi gain figures are 2.15 dB lower than dBd gain figures
B. dBi gain figures are 2.15 dB higher than dBd gain figures
C. dBi gain figures are the same as the square root of dBd gain figures multiplied by 2.15
D. dBi gain figures are the reciprocal of dBd gain figures + 2.15 dB

[G9C04]

First off we should probably define the terms “dBd” and “dBi”…

dBi is a measurement that compares an antenna’s gain with respect to an isotropic radiator (a theoretical antenna that radiates energy evenly in a spherical pattern. If you’ve perused the questions or exam study guide for the General and/or Extra exams you should have seen this term already. It’s based 100% on theory.

dBd on the other hand, compares an antenna’s gain to the gain of a reference dipole antenna.

I know, I know. We could deep-dive into the math but its probably going to be complicated and its easier to just recall the definitions.

The radiation pattern for a dipole is much much different than a spherical isotropic single point theoretical radiator. It resembles a toroid or “donut” going out from the wire of a dipole with the wire running down the center of the “donut.”

Further reading: Effective Radiated Power, which actually gets into the math.

For purposes of this question, just recall “higher” and you should be fine.

The answer (if you haven’t figured it out by now) is B. dBi gain figures are 2.15 dB higher than dBd gain figures

These are definitions, and basically you’re going to have to commit them to memory.