Below is an excerpt from a book:
Two common terms you'll hear in specifying the gain of an antenna are dBi and dBd. These terms are defined as dB of gain referenced to an Isotrope (dBi) and dB of gain referenced to a dipole (dBd). A dipole has 2.1 dBi of gain, but 0 dBd of gain. It is critical to know which reference the antenna manufacturer uses, because it will have an effect on the path loss and power calculations we will discuss later.
I don't get how dBd and dBi is related with the ratio of 2.1.
Can these units and 2.1 ratio be explained in an easier or pictorial manner?
Best Answer
An isotropic antenna would radiate equally well (or be equally sensitive) in all directions.
A dipole antenna is directional, radiating little or no power off the ends of the element, instead directing the power at right angles to the antenna element - this results in about 2 dB more power in the favoured direction than the isotropic antenna would provide.