What is an Isotropic Antenna and dBi gain?

What is an Isotropic Antenna and dBi gain?

Isotropic Antenna is a theoretical antenna which has a perfect spherical radiation pattern, which is never practically achieved in any antenna! Many commercial vertical antennas mention their gain in dBi, in comparison with the theoretical isotropic antenna. They have a 360 degree disk like horizontal radiation pattern towards the horizon all around, unlike the spherical radiation pattern of an isotropic antenna.

Dipole has strongest signal radiation perpendicular to the orientation of the radiating element. Radiation from a dipole is weakest from the ends of the radiator. Hence a dipole has a gain of 2.15 dBi compared to the isotropic antenna, in the direction of the main lobe. Gain comparison with a dipole is designated as dBd. Gain expressed in dBd will be 2.15 dB lower than a gain expressed in dBi.