What is an Isotropic Antenna?

An isotropic antenna is a theoretical antenna that radiates equally in all directions - horizontally and vertically with the same intensity. The antenna has a gain of 1 (0 dB) in the spherical space all around it and has an efficiency of 100%. An isotropic antenna is used as a reference antenna to evaluate antenna gain.

Antenna Pattern of an Isotropic Antenna in the Vertical & Horizontal

Many antennas specify gain in dBi (decibels over isotropic), which is the power transmitted by an antenna in a specific direction, divided by the power transmitted by an isotropic antenna emitting the same total power.

3-D Antenna Pattern of an Isotropic Antenna