Isotropic Radiator
Understanding Isotropic Radiator
The isotropic radiator is a mathematical construct that cannot physically exist but serves as a universal reference. Its radiation pattern is a perfect sphere with uniform power density at any distance. All real antennas have gain relative to an isotropic radiator in at least some directions.
The gain of an antenna in dBi is 10*log10 of the ratio of power density in the direction of maximum radiation to the power density from an isotropic radiator with the same total input power.
Frequently Asked Questions
Can an isotropic radiator be built?
No, it is a theoretical construct. A true isotropic radiator would violate electromagnetic boundary conditions. The closest practical approximation is a short dipole or small monopole.
Why use dBi instead of dBd?
dBi provides an absolute reference independent of any physical antenna, making it unambiguous. dBd references a half-wave dipole, which itself has 2.15 dBi gain.