The omnidirectional antenna is an isotropic antenna which radiates uniformly in all directions. Since an ideal spherical radiator cannot be realized in practice, the isotropic spherical radiator is used as a theoretical reference model for the calculation of the field strength and the antenna gain.
The isotropic omnidirectional antenna is a lossless antenna that radiates the magnetic waves as spherical waves uniformly in all directions. The level of an isotropic omnidirectional antenna is expressed in decibels isotropic( dBi) and the reference level of 0 dBi is used to calculate the antenna gain. The energy field radiated by an isotropic omnidirectional antenna is independent of the radiation angle and depends only on the distance to the antenna. It is calculated from the radiated power (Pt) of the antenna divided by the sphere surface at radius (r), `(Pt)/(4Pi) * r^2`.
In the isotropic omnidirectional radiator, the horizontal radiation pattern corresponds to the vertical one. Since the radiation is the same in all directions, there is no maximum with such a radiation pattern. All other antennas using the omnidirectional radiator as reference show a maximum in one direction or one plane, which causes the antenna gain compared to the omnidirectional radiator.
Inacoustics, the term omnidirectional is used for spherical loudspeakers.