An isotropic antenna is a theoretical antenna that radiates equally in all directions, with a radiation pattern that is completely symmetric around its axis. It is often used as a reference antenna in radio frequency (RF) engineering to compare the performance of other real antennas. In practice, isotropic antennas do not exist, but they provide a useful standard for measuring the gain and directivity of real antennas.
The isotropic antenna has a gain of 0 decibels (dB) in all directions, meaning that it radiates the same amount of power in all directions. By contrast, a real antenna typically has a directional pattern with higher gain in some directions and lower gain in others. The gain of a real antenna is expressed in decibels relative to the isotropic antenna, such as dBi (decibels relative to an isotropic radiator) or dBd (decibels relative to a dipole antenna).
Isotropic antennas are useful for comparing the performance of different antenna designs and for calculating the free-space path loss between two antennas. They are also used as a standard reference for measuring the effective isotropic radiated power (EIRP) of a transmitter, which takes into account the gain of the transmitting antenna and the power output of the transmitter.