Isotropic Antenna
An isotropic antenna is a theoretical point source that radiates the power fed to it completely equally in every direction in space. It cannot be built physically, but it is the common reference for all antenna gains and for the EIRP calculation. The letter "i" at the end of the unit dBi indicates this reference, and its gain is taken as 0 dBi.
To be able to measure antenna gain you need a zero point. The isotropic source provides that zero: it distributes power equally over the surface of a sphere, so there is neither gain nor loss in any direction.
- The free space path loss formula also derives from this model; the energy spreading over the surface of a sphere gives the dilution with the square of distance.
- EIRP is the answer to the question "how many watts would have to be fed to an isotropic antenna to produce the same field strength".
- Real antennas necessarily produce a pattern; a half-wave dipole has 2.15 dBi of gain relative to the isotropic reference.
Its practical benefit is this: you can compare antennas of different geometry from different manufacturers on a single scale. If you see dBi in a catalogue, this theoretical reference is behind it and the values can be written straight into a link budget.
Context where this term is used: 433 MHz or 2.4 GHz?