Plain-language answer: An isotropic radiator is an ideal mathematical point source that radiates equally in every direction and defines the 0 dBi reference.
Why it changes the result
It is not buildable; it provides a consistent reference for comparing pattern and gain in three dimensions.
What to verify
Check whether a gain uses dBi or dBd before comparing antennas or calculating EIRP.
The common mistake
Imagining the isotropic reference as a real perfect antenna.
Complete-system boundary
Match the exact radiator, feed system, return path, frequency, height, orientation, and environment. Connector or thread fit does not establish electrical compatibility, structural capacity, or safe operation.
