Plain-language answer: dBi expresses antenna gain relative to an ideal isotropic radiator in a specified direction.
Why the definition matters
It combines directivity and efficiency into a referenced gain value, but says nothing by itself about polarization, pattern shape, bandwidth, mounting, or installed loss.
Use the number correctly
Compare dBi values only at the same frequency, polarization, direction, and installed boundary, with the complete radiation pattern available.
The common mistake
Treating peak dBi as equal improvement in every direction.
Measurement and safety boundary
Keep reference units, bandwidth, detector, direction, averaging, and test conditions attached to every number. Current regulator requirements and the complete installed system control RF-exposure compliance.
