Plain-language answer: EIRP combines transmitter power, feed-system loss, and antenna gain referenced to an isotropic radiator in the stated direction.
Why it changes the result
It is directional and reference-dependent. Regulations may limit EIRP rather than transmitter output.
What to verify
Express terms in decibels, use dBi for antenna gain, and calculate from accepted power at a defined reference point.
The common mistake
Mixing dBd and dBi or ignoring feed-system loss.
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.
