Plain-language answer: Directivity describes how strongly an antenna concentrates radiation in a direction compared with an isotropic reference, without including dissipative efficiency.
Why it changes the result
It is pattern shape, not surviving transmitter power. Gain combines directivity with efficiency.
What to verify
Read the complete 3D pattern and reference, then identify lobes, nulls, beamwidth, polarization, and frequency.
The common mistake
Using directivity and gain as interchangeable numbers.
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.
