The change in signal strength caused by grounding the antenna
The gain of the antenna with respect to a dipole at ground level
To force net gain to 0 dB by grounding part of the antenna
An increase in signal strength from ground reflections in the environment of the antenna✓Bonne réponse
Explication
Energy leaving the antenna downward is reflected by the ground and, at low elevation angles, arrives in phase with the direct wave, adding to it. That reinforcement can raise low-angle signal strength by several decibels over what the same antenna would produce in free space, and it is what the term describes.