There are no absolute requirement but some guideline. General rule is that the further distance between object/ground plane and antenna the better. If you have to “fold” the antenna toward the ground plane, a distance equal to 1/16 of the wave length will keep you safe. Only small adjustment in antenna length needed to retune the effect of ground lines.
Last Update: August 8, 2017
January 11, 2017 2107 Antenna Q&A