The optimal mounting height for transmit and receive antennas in telecommunications links is determined by the Fresnel zones. The energy propagating between the antennas takes the shape of a spindle in space, with its widest point at the midpoint between them. The antenna mounting locations must be chosen so that no objects that attenuate radio waves (tree canopies, building roofs, hills, or the ground surface) fall within this zone. Otherwise, some of the energy emitted toward the receiving antenna will be irretrievably lost.
There are many Fresnel zone calculators available online that allow you to calculate the spindle cross-section at any point along the path between the antennas. This information, combined with the height of a potential obstacle, allows you to determine how tall a mast or tower should be to avoid energy losses in the link.