Compex Systems 802.11N - Manual (Page 16)

Antenna Alignment
The physical environment of the antenna must be examined when
aligning the antenna. Obstructions, available mounting locations,
and other factors must be considered. Many objects such as forests,
buildings, and hills, can obstruct the antenna, reducing the signal
strength. The antenna can be installed at a height above such
obstructions, and aligned so that antennas are directed at each
other by taking into account the horizontal angle and the vertical
angle of the antenna signal.
When the antenna is at the optimum alignment, there is less
possibility of encountering interference and of causing interference
to anyone else, and strong signal strength can be maintained.
NOTE
When the antennas are at the same height, it is quite simple to align the
antennas. However, when the antennas are at different heights, greater care
has to be taken to ensure that the antennas are properly aligned.