Yagi antennas and signal strength
We are using yagi antennas, but the terrain has substantial differences in elevation and is also covered with many trees. Will these obstructions impede our signals? Do we need a direct line of sight from antennas? Is there any thing we can do to increase our signal strength?
I think in your case, more than increasing signal strength, you need to properly plan your antennas without much disturbance from trees and elevations. Make sure you do a site survey, a signal quality, and a link test very carefully. If it still doesn't help, then you may want to increase the signal strength.
I also recommend that you read these articles by Lisa Phifer:
Understanding wireless antennas - part 1
Understanding wireless antennas - part 2
This was first published in April 2003