I noticed that in Link Planner, the subscriber module antenna uptilt that Link Planner recommended for some of my links seems excessive. For example, on a 900 MHz link between a subscriber module mounted 15 feet high, .5 mile away from an access point mounted 80 feet high, Link Planner recommended an uptilt of 18.5 degrees. I calculated the same link with Spat, which I used for years before Link Planner, and Splat said the elevation angle from the SM to the AP is 3.7588 degrees. There is a wooded area in front of the subscriber module that is about 130 feet deep, and then the path is clear the rest of the way to the AP. The only reason I could see tilting the antenna up 18.5 degrees would be to aim above the trees. What makes Link Planner recommend such an extreme uptilt? Should I go with the recommended 18.5 degree uptilt or something closer to 3 to 4 degrees?
The radio planning guy in me says 3 degrees - shoot through the trees and aim the center of radiation at the AP antenna. Link planner treats trees and building clutter as a wall, so I think its telling you to aim for the knife edge point of the tops of the trees. My guess is that if you vary the height of those trees, the uptilt recommendation will change along with it.
I can’t answer from an installation perspective, but Dave is correct and LINKPlanner treats all obstructions the same and tries to shoot over the obstructions using the knife-edge diffraction model (when clutter is disabled) or the Delta-Bullington diffraction model (when clutter is enabled).
The tilt angle is calculated based on the high points along the profile. The red line in the profile indicates the sight line that is used to calculate the tilt, when the direct line of sight is obstructed.