Free tool
Fresnel Zone & Link Calculator
Work out the Fresnel zone radius, required antenna height and free space path loss for a point to point wireless link, including earth curvature over long spans.
Line of sight is not enough
A radio link needs more than a clear straight line between the antennas. The signal travels through an elongated ellipse around that line called the Fresnel zone, and anything intruding into it reflects and cancels part of the signal. You can see both ends perfectly and still have a link that collapses whenever it rains.
The working rule is to keep at least sixty percent of the first Fresnel zone clear. The zone is widest at the midpoint and it grows with distance and shrinks with frequency, so a long 2.4 GHz hop needs far more clearance than a short 5.8 GHz one. A five kilometre link at 5.8 GHz needs roughly eight metres of clearance at the middle, which is a whole tree nobody accounted for.
Over spans beyond about ten kilometres the curvature of the earth starts stealing clearance of its own, and that is added here too. Trees also grow, so plan for what the obstruction will be in five years rather than what it is on survey day.