Calculate the Fresnel zone radius and clearance needed for a line-of-sight wireless link.
The Fresnel zone is an ellipsoid-shaped region around the direct line-of-sight path between two antennas; even without a solid obstruction blocking the direct path, an obstacle intruding into this zone can cause destructive signal interference through diffraction. The first Fresnel zone radius at any point along the path is radius = 17.3 × √(d1×d2 / (f×d_total)), with d1, d2 in km and f in GHz. As a rule of thumb, at least 60% of the first Fresnel zone should remain clear of obstructions for a reliable link.
First Fresnel zone radius
radius(m) = 17.3 × √(d1 × d2 / (f × (d1 + d2)))
The radius formula's d1×d2 term is maximized when d1 equals d2 (the midpoint), since for a fixed sum (d1+d2), the product of two numbers is largest when they're equal — so obstructions near the middle of a link need the most vertical clearance.
The radius formula has frequency in the denominator inside the square root, so as frequency increases, the required clearance radius shrinks — this is one reason higher-frequency microwave links can sometimes tolerate obstacles that would block a lower-frequency link.
Signal energy contributing to constructive interference is concentrated mostly within the innermost part of the first Fresnel zone; keeping at least 60% of the zone's radius clear captures the great majority of that useful signal energy while allowing some practical tolerance for towers/terrain.
It's necessary but not sufficient — the link also needs unobstructed line-of-sight itself, adequate signal power (link budget), and freedom from interference on the same frequency. Fresnel zone clearance specifically addresses diffraction loss from near-path obstacles.