Understanding Fresnel Zones
Fresnel zones are concentric ellipsoids of signal energy around the direct line-of-sight between transmitter and receiver. Obstructions within the first Fresnel zone cause significant signal degradation.
r_n = sqrt( n * lambda * d1 * d2 / (d1 + d2) )
For reliable links, at least 60% of the first Fresnel zone should be clear of obstructions. At microwave frequencies, this can be several meters in radius even for short links.