Fresnel Zone

A Fresnel zone is one of a series of concentric, ellipsoidal regions surrounding the straight line-of-sight path between two antennas. Radio waves do not travel in a pencil-thin beam; they spread outward and fill this three-dimensional elliptical volume. Objects that intrude into the zone, even when the antennas can technically see each other, scatter and phase-shift the signal and can sharply degrade the link.

The 60 percent clearance rule

The first Fresnel zone carries most of the signal energy, and the practical guideline is that at least 60 percent of it must stay clear of obstructions to avoid meaningful loss. If roughly 80 percent of the first zone is unobstructed, propagation loss approaches that of true free space. The zone is widest at the midpoint of the path and at lower frequencies, which is why a long LoRa link at 868 or 915 MHz can need surprising clearance over a ridge or treeline at its centre even when both endpoints sit high.

Planning around terrain

The nth-zone radius scales with the square root of wavelength times the distance product divided by total distance, so longer hops and lower frequencies demand more vertical room. This is why mesh-radio operators raise antennas above trees and rooflines rather than merely achieving bare visual sight of the far node.

Fresnel-zone clearance is a key input to path planning. See Link Budget for the full path accounting, Wavelength for how frequency sets the zone size, and Attenuation (RF) for obstruction loss.