Fresnel Zone

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, and energy arriving via slightly longer paths through the zone combines with the direct ray at the receiver. Objects that intrude into the zone — even when the antennas can technically "see" each other — scatter and phase-shift part of the signal, and the reflected components can arrive out of phase and cancel rather than reinforce. This is why a link with clean visual line of sight can still perform miserably: optics and radio disagree about what "clear" means.

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 its radius 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 — the ellipse bulges in the middle — and it grows 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 and proud. Obstructions that graze the zone edge cost a little; obstructions that block the direct path and much of the zone cost the link.

Sizing the zone

The nth-zone radius at any point along the path scales with the square root of (n × wavelength × d1 × d2 / total distance), where d1 and d2 are the distances to each end. Two consequences fall straight out of the formula. First, longer hops demand more vertical room: at 915 MHz, the first-zone midpoint radius is only a few metres on a 1 km path but grows past ten metres on a 10 km path — more than a rooftop mast may provide over intervening trees. Second, lower frequencies (longer wavelength) need proportionally more clearance than higher ones over the same span, one of the quiet trade-offs against LoRa's otherwise superb propagation. This is why mesh-radio operators raise antennas well above trees and rooflines rather than merely achieving bare visual sight of the far node, and why the best node sites are hilltops and towers rather than second-storey windows.

Ground reflection adds a subtler failure mode: a path skimming flat terrain or water can suffer cancellation from its own mirror image even with the zone nominally clear, which is why links over lakes sometimes behave worse than links over rough ground.

Planning around terrain

In practice you rarely compute this by hand: free path-profile tools overlay the first Fresnel zone on elevation data between two coordinates and show exactly where terrain or canopy bites into it. For a Meshtastic deployment across a valley or between homesteads, ten minutes with such a tool tells you whether a marginal path needs a taller mast, a relocated node, or an intermediate relay on the high ground — before anyone climbs anything. Remember too that the zone lives in three dimensions and changes seasonally: summer foliage that swells into a midpoint zone can take down a link that ran flawlessly all winter. Sovereign infrastructure means owning these details; the physics does not care who your ISP is.

Fresnel-zone clearance is a key input to path planning and one line item in the larger accounting of a link budget. See Attenuation (RF) for how obstruction loss combines with the other loss terms along the path.