Field Guide · term

Also known as: Fresnel zone, first Fresnel zone

A Fresnel zone is one of a family of concentric ellipsoids surrounding the straight line between a transmitter and receiver; the first Fresnel zone is the innermost, and it must stay largely clear of obstacles for a link to behave as clean line-of-sight.1 Even when the direct ray grazes over an obstruction, an object intruding into this zone diffracts energy and adds loss. Fresnel-zone clearance is the practical criterion that turns “we can see the far antenna” into “the link will actually close.”

TX RX first Fresnel zone intrusion → loss
The first Fresnel zone is an ellipsoid around the direct ray; an obstacle poking into it causes diffraction loss even with visual line of sight.

How it works

The zones are defined by path-length difference. The n-th Fresnel zone is the set of points for which a wave scattered from that point travels n·λ/2 farther than the direct ray, where λ is the wavelength. Contributions from within the first zone (0 to λ/2 extra path) arrive roughly in phase with the direct ray and add constructively; the second zone tends to cancel, and so on. Because the first zone carries most of the useful energy, it is the one that matters for clearance.

Its radius is largest at the path midpoint:

  • r ≈ √(λ·d₁·d₂ / d), where d₁ and d₂ are the distances from each endpoint to the point of interest and d = d₁ + d₂ is the total path length.

Two practical facts follow. The zone is wider at lower frequencies (longer λ), so VHF links demand more clearance than microwave ones over the same distance. And engineers apply the 60% rule: keeping about 60% of the first zone free of obstruction yields essentially free-space loss, so links are planned with antenna heights (and terrain allowance for refraction) that meet it.

Relevance to SDR

Fresnel-zone clearance explains reception that visual line-of-sight alone does not. A scanner may “see” a distant trunking site tower yet receive a weak, fady signal because a ridge or building row cuts into the first Fresnel zone, adding knife-edge diffraction loss. Raising the antenna a few metres can lift the zone clear of the obstruction and dramatically improve the signal — often more than adding gain would. Clearance also preserves a high Rician K-factor by keeping the dominant direct ray strong relative to scatter.

For microwave point-to-point backhaul (the links that connect simulcast and multisite infrastructure), Fresnel clearance is a hard design requirement: planners survey the terrain profile and set tower heights so the first zone stays open across the whole hop. GopherTrunk is a receiver and does no path planning, but the concept directly informs where to place a scanner antenna for the best capture.

In practice

The zone is why “just barely line-of-sight” links are unreliable: at grazing incidence half the first zone is blocked, costing roughly 6 dB versus free space, and any further intrusion drops the signal into the diffraction region analysed by the knife-edge model. Clear the zone and the link behaves as free-space path loss predicts.

Sources

  1. Fresnel zone — Wikipedia, on the elliptical clearance regions, the radius formula, and the 60% clearance rule. ↩

See also