Fresnel Zone

Radio waves spread through a volume around the straight line from transmitter to receiver. Fresnel zones are nested ellipsoids in which the path-length difference between the direct path and reflected or diffracted paths equals multiples of half a wavelength. Most of the signal energy travels within the first Fresnel zone, so link design is based on that zone.

The radius of the first Fresnel zone at any point is r = 17.32 · √(d1 · d2 / (f · D)), where d1 and d2 are the distances from that point to each end (km), D is the total path length (km), f is the frequency (GHz) and r is in metres. The zone is widest at mid-path: r = 8.66 · √(D / f). For a 10 km link at 5.8 GHz, the mid-path radius is about 11.4 metres.

In practice at least 60% of the first Fresnel zone should be clear, and more conservative designs aim for 80% or more. Hills, buildings, trees or roof edges intruding into the zone cause diffraction loss and signal fluctuation through reflections (multipath). Earth curvature must also be considered on long links. Under standard atmospheric conditions (k = 4/3) the ground bulges about 1.5 metres at the midpoint of a 10 km path.

The zone narrows as frequency rises, so a 60 GHz link can be built with lower masts than a 5 GHz link over the same distance, at the cost of greater rain fade. Seeing the far antenna with the naked eye (optical LOS) does not mean the Fresnel zone is clear. Antenna heights should be set from a path profile derived from terrain and surface models.