UpToWhereThe Fresnel zone
A radio link that has line of sight can still fail. The beam needs room around the straight line, and terrain that clears the line does not necessarily clear the room.
UpToWhere Studio includes the terrain-aware link check described on this page.
The first Fresnel zone is the ellipse of space around a radio path within which an obstruction changes the received signal. The working rule is that at least 60% of it must be clear of terrain, and a link that satisfies plain line of sight but not that rule will underperform for reasons that never show up on a map.
Why line of sight is not enough
Radio does not travel as a pencil line. Energy arriving at the far end has travelled by slightly different paths, and where those paths differ by half a wavelength they cancel rather than add. The region where that matters is an ellipse with the two antennas at its foci: the first Fresnel zone. Put terrain inside it and you take signal away, even when nothing is technically blocking the view.
The radius, and the criterion
The zone is widest at mid-path and pinches to nothing at each antenna. That shape is why an obstruction halfway along a link is far more damaging than the same obstruction 200 m from the mast, and why a 60% rule is a rule about a curve rather than a single number.
The formula
| Path length | F1 at 2.4 GHz | 60% of it | F1 at 5.8 GHz | 60% of it |
|---|---|---|---|---|
| 1 km | 5.6 m | 3.4 m | 3.6 m | 2.2 m |
| 5 km | 12.5 m | 7.5 m | 8.0 m | 4.8 m |
| 10 km | 17.7 m | 10.6 m | 11.4 m | 6.8 m |
| 20 km | 25.0 m | 15.0 m | 16.1 m | 9.6 m |
| 40 km | 35.3 m | 21.2 m | 22.7 m | 13.6 m |
Radius at mid-path, in metres. Note what the frequency does: the higher band needs a narrower corridor, which is the one respect in which 5 GHz is the easier link to clear. Note also what distance does: doubling the path only widens the zone by about 40%.
The atmosphere bends the beam, and the Earth gets in the way
Two more corrections separate a textbook answer from a usable one. The planet bulges up between the two ends, and the atmosphere curves the beam back down over it. ITU-R P.530 models the median refractive atmosphere as an effective Earth radius of 4/3, where the surveying convention for an optical sightline is about 1.15.
| Path length | Optical bulge (k = 1.15) | Radio bulge (k = 4/3) |
|---|---|---|
| 5 km | 0.4 m | 0.4 m |
| 10 km | 1.7 m | 1.5 m |
| 25 km | 10.7 m | 9.2 m |
| 50 km | 42.7 m | 36.8 m |
| 100 km | 170.7 m | 147.2 m |
Mid-path bulge, in metres. The difference is not cosmetic: running radio paths on the optical constant makes every long link read pessimistically, and the error is roughly 6 m of mast at 50 km and 24 m at 100 km.
What a terrain-aware check adds
The tight spot, not just a radius
A formula tells you how much room the beam needs. The terrain profile tells you where along the path the ground comes closest to needing it, which is the point any fix has to address.
The height that clears it
Given a blocked or marginal path, the useful output is not "blocked". It is the exact additional height, at either end, that brings the worst point back over the criterion, and the trade-off between raising one end and the other.
Every link at once
A single path check is a calculator. A network is a matrix: every pair tested, sorted worst first, with the ones that fail and the ones that only just pass separated from each other.
A margin that is about the data
Height advice here carries an allowance for the elevation model own error bar, shaped like the zone itself rather than added flat. Solving a mast to graze the terrain model is solving inside the noise.
What this does not do
The scope line is deliberate and stating it is part of being useful. This computes geometry: terrain, clearance, Fresnel, the height that fixes a path. It does not predict signal.
There is no received power, no link budget, no fade margin, no rain attenuation, no interference and no availability figure. Those depend on radios, antennas, cabling and licensing that a terrain model knows nothing about. If a tool offers you a dBm figure from a map alone, be careful about what it had to assume to get there.
Common questions
What is the Fresnel zone, in one sentence?
The ellipse of space around a radio path within which obstructions affect the signal, even when they do not block the direct line.
Why 60%?
It is the working convention: with at least 60% of the first zone clear, a path behaves essentially as it would in free space. Below that it degrades progressively, and at zero clearance, meaning a beam grazing the ground, you are already several dB down.
Does the second Fresnel zone matter?
For practical link planning, no. Obstructions in the even zones subtract and in the odd zones add, but the first zone carries most of the energy and clearing 60% of it is the criterion that planning practice is built on.
Which frequency should I use for the check?
The one the link will run on. The zone width scales with the inverse square root of frequency, so a 900 MHz path needs a corridor roughly 2.5 times wider than a 5.8 GHz one over the same distance. Checking at the wrong band is not conservative in a predictable direction.
Does rain or fog change the Fresnel zone?
No. Rain attenuates the signal, which is a link budget question, not a geometry one. The zone is set by wavelength and distance only.
Do trees count as an obstruction?
Yes, and this is where the elevation model matters. A surface model has canopy partly inside the terrain heights already; a bare-earth model has it stripped out and will report a path as clear that a stand of pines is sitting in. Which model was used is stated on every result.
Can this replace a path survey?
No. It narrows the list of paths worth surveying and tells you what to look for when you get there. A modelled result is not a survey, and it should be verified on site before anything is built on it.
Related
Check a real link against real terrain
UpToWhere Studio includes the terrain-aware link check described on this page.
Run this check on real terrain