Fresnel Zone

When doing wireless links you need to analysis the path between the two radios. The path must have a clear Fresnel zone–a cigar-shaped region all around the optical path.

‘Line of Sight’ is not nearly enough, there needs to be an open region all around the view.


In the above graph, the yellow line is the visual line (LOS), meaning you should be able to see between the two sites. Unfortunately in wireless that is not quiet enough to guarantee a good signal link.

The Purple line shows where the signal travels (Fresnel Zone) and the blue line is the ‘60% Fresnel zone. There should be nothing within that line (between it and the yellow line). That also includes trees and other obstructions that do not show on the graph. The green solid area is the shape of the land.

In the example picture above it would be needed to raise one or both ends so
that the ‘60% Fresnel zone is clear. Remember to count into the equation any trees/houses/Obstacles residing in this zone as well as the terrain shown.

Even with poor fresnel, it is possible to make a link, it depends on your expectations. Usually speed is the main thing reduced.

Good example of issues when beaming 5.8GHz through trees

Unlike wire line and optical communication, nearby all objects can affect an RF signal.

Any objects within the fresnel will either scatter, delay, attenuate or otherwise affect the RF that passes nearby.

The affect is different for different polarizations. Horizontal may be less affected than Vertical, or it could be the other way around.

The bottom line is that once there is Fresnel Zone incursion, there is no reliable way to predict what will happen to the RF.

Here’s a link with more information, if you’d like to learn……#x33-1220006.2

how much path (area) from side to side of LOS and Frensel Zone do you require for a link?

The Fresnel Zone is 3-dimensional. You need the same clearance from “left to right” as you need from “top to bottom”.

If you do not allow “left to right” space you will get the keyhole effect. The Fresnel Zone has to be clear on both azimuth points and down to ground elevation.





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