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The Fresnel Zone (pronounced FRA-nel Zone) is the
area around the visual line-of-sight that radio waves spread out into after
they leave the antenna. You want a clear line of sight to maintain signal strength,
especially for 2.4 GHz wireless systems. This is because 2.4 GHz waves are absorbed
by water, like the water found in trees.
Typically, 20% Fresnel Zone blockage introduces little signal
loss to the link. Beyond 40% blockage, signal loss will become significant.
The radius of the Fresnel Zone at its widest point can be
calculated by the formula listed below, where d is the link
distance in kilometers or miles, f is the frequency in GHz,
and r is the radius off of the center line of the link in feet.
All fields marked with an asterisks (*) are required.
Notes:
- The final value represents a first order approximation and should only be
used as a guide. No guarantees or warranties are implied accordingly.
- This calculation is based on a flat earth. It does not take the curvature
of the earth into consideration. The effect of this is to budge the earth
in the middle of the link. It is recommended for long links to have a microwave
path analysis done that takes this and the topography of the terrain into
account.
