Figure 25: Typical Obstructions in a Fresnel Zone, on page 64
illustrates an obstructed Fresnel zone.
Figure 25: Typical Obstructions in a Fresnel Zone
It is possible to calculate the radius of the Fresnel zone (in feet) at any particular distance along the path using
the following equation:
F1 = 72.6 X square root (d/4 x f)
where
F1 = the first Fresnel zone radius in feet
D = total path length in miles
F = frequency (GHz)
Normally, 60 percent of the first Fresnel zone clearance is recommended, so the above formula for 60 percent
Fresnel zone clearance can be expressed as follows:
0.60 F1= 43.3 x square root (d/4 x f)
These calculations are based on a flat terrain.
Figure 26: Removing Obstructions in a Fresnel Zone, on page 64
shows the removal of an obstruction in the
Fresnel zone of the wireless signal.
Figure 26: Removing Obstructions in a Fresnel Zone
Fresnel Zone Size in Wireless Mesh Deployments
To give an approximation of size of the maximum Fresnel zone to be considered, at a possible minimum
frequency of 4.9 GHz, the minimum value changes depending on the regulatory domain. The minimum figure
quoted is a possible band allocated for public safety in the USA, and a maximum distance of one mile gives
a Fresnel zone of clearance requirement of 9.78 ft = 43.3 x SQR(1/(4*4.9)). This clearance is relatively easy
to achieve in most situations. In most deployments, distances are expected to be less than one mile, and the
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Site Preparation and Planning
Fresnel Zone Size in Wireless Mesh Deployments
