Cisco AIR-BR1410A-A-K9 - Aironet 1410 Wireless Bridge Deployment Manual page 24

Avoid links that have obstructions in the Fresnel Zone
Figure 20.
To resolve Fresnel Zone issues raise the antenna or remove the
Figure 21.
obstruction
To improve a Fresnel zone obstruction you need to get above the obstruction which usually means
you need to get the antenna mounted higher. This may be a simple matter of mounting the antenna
at another point of the building such as an elevator room or other structure higher on the buildings
roof. If no such structure exists, consider using a taller mast pipe or determine if another nearby
building or structure would work better. If this Bridge is located on a radio tower you may want to
mount it higher on the tower or determine if more suitable tower exists. If a tree or other
obstruction is under your control perhaps you can remove it.
Technical jargon: It is possible to calculate the radius of the Fresnel zone (in feet) at any particular
distance along the path using this equation:
F1= 72.6 X SQR (Distance/4 X Frequency in GHz)
Where F1=the first Fresnel zone radius (ft.). D=total path length (mi.). f=frequency (GHz)
Note:
Fresnel zone, Free Space Loss, antenna gain, cable loss, data rate, link distance, transmitter
power, receiver sensitivity along with other variables that all play a role in determining how far your
bridge link will go.
Copyright © 2004 Cisco Systems, Inc. All rights reserved.
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