Cabletron Systems 802.11 Manuel D’Utilisation

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Site Preparation Guide
3
Antenna height requirements (mast):
At least 5 feet (1.5 meters) above the roof line if you are mounting it on a roof.
High enough to achieve a line of sight if you are mounting it on the wall of a 
building.
Note: The installer is responsible for local building codes.
Line of sight is defined as:
No obstacles in the direct path between the two antennas
No obstacles within a defined radius around the antenna beam
Clear of neighboring buildings, trees, power lines, and other obstructions