User's Manual

Hardware Installation Guide Radio Site Planning
02030311-10 Airspan Networks Inc. 5-3
Fresnel Zones define the amount of clearance required from obstacles. These zones
are composed of concentric ellipsoid areas surrounding the straight-line path
between two antennas. Thus, the zone affects objects to the side of the path and
those directly in the path. The first Fresnel Zone is the surface containing every
point for which the distance from the transmitter to any reflection point on the
surface point and then onto the receiver is one-half wavelength longer than the direct
signal path.
One method for clearing the Fresnel Zone (to use the free space model to calculate
link budget – see Section 5.6, "Calculating Link Budget") is by increasing the
antenna height.
The first Fresnel Zone radius is calculated by the following equation:
Where f is the frequency (in MHz) and d is the distance (in meters).
For example, using the formula above, a link of 4 km at 700 MHz produces a first
Fresnel Zone radius clearance of about 20 meters. This implies that to ensure the
ground does not enter into the first Fresnel Zone, both antennas (i.e. at Base Station
and subscriber) must be mounted at least 20 meters above ground level (or clutter
level). Typically, at least 60% clearance of the first Fresnel Zone is considered as
LOS. Therefore, in the above example, a height of at least 12 meters (i.e. 60% of 20
meters) above ground level is sufficient for LOS.