User's Manual

24 Link CX User Manual, Version A
Link CXs on each end of the radio link will be unable to communicate with each other. Make
sure the Link CX radios used in each radio link are within line of sight of each other. Refer
also to the Fresnel Zone Clearance section that follows.
• Fresnel Zone Clearance: There must be sufficient open space around the direct line of
sight to minimize interference with the radio beam. At a minimum, 60% of the first Fresnel
zone of the path should be clear.
Even with clear line-of-sight, objects still may be near enough to the transmission path to
cause problems. Obviously, objects that stand directly in the transmission path obstruct the
beam, causing a drop in signal strength at the receiving end; in addition, objects and
reflective surfaces that are in near proximity to the path can cause signal interference and
attenuation of the received signal.
Fresnel zones define the amount of clearance required for obstacles. These zones are series
of concentric ellipsoid surfaces that surround the straight-line path between the two
antennas. The first Fresnel zone is defined as the surface containing every point for which
the distance from the transmitter to any reflection point on the surface and then on to the
receiver is one-half wavelength longer than the direct signal path.
The following equation shows that Fresnel zones are a function of the transmission
frequency, path length, and location along the path:
Where:
Fl = First Fresnel zone radius in meters
d1= Distance from transmitter to reflection point in kilometers
d2 = Distance from reflection point to receiver in kilometers
D = Length of direct signal path in kilometers
f = Transmission frequency in GHz.
An envelope at six-tenths of the first Fresnel zone (referred to simply as the Fresnel Zone
Clearance) defines the minimum acceptable clearance of an obstacle (see Figure 3-1). The
formula for the Fresnel Zone Clearance is shown after Figure 3-1.
F1 17.3
d1d2
fD
------------ Fresnel Zones formula=