100
MDS 4790/9790 Series I/O Guide
MDS 05-3438A01, Rev. F
To ensure a highly reliable path, a line of sight between both ends of the
link is desirable. For short paths (up to 5 kilometers/3.1 miles), some
obstructions might be acceptable, but the performance of a blocked path
is always less predictable than a clear path.
Fresnel Zone Clearance
As the distance spanned by a link gets longer, it is necessary to have
more than just a grazing path between the two ends; the path must clear
the ground or other obstacles by some percentage of a Fresnel zone.
The Fresnel zone corresponds to the width or girth of the radio signal.
There are first, second, and third Fresnel zones, but the first
zone is the
only one that has substantial effects on signal strength.
The first Fresnel zone can be visualized as an oval-shaped volume
between two station antennas (
Figure 49
). As the width of the radio
wave front gets blocked by obstructions, less of the signal can get to the
receiver antenna.
In addition to blocking the signal, obstructions in the first Fresnel zone
may also cause multipath interference due to reflective and refractive
signal paths. The reflected or refracted signal might arrive at the receiver
out of phase with the desired signal and cause a cancelling effect.
Invisible place holder
Figure 49. Fresnel Zone Obstructions
As a matter of practice, 60 percent of the first Fresnel zone must be clear
of obstructions (0.6 x F) to allow a clear, unobstructed RF path.
Remember, the first Fresnel zone calculation is only one parameter
determining path quality.
Earth Curvature
As the distance of a communication link increases, the clearance
problem is compounded by the earth’s curvature. Radio waves traveling
through typical atmospheric conditions bend slightly, which is repre-
sented by treating the earth as though it were slightly flatter than it actu-
ally is. Experience has shown that if we consider the earth’s radius
4/3rds of its actual size, we get good agreement between theory and
measured propagation results.
FRESNEL ZONE
STATION A
STATION B
NOT
OBSTRUCTING
OBSTRUCTION
Distance D2
Distance D1
LINE-OF-SIGHT (LOS)
F
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