Rf Propagation Planning; Fresnel Zone Clearance - GE MDS 4790M Installation And Operation Manual

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8.2 RF Propagation Planning

Establishing a reliable point-to-point radio link requires system planning and
design. It is helpful to have an understanding of the physical parameters
affecting propagation. The following material discusses these factors and will
assist you in designing a dependable transmission path for your radio link.
NOTE:
This section is intended for use as a guideline when planning trans-
mission paths. It does not consider all of the local conditions that
may be present, nor does it guarantee that adequate signal strength
will be obtained in a given system. There is no substitute for an
on-the-air test to verify the predicted path results, and to check the
overall operation of the radio system.
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
may 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 36). 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 may arrive at the receiver out of phase with
the desired signal and cause a cancelling effect.
LINE-OF-SIGHT (LOS)
STATION A
Distance D 1
Figure 36. 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.
MDS 4790M (MPT 1411) I/O Guide
Invisible place holder
FRESNEL ZONE
Distance D 2
F
OBSTRUCTION
MDS 05-3628A01, Rev. B
STATION B
NOT
OBSTRUCTING