User's Manual

MDS 05-6399A01, Rev. D MDS Master Station 87
8 TECHNICAL REFERENCE DATA
8.1 RF Propagation Planning
Establishing a reliable point-to-point radio link requires system planning and design. You should 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 transmission 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 might be acceptable, but the performance of a blocked
path is always less predictable than a clear path.
8.1.1 Fresnel Zone Clearance
As the distance spanned by a link gets longer, it is necessary to have more than just a grazing path be-
tween 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
8-1 ). 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 inter-
ference 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 canceling effect.
Figure 8-1 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 al-
low a clear, unobstructed RF path.
Remember, the first Fresnel zone calculation is only one parameter determining path quality.