(7) Step 7 - Connect the dots to establish the required Fresnel zone clearance area for
the radio path between transmit and receive stations.
(8) Step 8 - Find any points of terrain inside the required clearance area.
d. First Fresnel zone exercise. This exercise shows how to determine first Fresnel zone
clearance, given:
d1 = 16 kilometer
d2 = 24 kilometers
f = 4 GHz
D = 40 kilometers
(1) First, multiply d1 by d2 (in this example, multiply 16 by 24, which equals 384).
Next, multiply f by D (multiply 4 by 40 which equals 160). Next, divide 384 by 160, which
equals 2.4. Then, determine the square route of 2.4, which is 1.549 or 1.55. Finally, multiply
1.55 by either 17.5 (determine the Fresnel zone in meters or by 72.1 to determine the Fresnel
zone in feet). 17.5 X 1.55 equals 27.125 or about 27 meters (C = 27 meters). 72.1 X 1.55 equals
111.755 or about 112 feet (C = 112 feet).
(2) Figure 2-10 illustrates plotting the Fresnel zone on nonlinear graph paper, where
d1 = 16 kilometer; d2 = 24 kilometers; f = 4 GHz; and D = 40 kilometers. In the illustration, at
the 16-kilometer distance mark, a line is drawn at a right angle to the straight line drawn between
points A and B (Step 2, LOS procedures for SHF microwave radio systems).
(3) Using the formula, the required Fresnel zone clearance at the 16-kilometer
distance has been determined to be approximately 27 meters (Step 3). Using the elevation scale
on the left margin of the graph paper for reference, a dot is placed on the line signifying the
required Fresnel zone clearance for that distance (Step 4).
e. As in VHF/UHF procedures, if LOS and first Fresnel zone clearance cannot be
obtained, communications could be degraded. To obtain first Fresnel zone clearance, the
required clearance. The alternative is to relocate one or both of the radio sites to achieve the
necessary results.
5.
Summary. In this lesson, you learned about the planning considerations for line-of-sight
(LOS) engineering, to include determining antenna heights for transmit/receive sites and
developing a manual LOS profile.
SS0121
2-14
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