ADTRAN TRACER 5045 - Manual (Page 21)

TRACER 5045 System Manual Section 2 Microwave Path Engineering Basics
612805045L1-1B © 2004 ADTRAN, Inc. 21
numbered Fresnel zones (2, 4, 6, ...) add destructively at the receiver.
The first Fresnel zone corresponds to the main lobe, 60% of which must be free of physical obstructions
for the path calculations to be valid. Since the main lobe contains the vast majority of the microwave
energy, this zone is typically used to determine proper antenna heights when placing antennas on towers or
buildings.
The curvature of the Earth becomes a legitimate obstruction for path lengths of 7 miles or greater, and must
also be accounted for when determining minimum antenna heights.
The aggregate expression for minimum antenna height that incorporates both the 60% first Fresnel zone
and the Earth’s curvature is given by
where f is in GHz and d is in miles.
Table 4 tabulates minimum antenna heights for given path lengths.
Table 4. Minimum Antenna Height for Given Path Lengths
Path Length
(miles)
Min. Antenna Height
(ft)
2 22
4 32
6 41
8 50
10 60
14 81
16 92
18 104
20 117
22 131
24 145
26 161
28 177
30 194
32 213
34 232
36 252
h 72.1 d
4f
----- 0.125d
2
+ =
(feet)