User's Manual

Section 2 Microwave Path Engineering Basics TRACER 4108/4208 System Manual
22 © 2003 ADTRAN, Inc. 612804208L1-1B
Fresnel Zones, Earth Curvature, & Antenna Heights
Fresnel zones correspond to regions in the microwave path where reflections of the intended signal occur
and combine in both constructive and destructive manners with the main signal, thereby either enhancing
or reducing the net power at the receiver.
In general, the odd numbered Fresnel zones (1, 3, 5, ...) add constructively at the receiver, while the even
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 5 tabulates minimum antenna heights for given path lengths.
Table 5. Minimum Antenna Height for Given Path Lengths
Path Length
(miles)
Min. Antenna Height
(ft)
222
432
641
850
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
h72.1
d
4f
-----
0.125d
2
+=
(feet)