Adtran TRC3202 用户手册
TRACER 2210/3202 System Manual
Section 2, Microwave Path Engineering Basics
61280012L1-1A
© 2002 ADTRAN, Inc.
21
Fresnel Zones, Earth Curvature, & Antenna Heights
The 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, and must be at least 60% 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 curvature is given by
where f is in GHz and d is in miles.
Table 2 tabulates minimum antenna heights for given path lengths.
Table 2. Minimum Antenna Height for Given Path Lengths
Path Length
(miles)
Min. Antenna Height
(ft)
2
22
4
32
6
41
8
50
10
60
12
70
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)