Справочник Пользователя для Adtran TRC4108

Section 2  Microwave Path Engineering Basics

TRACER 4108/4208 System Manual

 22

© 2003 ADTRAN, Inc.

612804208L1-1B

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.

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

(feet)