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Fresnel Zone
Fresnel zone is an elliptical area immediately surrounding the visual path. It varies depending on the length of the signal path and
the frequency of the signal. The Fresnel zone can be calculated, and it must be taken into account when designing a wireless link.
Figure 6 Fresnel Zone
Raise Antennas
Based on both line of sight and Fresnel zone requirements, the following table provides a guideline on height requirements for
antennas as various distances. This refers to height above any obstacles located in the middle of the RF path.
Table 3
Wireless Link Distance
(miles)
1
5
10
15
20
25
Cisco.com provides an Outdoor Bridge Range Calculation Utility that calculates the Fresnel zone and maximum range based upon
cable types and lengths; transmitter and receiver models; and antennas. The utility can be found at:
www.cisco.com/go/aironet/calculation.
Note that as path loss increases and distance decreases with frequency, these distances are only applicable to the 2.4 GHz band. A
10dB fade margin is included for dependable communications in all weather conditions. The distances given are only theoretical
and should only be used to determine the feasibility of a particular design.
Outdoors, every increase of 6 dB will double the distance. Every decrease of 6 dB will halve the distance. Shorter cable runs and
higher gain antennas can make a significant difference to the range.
All contents are Copyright © 2003 Cisco Systems, Inc. All rights reserved. Important Notices and Privacy Statement.
Approx. Value "F" (60%
Fresnel Zone) ft. at 2.4 GHz
10
30
44
55
65
72
Cisco Systems, Inc.
Page 11 of 29
Fresnel Zone
Approx. Value "C"
(Earth Curvature)
3
5
13
28
50
78
Value "H" (mounting Ht.)
ft. with no obstructions
13
35
57
83
115
150
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