Section 2 Microwave Path Engineering Basics | TRACER 4106/4206 System Manual |
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Antennas are also designed to radiate RF energy efficiently for a specific range of frequencies. Please consult the data sheet for your particular antenna make and model to ensure that it is specified to operate in the 2400 MHz to 2483.5 MHz frequency band for TRACER 4106 models, and the 5725 MHz to 5850 MHz frequency band for the TRACER 4206 system.
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
h = 72.1 | d | + 0.125d | 2 | (feet) |
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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 | Min. Antenna Height | Min. Antenna Height | |
@ 2.4 GHz | @ 5.8 GHz | ||
(miles) | |||
(ft) | (ft) | ||
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2 | 33 | 22 | |
4 | 48 | 32 | |
6 | 61 | 41 | |
8 | 73 | 50 | |
10 | 85 | 60 | |
14 | 111 | 81 | |
16 | 124 | 92 | |
18 | 138 | 104 | |
20 | 153 | 117 | |
22 | 169 | 131 | |
24 | 185 | 145 | |
26 | 202 | 161 | |
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22 | © 2004 ADTRAN, Inc. |
