Section 2 Microwave Path Engineering Basics | TRACER 5045 System Manual |
|
|
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) |
| ||||
| 4f |
|
|
|
where f is in GHz and d is in miles.
Table 4 tabulates minimum antenna heights for given path lengths.
Table 4. Minimum Antenna Height for Given Path Lengths
Path Length | Min. Antenna Height |
(miles) | (ft) |
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 |
|
|
22 | © 2003 ADTRAN, Inc. |