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3
Bridge Link Planning
Antenna Height
A reliable wireless link is usually best achieved by mounting the antennas at each
end high enough for a clear radio line of sight between them. The minimum height
required depends on the distance of the link, obstacles that may be in the path,
topology of the terrain, and the curvature of the earth (for links over 3 miles).
For long-distance links, a mast or pole may need to be contsructed to attain the
minimum required height. Use the following table to estimate the required minimum
clearance above the ground or path obstruction (for 5 GHz bridge links).
.
Total Link Distance
0.25 mile (402 m)
0.5 mile (805 m)
1 mile (1.6 km)
2 miles (3.2 km)
3 miles (4.8 km)
4 miles (6.4 km)
5 miles (8 km)
7 miles (11.3 km)
9 miles (14.5 km)
12 miles (19.3 km)
15 miles (24.1 km)
17 miles (27.4 km)
Note that to avoid any obstruction along the path, the height of the object must be
added to the minimum clearance required for a clear radio line-of-sight. Consider the
following simple example, illustrated in the figure below.
3-4
Max Clearance for
60% of First Fresnel
Zone at 5.8 GHz
4.5 ft (1.4 m)
6.4 ft (1.95 m)
9 ft (2.7 m)
12.7 ft (3.9 m)
15.6 ft (4.8 m)
18 ft (5.5 m)
20 ft (6.1 m)
24 ft (7.3 m)
27 ft (8.2 m)
31 ft (9.5 m)
35 ft (10.7 m)
37 ft (11.3 m)
Approximate
Clearance for
Earth Curvature
0
0
0
0
1.8 ft (0.5 m)
3.2 ft (1.0 m)
5 ft (1.5 m)
9.8 ft (3.0 m)
16 ft (4.9 m)
29 ft (8.8 m)
45 ft (13.7 m)
58 ft (17.7 m)
Total Clearance
Required at
Mid-point of Link
4.5 ft (1.4 m)
6.4 ft (1.95 m)
9 ft (2.7 m)
12.7 ft (3.9 m)
17.4 ft (5.3 m)
21.2 ft (6.5 m)
25 ft (7.6 m)
33.8 ft (10.3 m)
43 ft (13.1 m)
60 ft (18.3 m)
80 ft (24.4 m)
95 ft (29 m)
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