User's Manual
Wireless Bridge Link Planning 6 - 3
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 constructed 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).
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.
Figure 6–2. Establishing a Clear Line-of-Sight
Example
A wireless bridge link is deployed to connect building A to a building B, which is located three
miles (4.8 km) away. Midway between the two buildings is a small tree-covered hill. From the
above table it can be seen that for a three-mile link, the object clearance required at the mid-point
is 5.3 m (17.4 ft). The treetops on the hill are at an elevation of 17 m (56 ft), so the antennas at
each end of the link need to be at least 22.3 m (73 ft) high. Building A is six stories high, or 20 m
Table 6– 2. Required Clearance Above Obstructions
Total Link Distance
Max Clearance for
60% of First Fresnel
Zone at 5.8 GHz
Approximate
Clearance for
Earth Curvature
Total Clearance
Required at
Mid-point of Link
0.25 mile (402 m) 4.5 ft (1.4 m) 0 4.5 ft (1.4 m)
0.5 mile (805 m) 6.4 ft (1.95 m) 0 6.4 ft (1.95 m)
1 mile (1.6 km) 9 ft (2.7 m) 0 9 ft (2.7 m)
2 miles (3.2 km) 12.7 ft (3.9 m) 0 12.7 ft (3.9 m)
3 miles (4.8 km) 15.6 ft (4.8 m) 1.8 ft (0.5 m) 17.4 ft (5.3 m)
4 miles (6.4 km) 18 ft (5.5 m) 3.2 ft (1.0 m) 21.2 ft (6.5 m)
5 miles (8 km) 20 ft (6.1 m) 5 ft (1.5 m) 25 ft (7.6 m)
7 miles (11.3 km) 24 ft (7.3 m) 9.8 ft (3.0 m) 33.8 ft (10.3 m)
9 miles (14.5 km) 27 ft (8.2 m) 16 ft (4.9 m) 43 ft (13.1 m)
12 miles (19.3 km) 31 ft (9.5 m) 29 ft (8.8 m) 60 ft (18.3 m)
15 miles (24.1 km) 35 ft (10.7 m) 45 ft (13.7 m) 80 ft (24.4 m)
17 miles (27.4 km) 37 ft (11.3 m) 58 ft (17.7 m) 95 ft (29 m)
A
B
3 miles (4.8 km)
5.4 m
17 m
20 m
2.4 m
12 m
9m
1.4 m
Visual Line of Sight
Radio Line of Sight