Table of Contents
Advertisement
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).
Table 1: Required Clearance Above Obstructions
Total Link Distance
402 m (0.25 mile)
805 m (0.5 mile)
1.6 km (1 mile)
3.2 km (2 miles)
4.8 km (3 miles)
6.4 km (4 miles)
8 km (5 miles)
11.3 km (7 miles)
14.5 km (9 miles)
19.3 km (12 miles)
24.1 km (15 miles)
27.4 km (17 miles)
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
2.4 m
20 m
Example
A wireless bridge link is deployed to connect building A to a building B, which is located
4.8 km (3 miles) 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 (66 ft), so a 2.3 m (7.5 ft) mast or pole must
be constructed on its roof to achieve the required antenna height. Building B is only
three stories high, or 9 m (30 ft), but is located at an elevation that is 12 m (39 ft) higher
Copyright © 2012, Juniper Networks, Inc.
Max Clearance for
60% of First Fresnel
Zone at 5.8 GHz
1.4 m (4.5 ft)
1.95 m (6.4 ft)
2.7 m (9 ft)
3.9 m (12.7 ft)
4.8 m (15.6 ft)
5.5 m (18 ft)
6.1 m (20 ft)
7.3 m (24 ft)
8.2 m (27 ft)
9.5 m (31 ft)
10.7 m(35 ft)
11.3 m(37 ft)
Visual Line of Sight
3 miles (4.8 km)
A
17 m
Approximate
Total Clearance
Clearance for Earth
Required at
Curvature
Mid-point of Link
0
1.4 m (4.5 ft)
0
1.95 m (6.4 ft)
0
2.7 m (9 ft)
0
3.9 m (12.7 ft)
0.5 m (1.8 ft)
5.3 m (17.4 ft)
1.0 m (3.2 ft)
6.5 m (21.2 ft)
1.5 m (5 ft)
7.6 m (25 ft)
3.0 m (9.8 ft)
10.3 m (33.8 ft)
4.9 m (16 ft)
13.1 m (43 ft)
8.8 m (29 ft)
18.3 m (60 ft)
13.7 m (45 ft)
24.4 m (80 ft)
17.7 m (58 ft)
29 m (95 ft)
Radio Line of Sight
5.4 m
Wireless Bridge Link Planning
B
1.4 m
9 m
12 m
3
Advertisement
Table of Contents
