The APs must be positioned as instructed below:
- With hardware that the wind and ambient vibrations cannot flex or move.
- Where a tower or rooftop is available or can be erected.
- Where a grounding system is available.
- With lightning arrestors to transport lightning strikes away from equipment.
- At a proper height, such as:
- Higher than the tallest points of objects immediately around them (such as trees, buildings, and tower legs)
- At least 2 feet (0.6 meters) below the tallest point on the tower, pole, or roof (for lightning protection).
- Away from high-RF energy sites (such as AM or FM stations, high-powered antennas, and live AM radio towers).
- In line-of-sight paths:
- To the SMs.
- That is not obstructed by trees as they grow or structures that are later built.
NOTE: Visual line of sight does not guarantee radio line of sight.
Surveying Sites
Factors to survey at potential sites include the following:
- What pre-existing wireless equipment exists at the site. (Perform spectrum analysis.)
- Whether available mounting positions exist near the lowest elevation that satisfies line of site, coverage, and other link criteria.
- Whether you will always have the right to decide who climbs the tower to install and maintain your equipment, and whether that person or company can climb at any hour of any day.
- Whether you will have collaborative rights and veto power to prevent interference to your equipment from wireless equipment that is installed at the site in the future.
- Whether a pre-existing grounding system (path to Protective Earth ) exists and what is required to establish a path to it.
- Who is permitted to run any indoor lengths of cable.
Clearing the Radio Horizon
Because the surface of the earth is curved, higher module elevations are required for greater link distances. This effect can be critical to link connectivity in link spans that are greater than 8 miles (12 km).
To use metric units to find the minimum height required to reach the radio horizon use the following equation:
Radio horizon distance (km) = 4.12 (SQRT(h1) + SQRT(h2))
Where:
h1 is the height of the AP
h2 is the height of the SM
To use English standard units to find the angle of elevation, use the following formula:
Radio horizon distance (km) = 1.42 (SQRT(h1) + SQRT(h2))
Where:
h1 is the height of the AP
h2 is the height of the SM
Calculating the Aim Angles
The proper angle of tilt can be calculated as a factor of both the difference in elevation and the distance that the link spans. Even in this case, a plumb line and a protractor can be helpful to ensure the proper tilt. This tilt is typically minimal.
The number of degrees to offset (from vertical) the mounting hardware leg of the support tube is equal to the angle of elevation from the lower module to the higher module (<B in the example provided in Figure 18).
Calculating the Angle of Elevation
To use metric units to find the angle of elevation, use the following formula:
The angle of depression from the higher module is identical to the angle of elevation from the lower module.