|There are times when equipment specifications for wireless radios indicate that the combination of power output and antenna type provide coverage for many miles outdoors. Outdoor signal coverage is, however, dependent on more than the equipment installed at either end of a point-to-point link. To create an outdoor point-to-point or point-to-multipoint link requires no obstructions exist between the antennas that would cause degradation of the signal. The biggest obstruction is the Earth itself. Since the Earth is curved, the further apart two radios are the more the curve of the Earth becomes an issue.
Outdoor antennas must often be installed on towers to raise them high enough to avoid the natural curvature of the Earth. A cylindrical volume of space must be unobstructed around the direct line-of-sight between the two antennas. This cylindrical volume is equal to 60% of the radius of a larger cylinder commonly referred to as the Fresnel Zone. (Technically, it's the "First Fresnel Zone", because there are other zones that exist, but these other zones don't have a direct impact on the determination of antenna height).
Be sure to explore the Connect802 Antenna System Designer, an on-line RF calculator for link budget, path loss, near-field, antenna tower and mounting heights, earth bulge, refractive index, and more!
To determine the required height of an antenna tower over a hypothetically flat, spherical Earth, it's necessary to calculate the effect of the Earth's curvature, and then add a distance equal to 60% of the radius of the First Fresnel Zone. The table below shows the result of these calculations. Values have been rounded up to the nearest foot.
It can be seen in the table above that a wireless link between two points separated by 26 miles would require an antenna tower with a minimum height of 103 feet for an 802.11b/g radio and 95 feet for an 802.11a radio. In practice the heights would typically be 20 feet higher, or more. This is because the Earth is not smooth and flat and the tower height must be raised to compensate for buildings, trees, hills, or other obstacles. For example, in a suburban setting, with houses and small offices, it may be necessary to add 20 to 40 feet to the tower height to get over the homes, offices, and trees that would be in the line-of-sight between the two towers.
Before erecting an antenna tower of any height, and before mounting an antenna on a building's exterior, it's important that the local Municipal Code be checked to make sure the installation complies with local law. In most cases an antenna must be mounted to minimize visual impacts, particularly when they extend above the roofline of a building. Factors to consider in conjunction with the mounting of an antenna on a building, or when erecting an antenna tower include:
- Setback distance from adjacent property as per local ordinances
- Setback distances from overhead power lines as per local ordinances
- Wind force ratings (a typical antenna may be rated between 100 mph and 175 mph, indicating that it has been tested and found to be strong enough to not break apart or break of its mounting at that wind speed). This is a specification that may be stipulated by local ordinance.
- FCC lighting or registration requirements (normally not applicable on towers under 200 feet).
- FAA flight path penetration and adjacency limitations or notification requirements
- Possible requirements relating to the safe and acceptable energy levels as defined by OSHA and the EPA, particularly if the antenna will be co-located with other transmitting elements on, or near, the same mounting tower or mast.
- Insurance requirements (particularly liability insurance - antenna masts and towers tend to fail in a spiral motion downward with debris falling within 20-50% of the antenna's height).
- Loading limits (pounds-per-square-foot) for the building structure or supporting base for the antenna mast or tower. Local building codes place limits on the weight that a building roof or concrete pad of particular construction is allowed to support. It may be necessary to build a platform to disperse the weight load on a building roof, or pour a larger or deeper concrete pad for a freestanding mast to meet these limits.