The most common use of Fresnel zone information on a profile plot is to check for obstructions that penetrate the zone. While line of sight is important, it may not always be adequate. Even though the path has clear line of sight, if obstructions (such as terrain, vegetation, buildings, etc.) penetrate the Fresnel zone, there will be signal attenuation.
The Fresnel zone is computed along the path, usually for the distance of each of the terrain points, so the resolution of the computed and plotted Fresnel zone is comparable to the terrain data. The Nth Fresnel zone formula is a function of the wavelength (l) and the distance along the path from each endpoint (D1 and D2):
This value represents a perpendicular distance from the direct line of sight line. The combination of these points along the path can be viewed as the Fresnel zone plot. Typically the first Fresnel zone (N=1) is used to determine obstruction loss, with methods such as Bullington. Anytime the path clearance between the terrain and the line of sight path is less than 0.5F1 (half of the first Fresnel zone distance), some knife edge diffraction loss will occur. The amount of loss depends on the amount of penetration. Profiles are often drawn with the first Fresnel zone (N=1) and a ratio of 0.5 to provide a quick visual inspection of possible problems caused by obstructions penetrating that zone. Some engineers plot a ratio of 0.6 of the first Fresnel zone to add a bit of headroom for the path design.
