Q: When I plot a terrain profile, what number and ratio should I use for the Fresnel zone?

A: The line of sight (LOS) from the transmitter antenna center of radiation to the receiver antenna center of radiation and the Fresnel zone for the path at a given frequency can be added to a profile plot. The line of sight shows simply the direct path between the two antennas. The Fresnel zone is a theoretical "envelope" around the line of sight that has several important implications.

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 N^th 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.

Another important use of Fresnel zone information is to check paths (particularly microwave paths) for possible reflection points. The Fresnel zone formula shown above is the set of points where the distance from the transmitter to the Fresnel zone, then to the receiver, is longer than the direct path from the transmitter to the receiver. For even numbered Fresnel zones (N=2, 4, etc.), the difference between the direct path and the indirect path defined by the Fresnel zone distance, is a multiple of one-half wavelength. If the geometry of the path is such that an even numbered Fresnel zone happens to be tangential to a good reflecting surface ( a lake, a highway, a smooth desert area, depending on what wavelength is involved), signal cancellation will occur due to interference between the direct and indirect (reflected) signal paths.

You can set the Fresnel zone to even numbered values when plotting a profile to see if any potential areas of destructive signal reflection are present on the path.