A Fresnel zone is one of a series of concentric prolate ellipsoidal regions of space between and around a transmitting antenna and a receiving antenna system.
Obstructions within the first Fresnel zone can cause significant weakness, even if those obstructions are not blocking the line-of-sight signal path. For this reason, it is valuable to do a calculation of the size of the 1st, or primary, Fresnel zone for a given antenna system. Doing this will enable the antenna installer to decide if an obstacle, such as a building, is going to make a significant impact on signal strength. The rule of thumb is that the primary Fresnel zone would ideally be 80% clear of obstacles, but must be at least 60% clear.
By exporting the created .kml file to Google Earth you can check whether the desired fresnel zone is being obstructed by the terrain, for various radio systems, mainly for point-to-point radiolinks and wi-fi radio-links.
or by the 3D buildings in the cities (if available in Google Earth). Check the "3D Buildings" option.