The Fresnel zone
Radio waves spread out. The volume that contributes most of the energy is an ellipsoid around the line of sight whose radius at the middle of the path is r = 8.66 × √(d / f) metres, with the distance d in kilometres and the frequency f in GHz. For a 10 km link at 435 MHz the first zone is about 41 m wide in radius; at 2.4 GHz it shrinks to 18 m. The Fresnel zone calculator computes it at any point.
A good rule is to keep 60% of the first Fresnel zone free from obstacles. With that clearance the loss is practically the same as in free space.
Knife-edge diffraction
When a ridge rises into the path the signal still reaches the other side by diffraction, but with extra loss. A ridge that just touches the line of sight costs about 6 dB; a ridge that is 0.6 of a Fresnel radius above the line costs roughly 13 dB; deeper shadow costs 20 to 30 dB and more. The loss grows with frequency: a hill that hardly matters at 145 MHz can block a 2.4 GHz link.
The model used by the map follows the multi-edge method of ITU-R P.526: it finds the most obstructive point of the real terrain profile and then the dominant obstacles on each side of it.
Smooth Earth and ground reflections
Even on flat ground the surface matters. With low antennas the reflected wave interferes with the direct wave, and beyond a certain distance the loss rises with the fourth power of the distance instead of the square. Over the sea the reflection is strong and can create deep fades. The MFH Coverage Engine takes the larger of the terrain, plane-earth and spherical-earth losses.
