Fresnel noted that a ray of light passing near a solid object is subject to diffraction, or bending. This diffraction caused the intensity of the original light beam to increase or decrease depending on how near the object was to the beam. This characteristic of electromagnetic radiation is known as the Fresnel Effect. Light and radio waves are subject to the same laws of physics, including the Fresnel effect. If an object like a mountain ridge or building is close to the radio signal path, it can affect the quality and strength of the signal.
Radio waves diffracted by such objects can affect the strength of the received signal.
This happens even though the obstacle does not directly obscure the direct visual path. For this example, let's assume that both antennas are mounted at equal heights above the earth. This is not always the case, but it's good enough for the purpose of this discussion, and it makes the table easier to use. The network architect must make sure that all the proper clearances are observed. That means the earth curvature, depth of the Fresnel Zone, and height of objects in the radio path must be added together to get the antenna mounting height.