Figure 1: Fresnel zone.
d is the distance between the transmitter and the receiver,
b is the radius of the Fresnel zone.
In radio communications, a Fresnel zone is one of a theoretically infinite number of a concentric ellipsoids of revolution which define volumes in the radiation pattern of a usually circular aperture. Fresnel zones result from diffraction by the circular aperture.
Radio waves will travel in a straight line from the transmitter to the receiver normally. But if there are obstacles near the path, the radio waves reflecting off of those objects may arrive out of phase with the signals that travel directly and reduce the power of the received signal (Interferences). This effect is a reason of „Fading” in radio communication. On the other hand, the reflection can enhance the power of the received signal if the reflection and the direct signals arrive in phase.