Fresnel zone

A Fresnel zone (/freɪˈnɛl/ fray-NEL), named for physicist Augustin-Jean Fresnel, is a series of concentric ellipsoidal regions of alternating double strength and half strength volumes of a wave's propagation, caused by a wave following multiple paths as it passes by an object and is partially refracted by it, resulting in constructive and destructive interference as the different length paths go in and out of phase.

Fresnel zones are seen in optics, radio communications, electrodynamics, seismology, acoustics, gravitational radiation, and other situations involving the radiation of waves and multipath propagation.

This is the cause of the picket-fencing effect when either the radio transmitter or receiver is moving and the high and low signal strength zones are above and below the receiver cut off threshold.

Importance of Fresnel zones

Fresnel zones are concentric ellipses (1, 2, 3) centred on the direct transmission path (AB).

The 1st zone is the ellipse with chords 1/2 wavelength longer than the direct path (ACB).