What is the first Fresnel Zone?

The Fresnel (pronounced fre∙NEL) Zone is a three-dimensional volume around the line of sight of an antenna transmission. Objects that penetrate this area can cause the received strength of the transmitted signal to fade. Out-of-phase reflections and absorption of the signal result in signal cancellation.

The foliage of trees and plants in the Fresnel Zone can cause signal loss. Seasonal density, moisture content of the foliage, and other factors such as wind may change the amount of loss. Plan to perform frequent and regular link tests if you must transmit though foliage.

The first Fresnel zone is the locus of all points in space that results in a reflected ray half wavelength out of phase at the end of the receiver. The first Fresnel zone when plotted in three dimensions forms an ellipsoid. Simply put, the signal is shaped like a football between two transmitting devices. For best calculations, all obstruction should be kept out of the Fresnel zone. Most path calculations assume at least 60% unobstructed Fresnel zone clearance.