# Fresnel zone

From AMS Glossary

(Redirected from Half-period zone)

## Fresnel zone

A circular zone centered about the direct path between a transmitter and a receiver (

*or*between radar antenna and target), so defined that the distance along a path from transmitter to receiver through a point within the zone has a path length equal to some value between [*L*+*n*λ/2] and [*L*+ (*n*+ 1)λ/2], where*L*is the length of the direct path, λ is the wavelength, and*n*is a positive integer or zero.The first Fresnel zone is the zone defined by

*n*= 0 and containing the minimum path length. Fresnel zones are a useful concept for analyzing the interference between the direct signal on a propagation path and signals reflected by objects that are displaced from the direct path. Thus, for a given path, reflected radio energy arriving at the receiver from any point will have a phase determined by the particular Fresnel zone in which the point is located. Reflected signals from zones defined by even values of*n*will interfere constructively with the direct signal; those from zones defined by odd values of*n*will interfere destructively.