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 + nλ/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.