Fresnel zone

Fresnel zone: In radio communications, one of a (theoretically infinite) number of a concentric ellipsoids of revolution which define volumes in the radiation pattern of a (usually) circular aperture.

Note 1: The cross section of the first Fresnel zone is circular. Subsequent Fresnel zones are annular in cross section, and concentric with the first.

Note 2: Odd-numbered Fresnel zones have relatively intense field strengths, whereas even numbered Fresnel zones are nulls.

Note 3: Fresnel zones result from diffraction by the circular aperture.

Source: from Federal Standard 1037C and from MIL-STD-188