Fresnel zones result from diffraction by the circular aperture.
The cross section of the first Fresnel zone is circular. Subsequent Fresnel zones are annular in cross section, and concentric with the first.
Odd-numbered Fresnel zones have relatively intense field strengths, whereas even numbered Fresnel zones are nulls.
The concept of Fresnel zones may also be used to analyze interference by obstacles near the path of a radio beam.
Source: from Federal Standard 1037C in support of MIL-STD-188