Diffraction of cylindrical waves on annular periodic structures

We examine the diffraction of cylindrical waves on a grating of perfectly conducting rings of finite thickness. By the method of partial regions we obtain expressions for the field scattered by the structure. For gratings with narrow spacings we give approximate conditions for the resonant transmission of cylindrical waves through a structure with large outer and-arbitrary inner radii of the annular structure. It is shown that for small inner radii of the rings a more efficient interaction of the electron current with the harmonics of the retarded waves is possible than in the planar case.