  An approach has been developed where the Smith-Purcell radiation (SPR), i.e.
emission of electrons moving close to a periodic structure, is treated as the
resonant diffraction radiation. Simple formulas have been designed for the SPR
intensity for a grating having perfectly conducting strips spaced by a vacuum
gap. The results have been compared with those obtained via other techniques.
It has been shown that the intensity of radiation for the said gratings for a
relativistic case sufficiently exceeds the SPR intensity for the grating made
up by a periodically deformed continuous surface.
