This paper contains findings of authors, which concern reflection and transmission electromagnetic waves for dielectric slab with metallic structures inside. The metallic structure is periodic, with the relatively small period in comparison with wavelength. The idea of studied method is that the incident field creates current density in metallic frames of structure. These currents create reflected and transmitted waves. In addition the field in windows creates field behind grid in accordance to Huygens principle. The currents in frames and field intensity in windows are set approximately. Current radiated fields give little than 10% deviation for slab reflection coefficient for our selected geometry. The field, radiated by field distribution in windows, significantly changes transmission coefficient. Ill. 4, bibl. 6 (in English; summaries in Lithuanian, English and Russian).