A series of cross-shaped resonant grids

the resonant wavelengths of whichwere in the region of 55—90 um

have been constructed and the dependence of thespectral characteristics on the grid geometric parameters has been investigatedexperimentally in detail.Two empirical expressions which are useful for desig-ning the resonant grid have been found.Resonant interference filters which co-nsist of two resonant grids have been constructed either.The results show thattheir transmission in the pass-band and the rejection in the rejective-band arevery good and the interference filter is an excellent and practical bandpassSMMW filter.