A two‐grid approach for quasioptical bandpass filter design is presented. In this design, an inductive/capacitive grid on one side of a dielectric substrate is transformed to an effective capacitive/inductive reactance at the other grid plate, producing a parallel resonant circuit. Two bandpass filters for use in the millimeter region have been fabricated. Low‐loss power transmission at the design frequencies has been achieved with good quantitative agreement with theory. © 1992 John Wiley & Sons, Inc.