The potential use of dielectric grating waveguides as dichroic surfaces, in the millimeter waveband, is explored here. It is shown that the selective behavior of the structure may be conveniently changed by modifying the geometry of the periodic layer, by loading the periodic layer with homogeneous dielectric layers or by changing the angle of incidence of the plane wave that excites the structure. It is also shown that, essentially, the same selective response of a periodic layer with a step varying dielectric constant may be obtained of a periodic layer with a conveniently chosen sinusoidally varying dielectric constant. In addition, expressions are given to estimate design parameters for obtaining maximum reflection for the fundamental mode. (C) 1996 John Wiley & Sons, Inc.62128145