We derive the expression for the intensity of the diffracted waves from a symmetrical profile lamellar grading (SLG). Such symmetrical structure may improve the performance of an ordinary lamellar grating. If suitably designed, its applicability may be extended to the millimeter to radio wave regions apart from the normal optical and near infrared region in which it is mostly used. Such millimeter wave SLG can have significant application in astronomy and astrophysics. By proper choice of geometrical parameters, an SLG may be used as a microwave antenna in space communication as well. (author). 24 refs, 5 figs.