Decorative plate transparent to the emission waves of a radar system for a front grille of a motor vehicle, as a distinctive badge intended to close the opening that is made in said grille in correspondence of said radar system and anchoring itself to it, said decorative plate being of the monobloc and multi-layer type, comprising two main plastic layers (110, 120) obtained by multi-injection moulding wherein a first layer faces a front side, configured to be positioned towards the exterior of the vehicle, and is colourless and transparent to light, being configured to operate as an externally smooth lens comprising at the back decorative elements (111a, 111b, 111c) made in relief and protruding towards an internal side configured to face the interior of the vehicle, and wherein a second layer is opaque and is overmoulded on said first layer, on the interface configured to face the interior of the motor vehicle, limiting the thickness of said second layer exactly to the height of said decorative elements (111a, 111b, 111c) to be aligned leaving their head surface (112a, 112b, 112c) uncovered, with moulded fastening elements integrated in said second layer; and wherein the first layer is of colourless PMMA transparent to light, while the second layer is of opaque ABS-PC or opaque ABS; and wherein on the back surface of the said two overmoulded layers (110, 120), on the internal side, there is a decorative treatment that covers at least said head surface (112a, 112b, 112c) of each decorative element and is a metallization obtained by a vacuum deposition technology, such as the technology called Physical Vapour Deposition, PVD;; and wherein said plate (10, 110, 120, 130) has a total thickness that is constant at least according to the area functional to said radar system; and wherein said first layer has a mainly aesthetic and optical function, it covers the whole external surface of the plate and has a substantially constant thickness except for said decorative elements that protrude towards the internal side configured to face the interior of the vehicle, with the whole front surface that has a radius of curvature according to said externally smooth lens; and wherein said opaque second layer is of the multifunctional type, in such a way as to realize the contrasting background that delimits and highlights said decorative elements (111a, 111b, 111c), intrinsically acting as a masking for said decorative treatment and simultaneously realizing the supporting and fixing structure that allows to anchor and integrate said plate in said front grille; and wherein said anchoring of the plate in the grille occurs by means of said fastening elements that protrude towards the internal side, being obtained by moulding near the external edge of the plate in such a way as not to jeopardize the correct transparency to radar waves in its central part; and wherein said decorative treatment comprises the following layers superimposed in adhesion (132, 133): an ultra-thin metallization layer and a shadowing layer of the covering and opaque type with an anti-transparency effect, wherein said ultra-thin metallization layer is applied by said PVD technology in a thickness of less than 0.05 micrometres, and wherein said shadowing layer is applied on the back of said ultra-thin metallization layer, hence on the internal side, in such a way as to protect said ultra-thin metallization and simultaneously make it evident to sight, if considered from the front side -said decorative plate further comprising said decorative treatment also comprises an interface layer, which is colourless and transparent to light, and which is applied by spraying directly on the back surface of said first and second overmoulded layers (110, 120), in front of the ultra-thin metallization layer, to facilitate its application; said interface layer, consisting of an anchoring and smoothing layer, of the UV primer type with polymeric matrix, with a low content of solvents, with ultraviolet polymerization, that, once applied and hardened, forms a thickness between 5 micrometres and 20 micrometres, and with the surface facing the internal side that has an average value of surface roughness of less than 0.08 micrometres, that is to say, Ra < 0.08 □m, to make the layers superimposed to it (132, 133) reflecting to sight and not to alter the chromatic perception thereof, said interface layer being put in front (132, 133) considering said decorative plate from the front side.