The millimeter wave radiation of a dielectric leaky-wave antenna (DLWA) coupled with a metallic diffraction grating with a sinusoidal height profile is experimentally investigated. The interaction between dielectric and grating is contactless and it is from the broad-face of the dielectric line. The measured field patterns are compared with the simulated HFSS patterns. Antenna parameters such as polarization, HPBW, cross-polarization and side lobe levels are measured. The effects of the geometrical parameters of the grating to the radiation are also investigated. Some results are compared with a DLWA which has another diffraction grating profile shape (rectangular). The investigated antenna can be used as a frequency scanning antenna in mm-wave radars because it has low cross and side lobe levels and narrow beam width.