A composite patch array antenna with built-in polarizer has been developed to reduce road clutter noise by 20dB for 76GHz automotive radars. A polarizer is placed in front of Tx and Rx patch arrays within their reactive near-field region to suppress cross-polarized sidelobe radiation from their feeding lines while maintaining a low-profile characteristic with 4mm thickness. Additional metal-lined absorbers within the composite antenna structure, while terminating cross-polarized waves undesirably excited by the patch arrays, also serve as miniature clutter plates to further reduce sidelobes toward the road surface. The resultant composite antenna achieved sidelobe levels of -45dB, a 20dB improvement over standard patch arrays, at elevation angles close to 90°.