A vehicular radar sensing system includes a radar sensor disposed at a vehicle. The radar sensor includes a waveguide antenna and a printed circuit board (PCB) with an inboard side and an outboard side. A processor is disposed at the inboard side of the PCB and the waveguide antenna is disposed at the outboard side of the PCB. The waveguide antenna is partially disposed within a cavity at the outboard side of the PCB. Radio frequency signals are electrically communicated between the processor and the waveguide antenna through only a portion of the PCB. The waveguide antenna (i) guides the transmitted radio signals from the transmitter to the exterior environment and (ii) guides reflected radio signals from the exterior environment to the receiver.