An FMCW radar generates a chirp signal in the transmitter. The chirp is transmitted through a splitter device to both the TX antenna and the mixer in the receive path. Some object generates a reflected chirp captured by the RX antenna and the mixer produces the difference between the TX and RX signals - aka. the intermediate frequency (IF). A very simplified description of an FMCW radar.
The question is about the splitter in the transmitter: Some designs show a directional coupler as the splitter. A directional coupler has a 90° phase difference between the coupled port (going to the mixer) and the output port (going to the TX antenna). Is this not problem when the mixer produces the IF signal?