Henrik Forstén has a nice build log on his newest version of this homemade 6 GHz FMCW radar:
Frequency Modulated Continuous Wave (FMCW) radar works by transmitting a chirp which frequency changes linearly with time. This chirp is then radiated with the antenna, reflected from the target and is received by the receiving antenna. On the reception side the received signal that was delayed and undelayed copy of the transmitted chirp are mixed (multiplied) together.
Homemade 6 GHz FMCW radar – [Link]