This study presents a novel frequency modulated continuous wave (FMCW) radar system based on sub-harmonic generation, where a 24 GHz frequency divider-by-10 is used as an active reflector tag. A practical prototype is designed and fabricated on a GF45nm-Silicon on Insulator (SOI) technology for the 24 GHz building blocks, while a GF0.18 μm 7WL Bipolar Complementary Metal Oxide Semiconductor (BiCMOS) technology was used for the 2.4 GHz receiver and baseband. System measurement results show that as opposed to conventional primary radars, the proposed system is immune to strong multi-path interferences resulting from direct reflections of the interrogating signal. The system achieves a ranging precision of 3.7 mm with loop measurements. Moreover, when measured in an indoor environment, the ranging results show a ranging precision and accuracy of 5.8 and 22.3 cm, respectively, which outperform other FMCW radars in the literature.