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A new balanced Doppler radar front-end architecture with Tx leakage canceller for antenna impedance variation and antenna mutual coupling is proposed. Consisting of two quadrature hybrids, two ring hybrids and 4-port feed antenna system, the proposed balanced structure achieves high transmit/receive (Tx/Rx) isolation stable to antenna impedance variation and antenna mutual coupling. The proposed 4-port feed antenna is configured by dual fed 2 × 1 patch array having 6.97 dBi and 5.65 dBi of measured Tx and Rx peak gain, respectively. In addition, the proposed architecture undergoes no power loss in both transmit and receive paths, and obtains the measured Tx/Rx isolation of more than 45 dB stable at 24 GHz regardless of the load variation. The proposed architecture that can determine the speed as low as 0.5 mm/s (0.078 Hz Doppler shift) is theoretically and experimentally analyzed for high isolation despite of antenna mismatch.