This paper presents a novel Doppler radar sensor architecture with a quadrature hybrid mixer. Consisting of four branch-line couplers, two 45?? delay lines, and four Schottky diodes, the quadrature hybrid mixer demonstrates high transmission efficiency to pass the power from a local oscillator (LO) to an antenna without wasting received power at the oscillator port. A quadrature Doppler radar system is also implemented with the proposed quadrature hybrid mixer and defected-ground-structure LO. Compared to a conventional quadrature radar, the received power of the proposed architecture is enhanced by 3.3 dB when the LO power is 10 dBm, and is enhanced by 16.2 dB when the LO power is reduced to 0 dBm. Additionally, the improvement in received power is attributed to the improved transmission and lower receiving loss than that of the conventional ones. Capable of determining speeds lower than 0.05 km/h, which corresponds to a 1-Hz Doppler shift. The proposed Doppler radar architecture can identify the direction of the displacement due to use of the quadrature technique.