Abstract: A radar system using a quadrature signal includes a quadrature push-push oscillator for generating four harmonics with a 90-degree phase difference from each other, and producing two-balanced 2nd harmonic signals from the harmonics; a first coupler block for radiating one of the 2nd harmonics through an antenna; a second coupler block for terminating the other 2nd harmonic to ground; and a power combiner for combining a transmitted signal that is leaked from the first and second coupler blocks with the received signal that is radiated through the antenna. The radar system features an improved receiving sensitivity by offsetting the leakage signals of the sending end. Also, the radar system can be made very small by using a single antenna for transmitting and receiving.