Effects of quadrature channel imbalance on Doppler radar physiological measurements, and new compensation methods, are investigated in this paper. Parametric simulations and experiments were performed to investigate the effects of amplitude imbalance, phase imbalance, and initial phase on Doppler radar obtained pattern, displacement, and rate estimation. It is demonstrated that channel imbalance can introduce significant displacement error, and that error is sensitive to initial phase. Furthermore, it is shown that in-phase and quadrature mismatch can introduce error in heart rate estimation. Two methods to achieve accurate displacement estimation were proposed and demonstrated. Using several initial angle measurement points within quarter wavelength, displacement estimation with errors less than 1% was achieved without any imbalance correction. Alternatively, by combining data points at varying initial angle locations, imbalance factors are estimated with high accuracy. This technique provides another viable method of imbalance compensation without the need for hardware modification, or a target moving with a large displacement.