This paper presents a quantitative evaluation of the power efficiency decrease obtained by breaking the zero-current switching (ZCS) condition due to the alternate output occurring in a bi-level quadrature-modulation (QM) envelope-pulse-width-modulation (EPWM) transmitter. The power efficiency is evaluated by changing the Q factor of the series resonance circuit inserted in a power amplifier (PA). As a result, the power efficiency that includes signal and quantization noise rapidly decreases with increasing Q factor. Consequently, the power efficiency for the signal only becomes low. Therefore, a QM EPWM transmission system using quadrature polarized waves is proposed to satisfy the ZCS condition for high efficiency. A digital signal processing and microwave circuit simulation shows that the proposed transmission system can demodulate both sigma and delta components and improves the power efficiency of the PAs.