Abstract: The influence of unmatched antenna load on resonance characters of quarter wavelength coaxial oscillator, which determines the output pulse spectrum, was analyzed. Based on cascading dual-port microwave network model, the relationship between load reflectance and the spectrum of output pulse was derived by involving the reflectance of unmatched load into thequivalence coupler reflectance. Numerical simulation of the generator, which contains oscillator, coupler and antenna, was performed, showing consistent results with theoretical predictions. As predicted by theory, the spectrum of the radiation pulse creates two secondary peaks around the major peak, which was also observed in the high power experiment performed.