Computer simulations of whistler mode wave-particle interactions have been performed by using a free-boundary encounter model electromagnetic code. The free boundary is attained by adding a collision dominated damping region near one boundary to remove periodicities usually used in electromagnetic codes. In our model, a wave front encounters a resonant bi-Maxwellian hot electron beam. Spatial amplitude modulations are observed in our simulations. The dips of the amplitude, however, sometimes decrease to zero levels. The null points of the wave magnetic field and the associated phase reversals of the wave cannot be described by the usual particle trapping theory. A component of the field contains a feature reminiscent of a double sideband (DSB) wave without a carrier. This is interpreted as a beat between an incident wave and a generated wave. This phenomenon occurs if the amplitudes of both the waves are almost equal. The results of these simulations provide a possible explanation for the N events observed by Dowden et al. during a VLF injection experiment.