Sideband wave emissions are secondary peaks in wave power observed at frequencies above and below the primary peak. Observed sideband amplitudes are usually reduced compared to the primary wave amplitude. During the recent CHARGE 2B ionospheric rocket mission, sideband waves were observed during an electron beam injection experiment. The beam current was modulated causing a peak in wave power at the modulation frequency and at certain times sidebands were observed along with the primary wave emission. A nonlinear coupling mechanism between the primary wave and a lower-frequency wave is explored here to explain the presence of the sideband emissions. Comparison of the theoretical results with the wave data confirm this mechanism as a visible explanation for the observed sidebands. ¿ American Geophysical Union 1994