VLF transmitter experiments conducted at Siple, Antarctica (L?4) show that long (?1 s) keydown signals injected into the magnetosphere often generate sidebands as a result of nonlinear interactions with energetic particles. The spectral characteristics of observed sidebands are quite varied and complex. The sideband frequency spacing varies from ~2 to 100 Hz, but it bears no simple relationship to the carrier amplitude, in sharp contrast to the predictions of some theories. The sideband amplitude is usually 10 dB or more below the carrier amplitude, but sometimes it can exceed the carrier amplitude and also trigger emissions. Multiple sidebands are often observed, and their frequency seperations from the carrier may or may not be harmonically related. Sideband amplitudes may be symmetrical or asymmetrical about the carrier. In the asymmetrical case it is usually the upper sideband that is stronger. Various sideband generation mechanisms are discussed in the light of these new experimental data.