Spectral broadening of signals from a ground-based VLF transmitter was observed onboard the Magion-3 sub-satellite during electron beam injection from the lntercosmos-25 (APEX) satellite. Broadening of the order of 300–500 Hz was apparently correlated with the 2 s-periodic sequence of the electron gun pulses which were modulated in amplitude with frequencies 30.5 < fm < 31,250 Hz. No effects were found for those electron gun pulses with fm = 62.5, 125 and 250 kHz.