Coherent VLF (1.8–7 kHz) signals injected into the magnetosphere from Siple Station (L = 4.2), Antarctica, are amplified (30 dB) and trigger intense, narrowband whistler mode emissions. The mechanism of growth is believed to be wave-induced phase bunching of cyclotron resonant electrons in an interaction region near the equatorial region. A key parameter in these experiments is the spectral purity of the transmitted signal. If the width of the signal spectrum exceeds ∼ 10 Hz, there is significant loss of gain. In one experiment (f ≈ 3 kHz), two equal-amplitude signals are transmitted with a frequency spectrum of Δf. When Δf = 20 Hz, the gain is reduced as much as 20 dB below its single-frequency value. For f ≥ 100 Hz, the two signals tend to behave independently of one another, indicating that they interact with different electron populations. Other effects of interest include entrainment of strong free-running emissions by a relatively weak Siple signal and the frequent generation of sidebands (2–100 Hz) on Siple signals that have reached saturation. Future work at Siple Station will emphasize frequencies below 4 kHz in order to raise the energies (to > 10 keV) of the precipitating electrons and hence make them more readily detectable by ground-based techniques. To achieve this goal, the dipole length will be doubled (to 43 km), which is expected to increase the maximum radiated power at 2.5 kHz by 7 dB. Later, it is planned to install a circularly polarized crossed dipole that will double the effective radiated power and permit new experiments on the polarization properties of the ionosphere and magnetosphere at VLF.