It is well known that coherent VLF signals injected into the magnetosphere from Siple Station, Antarctica, often show temporal growth of 20--30 dB and emission triggering, as observed at the conjugate point, near Roberval, Quebec. In a new kind of experiment it has been found that when the input power to the transmitting antenna is reduced below a 'threshold' value Pt, growth and triggering cease. Below Pt the output is proportional to the input. In one typical experiment the peak received power increased by 24 dB when the input power to the antenna crossed the threshold value of 1000 W (estimated radiated power of 10 W). The value of Pt varied widely depending on the duct involved and on the time, changing as much as 10 dB in less than 1 hour. As the power was lowered during multipath propagation, the last duct to be cut off was found to terminate nearly overhead at Siple, as was expected, assuming all ducts to be equally active. Minimum radiated power for growth and triggering was 1 W. One possible explanation for the threshold effect is background noise (e.g., plasmaspheric hiss) that prevents the instability from getting started. Another is a drop in temporal growth rate to below zero at low signal level. Measurement of Pt might possibly serve as a ground-based diagnostic for magnetospheric flux levels, assuming calbration by satellite particle detectors.