Prerequisites for the application of a powerful source (radio set) of harmonic very low frequency (VLF) electromagnetic field to the monitoring of seismic activity are considered. The ACF-2 measuring instrument used for this purpose is briefly described. Experimental results obtained from the North Caucasus are analyzed. It is shown that the signals produced by the VLF radio set can be detected at a distance of 2700 km in the direction close to the equatorial region of the antenna system. The time stability of the impedance, which characterizes the prospects for applying the VLF radio set to earthquake prediction is assessed. Field measurements by the VLF radio set are compared with natural electromagnetic fields.