The whole radio frequency spectrum has been split into regions according to ITU radio regulations. The VLF region corresponds to frequencies between 3 kHz and 30 kHz (wavelengths between 100 and 10 km). Those frequencies are much lower than those used for AM broadcasts radio station (LW, MW, SW).

VLF waves are used for time signals and radio navigation beacons (such as the Russian hyperbolic radionavigation system RSDN-20. And, since they can penetrate water to a depth of tens of feets, they are used by militaries to communicate with submarines near the surface. Transmitters have a power of a few hundreds of kilo-Watts.

VLF frequency range is also home to natural electromagnetic emissions (called sferics, tweeks, whistlers...) emitted thousands of miles from the receiver. They can be turned into sound that we can hear. This is called "natural radio".

Many military stations around the world use VLF for communications with submarines. You can found in this website a list of VLF transmitters that can be used for SID monitoring.

You can found here a spectrum of all stations received at the SID monitoring station location (South of France). You will found HWU on 18.3 kHz, GQD on 19.6 kHz, FUO on 20.9 kHz, DHO38 on 23.4 kHz, and many more on 20.5 kHz, 24.6 kHz, 25.9 kHz, 26.9 kHz, 27.1 kHz, 27.5 kHz, 29.7 kHz, 31.25 kHz, 32.4kHz, ... More than enough for our purpose.