Yesterday morning the mistaken frequency field in PRIYOM scheduler made the prijemnik receiver to tune on to 0kHz for a while.
As there was a thunderstorm in progress (quite surprising for mid-September) the result was a beautiful display of EMI scattering, when surplus electrical fields discharged. The sound was more or less like a dolphin song, too bad I did not have a recorder on to provide an audio sample ..
The horizontal line at the bottom of the screen is a lightning strike happening, but please also pay attention to the very clear and neat VLF transmission happening at 18kHz region. This is likely a submarine communication and sounded like any other RTTY modulation.
Surprisingly, my magnetic loop antenna design seems to work very well down to the VLF region, making it a really nice and compact exploration tool as indoor urban antenna. The long aerial outside was absolutely mute below 50kHz and did not display any sign of VLF transmission other than static.