Amateur radio below 9kHz
Until a few years ago, the conventional wisdom was that you needed huge power and huge antennas to be copied “beyond the garden fence” at VLF and ULF. In recent years much has changed and time and again great distances have been covered by amateurs with quite low power and modest antennas.
The trick is great stability, very narrow bandwidths and long integration times, often of days. For several years just detecting a carrier was enough, but in recent times people have been sending simple messages using EbNaut software.
When at the old QTH I detected signals from several European countries using simple, homemade gear and free software.
Although not totally up to date at all times, I try to update news of amateur VLF and ULF experiments at https://sites.google.com/site/sub9khz/ .