Some years ago, I was quite keen on amateur radio below 9 kHz. I well remember people saying it was a waste of time and nobody would get beyond the garden fence. How wrong they were!! Some quite remarkable distances have been spanned.
Although for radiated DX below 9 kHz very big antennas are needed for TX, on RX some tiny antennas can work perfectly well such as an E field probe which is tiny. What is needed is very high stability and very narrow bandwidths, usually achieved by locking to a VLF MSK signal or GPS. Some years ago DJ8WX was spotted by locking to a VLF MSK signal (see photo). All this was possible with free software. Sometimes it is necessary for signals to be integrated for hours or even days.
It is some years since I have experimented at real VLF.
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