And as a follow-up comment on LF/VLF noise, I found this rather interesting scan of the LF bands from 14 to 62kHz, on some unidentified receiving system, using a 150' long wire:
https://www.radiomuseum.co.uk/VLF%20RECEPTION.html
The very low signal voltages in dBm may just indicate that the antenna factor of such a wire is very poor at such a low frequency. However I also note that the noise floor appears to be going in entirely the opposite direction from the one in Synchrodyne's graph. In other words the noise appears to be falling with falling frequency.
Richard