Re: Ultra VLF signals?
You may be referring to the Mizuho "Sky Coupler KX-3" like the one I have. Has a LF switch on it, that introduces a LPF giving a sharp cutoff above IIRC 500kHz.
Years ago, I used this and a comms receiver to decode a press fax transmission service on 139kHz, but that's long gone.
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John
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