Recently amateur radio hobbyist DE8MSH wrote in to let us know about how he was able to receive VLF (Very Low Frequency) signals using a very rare Refcom FC-VLF upconverter and his direct sampling modified RTL-SDR.
His antenna is the PA0RDT mini whip which requires 12v of power that is delivered directly by the Refcom FC-VLF. He writes that the Refcom upconverter is used to upconvert the 9 to 50 kHz range into the 14.009 to 14.050 MHz range which is receivable by a direct sampling modified RTL-SDR. Using this set up he was able to receive several VLF stations as shown on the waterfall image below.
DE8MSH also writes
Note that I’m not living in a quiet area. As you can see there is a lot of men made noise like from tv sets, lights bulbs etc. pp. Some signals are not very strong becaus I switched the Refcom to 9-50kHz filter. So some stations above 50kHz could be stronger.
Below are some close up shots of VLF signals being received.