brief rundown of using a large FERRITE CORE LOOPSTICK tuned ANTENNA to hook up directly to the sound card input...and then use some SDR software modules to get rid of HUM, NOISE, HISS and other EMF QRM from the house wiring emf'N into the RX antenna etc...
the ULF CW TRANSMITTER is a PI 4's audio output - going to a 30 watt audio amp - going to a 4 to 1 iron powder toroid - going to a 40 foot wire loop antenna the SDR MODULES
1. at MIC JACK INPUT first - is a 2500 hz bandpass filter
2. then a HILBERT MODULE to take the mono input and output I & Q
3. then to a 2nd 2500 hz bandpass filter
4. then to an LO-MIXER that converts the 2500 hz to @700 hz
5. then to a GAIN MODULE ( about 30 dbs worth) all SDR MODULES are from RECRI KEYER https://github.com/recri/keyer
for more info on these elf,slf,ulf,vlf,lf experiments, please see this playlist: