Re: Bad idea to use RG-6 coax for the VLF/HF loop antenna amp?
Either version of the Loop Antenna Amplifier will work well with 75 ohm coax.
What I would suggest at the receiver end would be to add a 75 to 50 ohm matching attenuator. This would give the coax a good 75 ohm load relatively unaffected by whatever the input impedance of the SDR receiver is.
A simple 10 dB tee attenuator would have a 47 ohm resistor from the 75 ohm input, 18 ohm resistor from the 50 ohm output with a 43 ohm resistor from the tee junction to ground. These are preferred resistor values so that it's easy to buy the parts.
Here's a very useful online calculator to calculate resistor values for other impedances or attenuation levels: