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:

https://chemandy.com/calculators/matching-t-attenuator-calculator.htm

Regards,

Chris