The loop amp + fed by the bias-t of USB connected sdr receivers seems a good idea, but does have a few pitfalls. My initial experience of feeding the LA+ from my RSPduo was disappointing. Performance was way below what I expected. My setup is a loop made of 10 mm copper water pipe fed with approximately 15 metres of RG58u coax. Here lies the possible problem, and the cause of the poor performance.
I measured the voltage presented to the LA+ at the connection to the amp as 4.49 volts. The open-circuit voltage was 4.69 volts.
I then fed the LA+ from a separate (linear) 5 volt supply, via a bias-t of Chinese origin and the performance improved remarkably.
The measured voltages at the LA+ were 4.78 volts connected, and 5.14 volts open-circuit. The higher voltage delivered by the separate 5 volt PSU and bias-t obviously made all the difference.
The situation could be improved by the use of RG8 coax, but there still might not be a high enough voltage to power the LA+ adequately. I do not have the facility for varying the voltage to the LA+ to determine the minimum voltage required. A much shorter run of RG58 may be OK, but I have not tried this as I need to locate my loop some distance from the operating point. Using a separate bias-t does have the advantage of being able to locate it much nearer the loop. There does seem sufficient signal presented to the RSP when fed by the 5 volt PSU/bias-t arrangement. Note that I have only checked this on LF and MF frequencies, but I would expect similar results at HF.
Any other experiences with this setup?