Hi Alex,
this is all very interesting!
> I still cannot explain this, why the signal is perfectly visible in the transition zone and is completely absent at the beginning of the far zone.
Well once again we see that the hardest part is getting out of the nearfield ;-) Normally we would expect a r^-3 dependence up to 5.7 km (18 dB drop at double distance). So yes, if you could **hear** the signal at 3.7 km, it should be detectable in some mHz bandwidth at 7.8 km.
Note that the pattern and orientation of the quasistatic magnetic field lines are quite different from the far field. There is also an axial component which is twice as strong as the transversal component, but disappears completely in the far field. This is probably what you picked up at the second location, providing another 6 dB signal strength and unusual RX-loop orientation. It is also possible that the signal was locally enhanced by reradiation from underground wires or metal pipes.
> the frequency of the signal continuously and irregularly changes within small limits
This is normal behaviour of the PLL. Depending on the measured phase and small samplerate errors, it steers di.freq, such that on average the phase is held for the correct frequency (8270.00625). This was confirmed by the green trace, so I believe you were transmitting on the intended frequency.
> another laptop with rx-loop + DHO38-calibration.
Samplerate calibration based on DHO38 also has some difficulties. First it can be affected by soundcard glitches, wich cannot be corrected and can cause a residual phase change which will disturb the frequency. Using 1pps + GPS-phaselock is the only method available in SpecLab which can correct such glitches, because it knows absolute time and can calculate the phase which the signal should have, even after an interruption.
In addition, the DHO38 transmitter is not exactly on 23.4 kHz. When Wolf and I measured it ~ 12 years ago it was approximately 0.03 ppm high, which is why it is announced as 23400.00006 Hz in the table. But I do not know if this is still correct today.
> - should Input Calib and Output Calib be equal?
Normally yes. I know that there are some soundcards which have different ADC and DAC samplerates, but these seem to be rare.
> - do I need to enable the "resample to nominal output SR" checkbox?
No, this is not necessary for this application.
73 and good luck,
Markus
