Results were interesting: although signals were copied at 1.6km and 3.6km on both test frequencies, nothing at all was copied at 6km, where very good signals were copied on 8.97kHz a few weeks ago. Ground conductivity and weather conditions were identical on both test days: dry for the last several days, so conductivity likely to be lower than when the fen soil is saturated.
I have no real idea why 8.97kHz should appear to be a "sweet spot" in frequency. It is possible that other frequencies lower than 8.97kHz and higher than 1.1kHz may be even better. In all cases the RX loop was resonated and positioned on the ground varied for best signal.
|1.12kHz earth-mode signal at 1.6km|
|17.952kHz earth-mode signal at 1.6km|
|1.12kHz earth-mode signal at 3.6km|
|17.952kHz earth-mode signal at 3.6km (note MSK signals close by)|