Today I continued "by ear" testing local induction and conduction communication using my 4W PA at 1kHz with 10m spaced grounded electrodes. Using the 80cm loop, HPF and audio amp I was able to achieve 0.3kms today, slightly less than on Monday. Everything else is unchanged, so I'm wondering if the effective TX loop area formed by the 2 earth rods in the ground changes with soil moisture? I.e. when the soil is damp is the loop smaller? Logically this makes sense. Copy using my 80cm loop and VLF up-converter was disappointing: I was unable to get further than 0.2km. This system needs further work as this should be every bit as good or better than the audio amp receiver.
Also today I checked the resistance of the TX grounded electrode system using an audio oscillator and potential divider method: it measures approximately 50-60 ohms from 1-10kHz, which is not too bad. I also tried receiving my signal 0.2km away using another pair of grounded electrodes spaced by 10m. Although copy was just possible, the signal was buried in mains hum. Mains hum pick-up on the loop is much lower. This again suggests that the main means of communication is induction, not conduction through the soil.