20 Jun 2013

VLF earth-mode "propagation" variations

In the last few weeks I have been doing QRSS3 (slow CW) tests at approximately 1kHz, 4.48kHz, 8.97kHz and 18kHz transmitting 5W into 20m spaced earth electrodes (1 electrode at the bottom of the garden and the other connection to house copper pipes) and checking signal strengths at 1.6, 3.6 and 6km away from the home QTH using my portable loop antenna, preamp and a small netbook PC running Spectran software. Propagation is by utilities assisted earth-mode i.e. the main means of signal propagation is (I believe) conduction through buried pipes and cables with the induction field at the RX point being picked up with the loop. I have also used an E-field probe to detect the E field signal at some distance.

The recent tests were to see how signal levels varied with frequency, but I am finding VERY large differences in signal level day-to-day. The 8.97kHz signal was around 10dB S/N today at 1.6km whereas it was around 20dB S/N a week or so ago. I was unable to copy a signal at 1kHz and 8.97kHz at 6km at all when I tried a few days ago yet the 8.97kHz signal was quite decent a few weeks ago at the very same spot.
Weaker 8.97kHz received signal at 1.6km today
Weak 4.485kHz received signal at 1.6km today
There will be some dBs variation depending on the exact positioning of the RX loop on the ground but the variations seem suggest something else. Today there were a lot of static crashes and I don't know if this upsets Spectran's DSP processing? The other variation could be soil conductivity: today was wet (raining) whereas the best results seem to be with dry settled conditions when the soil has dried out a bit.

Conclusions so far? Signal strengths between 1-17kHz at 6km range don't vary that much over the frequency range, but signal levels can be up to 20dB different day-to-day as a result of other (as yet not understood) variables in the system or path.

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