LF: Earth mode ranges

[Roger Lapthorn <rogerlapthorn@gmail.com>]

A further observation from my 8.76kHz earth mode test today: with 17dB above the noise 
signals at 5.1km using QRSS3 there is plenty of (theoretical) range 
still to be had with utilities assisted earth mode, even using just 5W.

Assuming 18dB loss every time distance is doubled (an inverse cubed 
attenuation rate for induction/conduction - is this right?) then almost 
10km should just be possible, assuming the utilities that are aiding 
propagation are still there in the ground. Going from QRSS3 to QRSS30 
should give another 8-10dB (see http://www.qsl.net/on7yd/136narro.htm#QRSS).
 Increase power from 5W to 100W gains another 13dB, so QRSS30 and 100W 
could give a range of some 20km by utilities assisted earth mode in 
favourable places based on my limited tests so far.

So, in some locations using the utilities buried in the ground all 
around us could offer some quite decent ranges with an earth electrode 
"antenna" at the TX end that requires minimal matching and no large 
loading coils, just a step-up transformer.  

What is less clear to me is what exactly is the signal being carried 
along? Water pipes (often these are plastic now), overhead mains cables,
 gas pipes (usually non-metallic), phone lines, rivers even? 
And what about the propagation along a sea coast with one electrode on a buoy 100m out from the shore?

Not radiated VLF DX, but fascinating.

73s
Roger G3XBM

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