Mal,

Please, I make no magical claims for the earth electrode antenna I use. All I say is that, in certain circumstances, it works rather better than one might expect both on RX and TX on VLF/LF/MF.

For cave-to-surface communications we have found that, in general, an earth electrode system is much better than a tuned loop.   When we tried both systems at Kingsdale Master Cave  Valley Entrance (a standard CREG test site) the earth current antenna gave a 32 dB greater signal at the surface than did the loop.

It all seems to depend on the geology.  If the top layer of the overburden is essentially organic, then the loop tends to give better results.

We still do not have a theoretical explanation for all this, but it works.

John F5VLF