[Fwd: LF: earth bipole news]
Sat, 04 Apr 1998 08:41:03 -0500
> I am continuing to peruse the literature re the earth current transmission
> method and possibly may have a better idea of how it functions. Information
> is being gleaned from VLF Radio Engineering as well as from various texts
> in geophysics (one method of prospecting involves the injection of VLF into
> two electrodes in the ground). It would seem likely that the system may be
> regarded as a loop in the vertical plane, the size of which is a function of
> the skin depth. Skin depth is dependent on both the frequency of the signal
> applied and the conductivity of the medium. For 136.5kHz, skin depth in
> ground of poor conductivity is of the order of 45 metres, whereas in very
> conductive ground it is 10-15 metres. Rain on the surface may reduce the
> current flowing at depth considerably. This assumption of a notional loop
> gives rise to a calculation of the inductance of such a loop (around 2mH).
> This agrees very well with my actual measurements.
> I have concocted the 'earth bipole' name as a compromise between the 'earth
> dipole' terminology referred to by the British Cave Radio Research group and
> the 'electric bipole' referred to in geological texts. The term 'ground
> antenna' which I was using before seems, on reflection, rather too
> transatlantic and 'earth aerial' just does not feel right.
> John G0AKN IO91LR email@example.com