Jim Moritz wrote:
> ...single-turn loop current would be about 32A
> ...10m diameter

> B = 6*10e-15 T

> d = cube-root-of( (mu0*r^2*n*I)/2B ) = 4.4km

> So quite a long distance (have I got it right?!)

Yes, I get the same answer for that loop and current.

> 6dB SNR, we need to generate about 6 femto-teslas of
> magnetic flux density at the receiver to get a readable
> signal.

I am finding it is well worth orienting the loop to null
the average direction of the prevailing VLF background.
There is nearly always around 6dB reduction of background
as a result.  At times when the background is quite
concentrated in one direction, I've seen 10dB reduction.
On top of this, blanking the sferics gives another couple
of dB or so, depending on level of sferic activity.

With this in mind, range could go even further, but then
the far field is starting to take over and you can no
longer call it just induction.
--
Paul Nicholson
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