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Message-ID: <9709F1D41FF74661B0771F657988884F@JimPC>
From: "James Moritz" <james.moritz@btopenworld.com>
To: <rsgb_lf_group@blacksheep.org>
References: <9afca2641003111254v47a0363eicdee899d7c2b3431@mail.gmail.com>
In-Reply-To: <9afca2641003111254v47a0363eicdee899d7c2b3431@mail.gmail.com>
Date: Thu, 11 Mar 2010 23:57:10 -0000
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Subject: LF: Re: Induction DX?
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Dear Roger, LF Group,

It is interesting to try to work out what might be possible by induction...

First, how much magnetic field is needed for reception? Assuming we can make 
a receiver where external noise dominates, Paul Nicholson gave a typical 
figure of magnetic flux density of 30fT/sqrt(Hz) for the noise level. 
Assuming we use QRSS100 with about 0.01Hz BW, and 6dB SNR, we need to 
generate about 6 femto-teslas of magnetic flux density at the receiver to 
get a readable signal.

Now the transmitter - assume a "largish" loop means 10m diameter, and it is 
a single turn with resistance of 0.1R, or a multi-turn loop with 
proportionately less current. With 100W, the single-turn loop current would 
be about 32A when matched. My old physics textbook says that the magnetic 
field on the axis of a circular loop, at a distance that is large compared 
to the loop diameter, is

B = (mu0*r^2*n*I )/  2d^3

with mu0 = 4pi*10e-7, r = 5m, n =1, I = 32A, and B = 6*10e-15 T, d = 
distance in m

Rearranging to find d gives

d = cube-root-of( (mu0*r^2*n*I)/2B ) = 4.4km

So quite a long distance (have I got it right?!), making some quite 
optimistic assumptions. You could get a bit more by reducing the loop 
resistance, or increasing the loop size, power etc., but the cube-root means 
it would be hard to make a big increase in range. To reach DF6NM's 12km, you 
would need to increase the loop current to about 650A! The presence of the 
ground would probably have some effect. As has previously been pointed out, 
conductors such as mains cables or telecomms lines near the transmitter 
would have significant currents induced in them, and these could act as 
transmission lines carrying the signal much longer distances, but this might 
be thought of as "cheating"!

Cheers, Jim Moritz
73 de M0BMU


----- Original Message ----- 
From: "Roger Lapthorn" <rogerlapthorn@gmail.com>
To: <rsgb_lf_group@blacksheep.org>
Sent: Thursday, March 11, 2010 8:54 PM
Subject: LF: Induction DX?



> It would be quite easy for anyone to generate 50-100W or so and feed this
> into a largish diameter multi-turn loop. Correctly aligned and with a
> similar loop at the far end and using decent weak signal processing just
> what range might be possible using modes like QRSS or WSPR? Has anyone 
> tried
> this to see just what is practical, simply using induction fields?
>