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" <[email protected]>
To: <[email protected]>
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?
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