Steve and group
Aluminium kitchen foil is much thinner than the skin depth at these
frequencies, so you are getting nowhere near the full screening effect.
Skin depth is given by 503.SQRT(Rho / Freq) where Rho is the resistivity
in Ohm metres, 2.7E-8 for Aluminium, and Freq in Hz So the skin depth at
137kHz is 0.22mm.
I measured the thinkness of some (Sainsbury's own brand) foil at 0.03mm.
This equates to 0.13 skin depths. The skin depth is defined as the point
where the current penetration has fallen to 1/e or 37% of its surface value
[ EXP(-1) ] , so in this foil the field is only reduced by a factor of
EXP (-0.13) = 0.87. Therefore only 13% screening. You would get a more
significamnt effect by using sheet aluminium , or several layers of Al foil.
I wouldn't have thought it was necessary to use such a large area either.
All you are really trying to do is prevent penetration of the fields from
the coil into lossy ground (skin depth many metres|), so a diameter not much
more than twice the coil radius would probably do.
From: Steve Rawlings <[email protected]>
I then moved the loading coil to one side, and put down two
of aluminium foil, each measuring 450 mm x 1600 mm. I left a
gap of 30 mm between the strips, so the overall area covered
measured 930 mm x 1600 mm. Four lengths of timber were then
placed along the edges of the aluminium foil to hold it