Re: Thickness of copper and largest holes allowed when blocking 13kHZ RF signal

The thickness of copper depends on the permeability and permittivity to 
block the radio waves.The waves can penetrate water to depth of 40m.
The wavelength of 13Khz radio wave being 2.3 myriametres(23060m),one 
concept is the Beer-Lamber law:

 Intensity after penetrating the surface = original intensity multiplied 
by exponential power of -(u/p)pl.
u=Attenuation coefficient of copper.
p=Density of copper=8.94 g/cm^3.
l=Distance penetrated by the wave.

The mass attenuation coefficient of copper(u/p) is 1.057E+4 to 9.413E-2.
So the attenuation coefficient is 94495.8 to 0.8415222 per cm.
I=I(original)e^-ul.
For the radiation to get reduced to its half value,
0.5=1.0*e^-(0.8415222*l)  [u taken as 0.8415222 for better blocking].
ln 0.5=-(0.8415222*l),
for which the thickness is 0.82 cm[Half layer value]

To get reduced to 90%,the thickness is 2.73 cm.
And for 99% reduction , it is 5 to 6 cm.
More the value of attenuation coefficient,more is the radiation weakened.
Copper in continuous layer is negotiable for hole size,since the hole size 
should be smaller than the amplitudes of electric and magnetic fields of 
the radiation(for the Faraday's cage used).


References:
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Frequency wavelength calculator.
Physical measurement laboratory tables.