Art Unwin wrote:
If on places a am/fm radio inside a box made of thin aluminum foil the
radio will be able to hear am broadcast band but not the fm band.
(Experiment by Harvard in Boston)

Yes, because the attenuation from an enclosure is made up of a
reflection loss and an absorption loss. The absorption loss is
proportional to both the thickness of the material and the frequency (
amongst other things). So a thin shield will have less attenuation at
low frequencies.

Intuition tells me that when using a perforated plate the lower the
frequency then the smaller the perforations in the shield to create a
blocking effect. This would, I believe, opposes the progression of
skin depth with respect to frequency.

Why does intuition tell you that? My intuition tells me that you need
smaller holes as you increase the frequency?

Also when you refer to the size of holes are you referring to their
diameter, spacing or both?


The books state for a mesh shield the perforations should be less
than 1/10 of a WL which on the surface opposes the results obtained by
the box experiment!

Why? The box had no holes! Adding holes will just degrade the screening
from the starting point of a continuous screen. The larger the
diameter of the holes the worse the screening will be degraded at a
particular frequency, and the degradation will also be proportional to
wavelength, large holes wrt wavelength, the more signal will leak through.


Jeff