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

On Mar 21, 5:42*am, Jeff wrote:
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.

Thanks first for your thoughtful response. The article mentions only
skin depth and frequency applied so in effect frequency is the only
variable
Agreed
The above states that as you go down in frequency (a longer
wavelength) that it becomes
easier for propagation to be available at the other side. Stated
another way the deeper the
the skin depth the more penetration occurs

* *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?

The experiment shows that it is easier for the lower frequency to
penetrate to the other side than the higher frequency. Or another way
as you move higher in frequency a better blocking effect occurs. If I
add perforations the same progression arises with respect to hole
diameter. (Note in the experiment we only have two variables, skin
depth and frequency, everything else is seen as a constant by the
experimenter. Not knowing the thickness of the screen could possibly
bring us into the situation of circuit boards where the skin depth is
deeper than the trace thickness but that may be a red herring)
From my perspective adding holes will provide the FM wavelengths more
leverage
to penetrate to the other side!


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

I was comparing voids to bulk.



* * 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..

agreed

Jeff

I used the perforations as a method of reducing the screening in both
cases to make a point

Jeff, I am stating that my path of thought took me into a different
direction from the books.
Thus I have to assume that my logic or aproach is in error. The
question thus is where is my error , That is where I need direction?