Posted by bbin37 on
URL: https://www.es-forum.com/shielding-Notebook-tp1536902p1536952.html

--- In [hidden email], "Charles" <charles@c...> wrote:
> Hello Beau,
>
> I still maintain my meaning about those equations.
> Let me go further.
> I think it is a lot of bullshit.

Hi Charles,

I'm glad we can be frank with each other. I agree to disagree with
you. :-D

Note that another person in our discussion group, David Fancy, found
measurable reduction in his modem's transverse EM emissions by
wrapping it in metal window screen mesh. This mesh's aperture spacing
is much smaller than what is nominal for DSL modem wavelengths.
Overlapping of layers was probably a factor, too, though.

> No pun intended, but you cannot calculate the shielding effect.
> You can only MEASURE the shielding effect, and must do that with
> several different frequencies.
> As is done by prof. Pauli of the University of the Bundeswehr (Army)
> in Germany.
> He is the authority for testing shielding materials.
> And a lot of materials do react sometimes quite differently as
> expected!

I don't think we are really saying different things here.

> But a shielding material is not enough.
>
> We have to look very hard at the longitudinal waves, which pass
> right through shieldings.

My post should be taken as limited to non-ionizing, transverse EM
radiation.

I value our dialog and the wealth of new information you share.

Thanks,
Beau
note
> > that I conditioned the equation's application by an a priori
knowledge
> > of frequencies by saying "*If we can take* the computer's CPU
clock
> > speed as its highest frequency component...". Of course, I do
agree
> > that we can't know all the frequencies without measuring them
first.
> > However, if we assume a maximum for the frequencies a body is
emitting
> > we have concurrently placed a minimum on the emitted wavelengths.
> > Meshes with spacing smaller than this minimum wavelength begin to
> > appear as continuous surfaces without holes for transverse EM.
> >
> > Also, I want to emphasize that I am not saying a mesh provides
*total*
> > shielding of the transverse EM from a notebook computer.
Everyone,
> > please pardon me if I inadvertently gave that impression by saying
> > near the end of my previous message "...should readily shield the
> > transverse emissions in the above example."
> >
> > Let me be more complete. Given a mesh with spacing much smaller
than
> > the minimum radiated wavelength, the level of shielding is
dependent
> > on the characteristics of the mesh material -- its depth, its
> > permeability, its conductivity -- as well as the geometry and
> > frequency of the radiated field encountering it.
> >
> > If we have an aluminum mesh enclosure fitting our spacing criteria
> > with wire thickness of 0.25 mm, a 1 GHz transverse EM plane wave
> > encountering it will be approximately attenuated to around
> > 1/(2.718)^(100) its original amplitude after penetrating the
mesh. A
> > 100 MHz transverse EM plane wave will suffer attenuation to around
> > 1/(2.718)^(33) its original amplitude. In near-field conditions,
such
> > as a mesh around a notebook computer, the level of shielding is
more
> > complex to calculate, but it will exist to some extent for these
> > frequencies for both the electric and magnetic field components.
> >
> > Unfortunately, low-frequency transverse EM won't be hampered by
this
> > mesh even though it 'looks' like a smooth surface to it. For a
100 Hz
> > transverse EM plane wave would require the mesh's depth to be 36x
> > thicker (around 8.5 mm) just to attenuate it down to 1/3 its
original other for
> > the
> > > > computer will begin is:
> > > >
> > > > (0.3 m)/(CPU speed in GHz) or
> > > > (300 m)/(CPU speed in MHz)
> > > >
> > > > Smaller aperture sizes than this work much better. For
typical
> > square
> > > > mesh, divide by the square root of 2 to account for the
diagonal
> > of
> > > > the mesh being its largest aperture width.
> > > >
> > > > For example, if we had a 1 GHz CPU, then square mesh below (30
> > > > cm)/(sqrt 2) ~ (21 cm) will begin to confine the transverse EM
> > > > radiation. A 1/4" square mesh has as its largest aperture
width
> > > > (0.25") * (1.414) ~ (0.35") ~ (0.89 cm) which should readily
> > shield the
> > > > transverse emissions in the above example.
> > > >
> > > > Beau
> > > >
> > > > --- In [hidden email], "Glenn Coleman"
<glennhcoleman@h...>
> > > > wrote:
> > > > > Hello,
> > > > >
> > > > > I have thought of something similar, but to hook an external
> > > > keyboard, and
> > > > > use a wire-mesh fence between me and the notebook. I
figure if
> > wire
> > > > mesh
> > > > > works on microwaves it may work here too. I may even make a
> > whole
> > > > box
> > > > > around my notebook with notebook sitting in cage.
> > > > >
> > > > > If it does work, then I can find a finer quality mesh so I
can
> > see
> > > > monitor
> > > > > better. I was going to start with some wire fencing with
1/4" with
> > > > > wallpaper on its outside. I tested and know shielding it
will