Subject: Re (T) ground/earth/soil
Date: Wed, 30 Jul 1997 043313 -0500 (CDT)
From: Clas Tegenfeldt <tegen@bemi.se>
To: Multiple recipients of list <emf-l@mail.llion.org>
--------------------------------------------------
At 14:46 1997-07-29 -0500, you wrote:
>>[Clas]
>>The idea of shielding electric fields from eg. monitors by using a earth
wire
>>works for LOW frequencies but will not work for high frequencies (above
some
>>tens of megahertz).
>[Haldun]
>Is it correct to say that it works as long as we can think of fields
>rather than waves?
>10 MHz is 30 m and 100 MHz is 3 m, so the quasistatic (field) thinking
>starts to be wrong. A conducting shield smaller than the wavelength
>will not stop radiation because the waves will just diffract around it.
>At very smaller frequencies though, when we can think of them as
>quasistatic fields, it is possible to terminate the field lines with
>the shield.
[Clas]
I would like to say the earth wire and a partial shield does not work for
"radiation" since radiation can be defined as energy in form of particles
or waves that has /left/ the source. That requires that we are in the
far field, and the waves will both diffract around the edges as well as
being reflected throughout the enviroment and thus coming from almost all
directions.
>[Haldun]
>But it should shield again at higher frequencies:
>At 1 GHz, it is 0.3 m, at 3.33 GHz, it is 0.1 m and the shield may
>start to be able to reflect back the waves. This is the case for
>a solid conductor or fine mesh. But common monitor shields do
>not shield at these frequencies. What is the reason? Perhaps
>the material is no longer a good conductor at these frequencies?
>(It is certainly not a shield at visible frequencies!)
>
>Incidentally, NoRad claims high E-field shielding up to 1 GHz.
[Clas]
To some extent high frequencies may reflect back from a monitor filter
glass,
but then again it will be reflected by metals inside the monitor, the
tabletop etc.
I would like to clarify that the talk about distances isn't relevent here
when
looking at the filter hooked onto the monitor, that distance is very small
and
we are only talking about near fields up to quite a few gigahertz.
When we are looking at the length of the earth wire it should be clear that
we in almost no circumstances have less than 50 meter wire to a grounding
rod into the soil, this means that our (PE) ground is almost useless for
radiofrequency signal grounding.
Measuring the attenuation of the filter glass is done by transmitting
a signal with an antenna on one side and receiving it on the other
side of the glass. The reduction measured is the attentuation reported.
However, if you look what this really means, it is not that fun anymore!
The ground for the antennas and the filter glass as well as the
measurement equippment are common, often to a ground plane that everything
is standing on. Anyway, you make the measurement relative to the filter
glass ground, and THAT makes the attenuation keep up even for high
frequencies! In practice, in real life, you will NEVER get that kind
of attenuation for high frequencies!!!
It is like measuring the capacitance of an isolation transformer and
getting a result of less than one picofarad, you only get that kind
of result because you refer your measurement ground to the shield itself.
>>>[Haldun]
>>> *) However, to shield yourself from a source by using a conducting
>>> finite barrier, that barrier must be grounded to be able to attract
>>> from earth the charges to terminate the E-field lines.
>>> (Example: grounded conducting monitor shields)
>>> Haldun
>>[Clas]
>>You talk about generalized shields here, and even they do NOT need "earth"
to
>>work! What we are talking about is equalisation of charge distribution.
>>I would again like to stress that earth has no
>>special property (okey, we stand on it, but what the heck) but anything
that
>>has a large amount of free electrones will do. Why we use the earth wire
is
>>because it /also/ is connected to the electrical system as a reference.
The
>>only reason that reference also is connected to a grounding rod into the
soil
>>is because we want protection against lightning! OK?
>>Clas Tegenfeldt ,,,
>[Haldun]
>I agree that earth or soil is nothing special. Anything with a large
>amount of free electrons should do. How large is large enough I am
>not sure though. For instance, if the radiators and boiler were
>isolated from the earth/soil, would they be sufficient to ground
>digital equipment or a E-field shield? (Apart from lightning protection.)
>
>Are you implying that the monitor shield would work even without
>grounding? I believe that is not the case (?). An isolated metal barrier
>will not shield E-fields, in fact it may focus them.
[Clas]
Yes, everything with a large amount of free electrons will do. However,
think
about what happens. When connecting a shielding glass in front of the
monitor to the protective earth (PE), what you do is making it possible for
the
electric field to drive the electrons into that PE wire. What is that?
It is an electric current and thus we get a magnetic field around the PE
wire.
For successively higher frequencies the field around the PE wire will more
and
more easily radiate, which is just another way of saying that you just don't
put
high frequency signals into the "ground".
The radiator pipes would probably be too small, what you actually are doing
is to
spread the field out, it doesn't just disappear... If you want a really low
resulting field you will have to have a large body (surface) to connect to.
The monitor shielding glass works perfectly fine without PE if you have a
metallic
enclosure around the whole monitor and the shielding glass is grounded to
that
enclosure. The enclosure IS the ground in this case. The enclosure itself
doesn't need to be grounded, but according to safety rules it must be
connected to PE. By the way that enclosure is NOT a Faradays cage since it
has
cables leading into it.
By the way, the BEMI monitor I have constructed is NOT connected to
protective
earth. The electric fields around that monitor are non-existant by all
instruments
I have from 50Hz up to 1.2GHz... and it is not earthed, OK?
Clas Tegenfeldt ,,,
(o o)
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