Absorptive Shielding?
 

I was wondering if there is a radio frequency absorptive or otherwise lossy
material with which one might line a conductive shield or use instead of it.
I have noticed when tinkering with my DX-394 receiver that when I add a
conductive shield so that it is largely enclosed there is an increase in the
coupling of spurious signals from one part of the radio to another, e.g.,
from the 455kHz IF stage into the LW internal antenna and other parts of the
input circuitry. I suppose this is because the energy that would ordinarily
escape through the plastic lid is now reflected back and is trapped inside,
thus raising the intensity. This may amount to a degradation of 3 to 6 dB.
I use galvanised steel for the shield because it is cheap, easily worked,
highly conductive and ferro-magnetic so it is a good material for keeping
out both electric and magnetic external fields. Likewise, it keeps the
internal ones in when it would be preferable to dissipate them. Any advice
on an absorptive or lossy material for use from LF to VHF?

Would a carbon spray (if there is such a thing) do the job?
How about the antistatic foam material semiconductors and other devices are
packaged in?
How thick would it have to be?

73, Tom

On Fri, 7 Nov 2003 21:52:55 -0500, "Tom Holden"
wrote:

I was wondering if there is a radio frequency absorptive or otherwise lossy
material with which one might line a conductive shield or use instead of it.
I have noticed when tinkering with my DX-394 receiver that when I add a
conductive shield so that it is largely enclosed there is an increase in the
coupling of spurious signals from one part of the radio to another, e.g.,
from the 455kHz IF stage into the LW internal antenna and other parts of the
input circuitry. I suppose this is because the energy that would ordinarily
escape through the plastic lid is now reflected back and is trapped inside,
thus raising the intensity. This may amount to a degradation of 3 to 6 dB.
I use galvanised steel for the shield because it is cheap, easily worked,
highly conductive and ferro-magnetic so it is a good material for keeping
out both electric and magnetic external fields. Likewise, it keeps the
internal ones in when it would be preferable to dissipate them. Any advice
on an absorptive or lossy material for use from LF to VHF?

Would a carbon spray (if there is such a thing) do the job?
How about the antistatic foam material semiconductors and other devices are
packaged in?
How thick would it have to be?

73, Tom

Tom,
I remember a carbon based spray coating that was available a few years
ago - it was for adding shielding to monitors and such. Don't know if
it's still available and can't remember who made it. Lord Corp makes
a conductive paint (Z307) that is used by satellite manufacturers; the
last time I purchased some about 4 years ago (I work for a satellite
manufacturer) the cost was about $50 a gallon with a $400 minimum
purchase. Probably not a cost effective option.

Antitstatic foam (typically pink in color) would not be a good choice
- it is made antistatic from adding a humectant (spelling may be off)
to the material so that it forms a mildly conductive layer on the
surface of the material. Surface resistivity is on the order of 10E12
to 10E14 ohms. The humectant can leech out over time and heat
accelerates the process; many of the materials use an amine that
deposits on the packaged item. In the electronics industry this has
caused solderability problemsand there is the chance of corrosion to
exposed leads as well.

Conductive foams are carbon loaded and are black in color, surface
resistivity is on the order of 10E5 to 10E7 ohms. While being useful
in the packaging & handling of SSD's (Static Sensitive Devices) a
pitfall is shedding of carbon particles - which in a powered device
can, in the worst case, become a source of short circuits. If you
want something easier to work with than metal you might consider a
Static Shielding film (3M makes this as does SECO and several other
manufacturers) which has an inner layer of antistatic film (thin gage
usually less than a mil) with a layer of vacuum deposited metal and
then a thin gage film (often polyester) on the outer surface.
However, unless you know of a local packaging company that is willing
to sell small amounts expect to spend some $$$.

There is of course, the aluminum or coppe foil option which is cost
effective and eliminates problems from particulate contamination and
outgassing (leeching) of humectant plus it offers increased
conductivity and shielding. Antistatic and conductive films & foams
do not provide shielding. So, in a nutshell - what you are using or
copper or aluminum foil would be your best bets.

Howard

Tom Holden wrote:

I was wondering if there is a radio frequency absorptive or otherwise lossy
material with which one might line a conductive shield or use instead of it.
I have noticed when tinkering with my DX-394 receiver that when I add a
conductive shield so that it is largely enclosed there is an increase in the
coupling of spurious signals from one part of the radio to another, e.g.,
from the 455kHz IF stage into the LW internal antenna and other parts of the
input circuitry. I suppose this is because the energy that would ordinarily
escape through the plastic lid is now reflected back and is trapped inside,
thus raising the intensity. This may amount to a degradation of 3 to 6 dB.
I use galvanised steel for the shield because it is cheap, easily worked,
highly conductive and ferro-magnetic so it is a good material for keeping
out both electric and magnetic external fields. Likewise, it keeps the
internal ones in when it would be preferable to dissipate them. Any advice
on an absorptive or lossy material for use from LF to VHF?

Would a carbon spray (if there is such a thing) do the job?
How about the antistatic foam material semiconductors and other devices are
packaged in?
How thick would it have to be?

73, Tom

Try some of that gray (colored) coated paper that comes with certain
microwaveable foods. It's used to brown the food inside it. The
technical name for it is a 'susceptor'. Don't know if it will work down
to LW though.


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Any advice on an absorptive or lossy material for use from LF to VHF?

Would a carbon spray (if there is such a thing) do the job?
How about the antistatic foam material semiconductors and other devices
are
packaged in?
How thick would it have to be?
=======
It is perhaps somewhat expensive ,especially for larger areas , but
conductive paint would do the trick.

Frank GM0CSZ / KN6WH

Tom Holden wrote:

I was wondering if there is a radio frequency absorptive or otherwise lossy
material with which one might line a conductive shield or use instead of it.
I have noticed when tinkering with my DX-394 receiver that when I add a
conductive shield so that it is largely enclosed there is an increase in the
coupling of spurious signals from one part of the radio to another, e.g.,
from the 455kHz IF stage into the LW internal antenna and other parts of the
input circuitry. I suppose this is because the energy that would ordinarily
escape through the plastic lid is now reflected back and is trapped inside,
thus raising the intensity. This may amount to a degradation of 3 to 6 dB.
I use galvanised steel for the shield because it is cheap, easily worked,
highly conductive and ferro-magnetic so it is a good material for keeping
out both electric and magnetic external fields. Likewise, it keeps the
internal ones in when it would be preferable to dissipate them. Any advice
on an absorptive or lossy material for use from LF to VHF?

Would a carbon spray (if there is such a thing) do the job?
How about the antistatic foam material semiconductors and other devices are
packaged in?
How thick would it have to be?

73, Tom




Carbon spray might work at microwave frequencies but I don't think it
will be thick enough
at the frequency you are working at. Tire rubber and bicycle inner tube
material works at microwave
but again I don't know about lower frequencies..

Bill K7NOM