Carbon Black EM Enclosure

[rosetta]

Is it possible to use "carbon black" or materials containing it to produce an EM enclosure that will act as a cavity resonator? If so, any info on permissible freq ranges?

 

[nbucska]

A cavity must be good conductor -- carbon black isn't.

<nbucska@pcperipherals.com>

 

[rosetta]

First, I do not expect to observe the same &quot;energy&quot; efficiencies as with metal structures, just the feasibility at low power levels.

Second, many electrically conductive polymers and composites are effective EMI shields, thus demonstrating they have electrical properties that emulate metals, and many of these do not contain metal additives.

Third, I am constantly reminded that metals have very low DC resistance, but then at even moderately high frequencies it is the &quot;skin effect&quot; conductance that is important, thus the AC resistance is the factor to be considered.

In conventional metal waveguides the following has been found to be true, &quot;The distribution of current density in the surface of the conductor will vary with depth into the conductor according to an exponential law (Skin depth).&quot;

One might think the conductive bonds in non-metallic conductors act in the same manner. I find that this issue is being studied at the following sites:

http://www.physics.ohio-state.edu/~ppl/shield.html

&quot;Applications of polymers and molecular magnets - low frequency electromagnetic shielding and resonators/absorbers&quot;

and at:

http://www.zipperling.de/Research/Synthmet/Part_A4.html

where they have found, &quot;At high frequencies, by contrast, the measured shielding efficiency tends to increase with frequency earlier than theoretically predicted.&quot;

 

[CapnGene]

While carbon black is an excellent conductor (carbon is a metal), when you support it in a plastic medium (epoxy, polyester) the dielectric of the prinforcing pastic with the carbon black creates an artificial dielectric with lossy properties. I have used this to make microwave loads very effectively.

I suggest you use a carbon filament or carbon mat material. These, when embedded in a resin still mmaintain their electrrical conductivity properties.

have used carbon tissue and carbon mat for years to make carbon reinforced plastic reflectors and waveguides. I have used the waveguides through 5 Ghz and the reflectors through 40 gHz. I have not tried to use them as waveguide above g5 gHz because I had no need. I see no reason why they would not work.

This should work efficiently as a screening material.

Gene Augustin
TECHNICAL SYSTEMS ASSOCIATES

http://www.tsa-orlando.com

 

[rosetta]

Your use of carbon in the form of tissues and mats confirms what I have been reading from a number of sources. Some of the literature I reviewed discussed its use as shielding and this indicated that it would &quot;contain&quot; an EM field, but no one identified it was suitable to use as a waveguide.

It appears that a crude waveguide could be made by &quot;painting&quot; a layer of carbon on a dielectric, and it could be very thin.

Thanks FrankM

 

[zeitghost]

Do you mean a coating like that Aquadag stuff they put on the outside of CRTs?

rgds
Zeit.

 

[CapnGene]

I believe Aquadag is a brand name for carbon disbursed in a water base medium to allow it to be painted on a surface. I've used it to make microwave loads about 35 years ago, but have not used or seen it since.

I you want an inexpensive shielding material, you can buy aluminized mylar film in rolls. It is a vapor deposited film of aluminum on mylar. If you dont crinkle it, it works fine. It becomes relatively transparant if you crinkle it.

Gene Augustin
TECHNICAL SYSTEMS ASSOCIATES

http://www.tsa-orlando.com

 

[rosetta]

The carbon based &quot;paint&quot; seems to be used for a number of applications, one of the biggest being inside and outside coatings on CRTs. That makes them pretty good sized Leyden jars, a convenient filter capacitor and a gatherer of secondary electrons.

http://www.marmoset.net/radio/howto/aquadag/

http://www.tpub.com/content/neets/14178/css/14178_89.htm

Using &quot;carbon paint&quot; in a search identified many product varieties. Thanks 'zeitghost' and Gene Augustin for your help.