Hello.
I have a plan to make new version of Faraday cage for gas discharge circuit. Unlikely to current cage in which radiation sources such as Thyratron and capacitor are just proximate to some electronics related to discharge triggering, This new box will have additional shielding room for these sources as seen in the attached images.
At this moment, I don't know which is good way to get more effective shielding. one image (1st case) shows additional shielding room shares one side to whole Faraday cage while the another (2nd case) is isolated type which is connected to inner surface of the cage via wire.
In 1st case, What I'm worrying is the possibility that EMI directly hits the cage thus cages are all fluctuated in voltage if EMI is severe.
Actually EMI is severe. When we enclose the capacitor with aluminum foil, foil is shaken!
Could you tell me anything about these two design?
In 2nd case, does external surface of the isolated shielding room keep itself equipotent no matter what happens inside?
Dear Lady,
of course we have unwanted effects. We have the resonances in the Farraday shielded box. It is important, that you know the geometry and the mode and the source frequency, so you can calculate the resonance frequency. In this resonance frequency the shielding have lower values. This is bad.
All the best
F. Gräbner
The second choice would be better I think at least from the viewpoint that the lead connecting the two shielded enclosures would be removed. If it is there it can carry current from one zone to the next and also it can radiates in the zone between the two enclosures and also pick up any EM field that is there and induce currents on the lead that can be propagate into the "shielded" zones.
Also, (of curse dependent upon the frequency of the EMI and the dimension) aluminium foil is rarely the good to use as there are often gaps in it wherein EM fields can penetrate. In same cases the aluminium foil can act as an antenna and worsen the shielding effectivness!
In addition, (and this is often forgotten) any EMI that leaks into the zone between the boxes can be amplified (if the outer shield is very good) due to a large Q value. basically we will have get a reverberation chamber.
Use proper solid, metallic enclosures to get best results.
Hm..good points. Unwanted noise amplification within the cage is my another concern. I hope there is a sheet or some material to absorb fre MHz radiation.
I have one fundamental question. If the cage surrounds radiation, what happen on external surface of the cage? Is it fluctuated in voltage? I guess it is. This is opposite case that electrical field inside the cage is zero when radiation is externallu incident on the cage.
Hi
You can use copper or galvanized steel sheets for the inner chamber, with fully welded edges or alternatively assembled with many small bolts with distances of 40 mm or so. The copper or steel will reduce both electric and magnetic fields. The inside of the inner chamber will be electrically separate from the outside of the inner chamber. There will be some currents, but only in the very thin surface "skin" of the material. I think the inner box and the outer chamber should be kept separate, no need for grounding unless it is for safety reasons. If you need to pass any supply wires through the copper sheets then you must use two high quality "through hole" type of capacitors. If you use two wires in the vicinity of the noise source, the two wires must be twisted so they have less loop area. Optical cables are always smart to use for measurements etc. as they will not conduct any noise. You can drill a small hole like, say, 6-7 mm in the metal sheets, this will not let the electric noise escape and it will let the optical cable pass the enclosure wall. If you have sensitive equipment outside the room then this can be protected against the magnetic field with boxes of "mu-metal" which is a special, optimized kind of soft-iron plate, which is heat treated after the construction of the box. It will not work well near the source because the impedance of the field is very low there
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