Power line EMI filter + feedthrough filter for a Faraday cage

  • #26
berkeman
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I am beginning to think that only criminals and government cipher machines actually need screened rooms.
We rent time at a local lab's shielded anechoic chamber for our FCC-type radiated EMI testing and certification. It is used both for radiated emission compliance, as well as EN 61000-4-3 RF Radiated Immunity testing.
 
  • #27
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The wall of a shielded room will need to be a double wall.

Is that because of "the futility of trying to reach perfection"? Again, what is the problem by following theory? Papers show that double walls don't double the shielding effectiveness.

When it is needed, the required specifications will be known, and will certainly not require attenuation of the band from DC to Daylight

Good point actually, and you can see that I told berkeman that I can answer it in private only.

One needs such a shielding enclosure when the adverse frequency isn't really known in advance.
Criminals require a shielding enclosure if (and only if) they use some kind of electromagnetic weapon against their target (but it is widely accepted that is a conspiracy theory isn't it?). So the priority would be to see who have such weapons and then who builds shielding.
 
  • #28
Baluncore
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I accept that there are situations where EMI testing and certification actually specify that a screened anechoic environment be used. It is only the certification process that requires the controlled environment. Later, the instrument and the certificate work quite well outside.

It is commercially very sensible to only rent a certified instrumented screened room when required by certification standards. The premium instruments used in screened rooms are expensive to buy and maintain, partly because they must be repeatedly certified as meeting specifications, which takes time and requires several other certified instruments to complete the task.
 
  • #29
Baluncore
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Papers show that double walls don't double the shielding effectiveness.
Can you reference those papers.
Two separated walls, having a separate medium in the middle, with the inherrent two mismatched impedance interfaces, will usually outperform one wall having twice the thickness.

Criminals require a shielding enclosure if (and only if) they use some kind of electromagnetic weapon against their target (but it is widely accepted that is a conspiracy theory isn't it?). So the priority would be to see who have such weapons and then who builds shielding.
No. The use of RF equipment may radiate energy and information that can be easily followed by law enforcement. Wrong-doers get caught because they are naïve and so do not have sufficient shielding. They foolishly transmit using codes or scramblers that draw attention to themselves.
 
  • #30
27
4
Can you reference those papers.

I remember reading that in more than one document, those showed that when doing precisely, the 2nd layer improves SE by some (30 or so) dB, but it will be less than double.

"LAMINATED AND NESTED SHIELDS Occasionally, it is assumed that the SE of enclosures can be doubled by doubling the walls or by adding shields contained within the enclosure, Fig.9. It will be demonstrated that the resulting SE is less than double."
https://www.ipen.br/biblioteca/cd/ieee/1999/Proceed/00350.pdf

Also here: http://www.jpier.org/PIERL/pierl01/06.07110706.pdf

No. The use of RF equipment may radiate energy and information that can be easily followed by law enforcement.
It depends on what type of criminals we are talking about. Some of them keep up with times. I agree that a full-blown energy weapon would burn things like metal objects, or radar frequencies would bounce off metal but there are other ways to commit crimes undetected (implants etc..)

btw. Wouldn't a two-layered wall cause resonances and worsen SE at specific frequencies?
 
  • #31
Baluncore
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I remember reading that in more than one document, those showed that when doing precisely, the 2nd layer improves SE by some (30 or so) dB, but it will be less than double.
Double the energy is a change of 10*Log(2) = 3dB. I think you were expecting doubling the dB which is squaring the SE?

According to your ref; Kistenmacher and Schwab, 1996. IEEE. Page 351.
My statement is shown to be true for conductive non-magnetic materials, for Cu see fig. 12.
The effectiveness increases with frequency. Note the plot there assumes the same total mass of material.

My statement is false for magnetic resistive screen materials. See fig 13. Which is what you expect from ferrite slabs that are designed to allow EM propagation between the magnetic grains.

Your reference, the Bahadorzadeh, Moghaddasi and Attari, 2008, paper, appears to be discussing cascaded apertures in the wall of a cavity. Figure 2 shows two walls give about a 12dB advantage over one. That is significantly better than a factor of two and supports my suggestion that the same mass distributed amongst several walls is better than one thick wall. The paper goes on to show increased wall separation increases screening.

btw. Wouldn't a two-layered wall cause resonances and worsen SE at specific frequencies?
Resonant cavities need coupling to be part of the circuit. A 100mm thick timber stud wall supporting and separating two conductive sheets will not support a significant resonance. Galvanised steel is cheaper than copper and can be easily soldered along the seams.
 
  • #32
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4
Double the energy is a change of 10*Log(2) = 3dB. I think you were expecting doubling the dB which is squaring the SE?

Yes, for power 10*Log(2) = 3dB is the double.

Let's say that an enclosure with a single wall has an SE of 60dB. It seems logic that a nested enclosure (a double walled one) have an SE of 120dB, which is the doubling of dB (that is what I was expecting yes).

We can agree that double walls (the nested ones) improve SE anyway. What I wanted to ask that when done correctly, do I really need nested enclosures?

Resonant cavities need coupling to be part of the circuit. A 100mm thick timber stud wall supporting and separating two conductive sheets will not support a significant resonance.

What about something smaller like 20mm? Are there any rules on this, or do I have to do simulations to know the resonances? I didn't know that resonant cavities need coupling (you mean electrical connection?).

Galvanised steel is cheaper than copper and can be easily soldered along the seams.

+1 Yes, galvanised steel seems to be solderable.
 
  • #33
Baluncore
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It seems logic that a nested enclosure (a double walled one) have an SE of 120dB, which is the doubling of dB (that is what I was expecting yes).
When you cascade two attenuators you square the attenuation, unfortunately that is not directly applicable to double walls. Is it better to use two conductive walls of half the thickness or one thick wall? Probably more walls, but it involves understanding SE as a function of wall thickness, which is frequency dependent.
A conductive wall has external circulating RF currents due to the external world on the outer surface, with internal circulating RF currents due to your activities on the inner surface. Those currents induced in one face only diffuse very slowly (100 m/s) through the material due to back emf, hence skin effect. You need to keep those surface currents separate which in reality comes down to the quality of the aperture seals, which provide the only short path around the wall.

That means your door and vent seals must be very reliable, because any break in conductivity concentrates surface currents and allows them to wrap around the edge of the conductive screening sheet. Keep apertures away from external edges where circulating currents are naturally higher, so the door should be in a face. It must have metallic wipers that eliminate the conductivity gap when closed. Any imperfect seal becomes a slot antenna or an aperture coupling inside to outside.

With double walls the door will also have two insulated conductive surfaces, each with it's own wipers. The only ground connection between the double walls will be the conduit used for the power supply which is the middle part of the cascaded filter box.

There is a construction requirement. A timber stud wall, floor and roof will all be self supporting. That suggests independent inner and outer conductive walls can be attached. Fold the material around the edges so your conducting joints are on less stressed flat faces. Avoid TIG or MIG welding, use plumbers solder along all the seams.

What about something smaller like 20mm?
You are talking waveguides with cutoff frequencies. But you must inject the energy and then couple it out again so avoid apertures.
 
  • #35
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This is a great link, thanks. I like the way the panels joined, you're right, this method is excellent.

What about something smaller like 20mm?

I meant, what if the space is smaller than 100mm between the walls. 20mm would be a bit small but something like 40-50mm? Is there any rule?
 
  • #36
Baluncore
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The shielding problem is hardest to solve in the audio frequency magnetic range. A current induced to flow around an outer face will couple into a parallel inner wall in the same way that a single turn transformer operates. Separation distance is not going to be critical so long as there is only one ground connection to an adjacent shell to prevent ground loops.

I think the design key is that a structural wall needs some thickness. Use that thickness to your benefit but do not exaggerate it. If you are building a portable room it will have cellular construction. If your room is fixed inside an existing building it can be an isolated double wall enclosure. There is a limit to the attenuation required, somewhere around the thermal noise floor in your instruments. Enough is sufficient, an extra layer of foil will not hurt, but it will cost time and money, and makes no difference to the shielding.


You should take a look at these two publications I found on the web;

1. Theory, Design And Engineering Evaluation Of Radio-Frequency Shielded Rooms. Aeronautical Electronic and Electrical Laboratory. REPORT NO. NADC-EL-54129 13 AUG 1956. BUREAU OF AERONAUTICS TED Project No. ADC EL-538. 125 pages, 10.2Mbyte.

2. Military Handbook. Radio Frequency Shielded Enclosures. MIL-HDBK-1195. 30 SEPTEMBER 1988. U.S. Navy. 86 pages, 370kbyte.

Also, if you can find a copy, see; Architectural Electromagnetic Shielding Handbook. A Design and Specification Guide. Leland H. Hemming, 1992, IEEE Press. 229 pages.
 

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