Optimal Faraday cage

For similar purposes I have a large thick corrugated carton that is covered with copper foil. Draped around the edges so the lid makes a tight seal. So far any sort of radio goes totally silent in there.

A word of caution: In certain weather conditions removing and closing the lid can generate an impressive blue spark, often accompanied by a succinct *BANG*, so be careful. Maybe keep it grounded together.

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Wouldn't the aluminum foil be passive to the magnetic component?

"Greegor"

Wouldn't the aluminum foil be passive to the magnetic component?

** Aluminium or other non-magnetic foils will have no effect on a static or slowly varying ( ie 50 /60Hz) magnetic field.

A static electric field is defeated because the charges on the surface of the foil become re-arranged so as to cancel any static field INSIDE the enclosure.

A radio frequency wave is very much attenuated inside the enclosure too - the surface charges respond to the wave's electric field and MOVE in synch with the wave thereby creating a magnetic field that opposes the magnetic part of the arriving wave.

..... Phil

That should work fine. You'll need good feed-through filters to get signals and power in and out. The "door" is the biggest problem.

One advantage of screen is that it lets light and air in, and is about as good an EMI shield.

OK, or just use a "screen window."

That won't help.

John

There's almost no practical way to shield from external low-frequency H fields. Even steel boxes don't help much unless they are insanely thick-walled. Just keep the whole thing away from any transformers and such; unlike EM fields, H fields fall off very rapidly with distance.

Foil will shield against higher-frequency H fields, the skin depth thing.

John

It is no simple to make good shielded chamber. Problem with self adhesive copper tape is unreliable surface contact; aluminum is MUCH worse in this regard. After some time, tape can start leaking although the contact could seem good. The most difficult part is RF sealed door. It must have perfect contact over all perimeter; otherwise the chamber will leak badly.

VLV

Fingerstock will help a lot, but the door and the frame needs to be tightly matched with each other, so if the door i damaged, you can not just replace the door, but also the frame must be replaced that fits tightly with the new door.

Iron filings are of little use; suggest Nickel Carbonyl. Better yet, use carbon loaded foam like what Jameco carries.

...Or have insanely high permeability. There should be a reason why particle accelerators stainless steel pipes (in which beam travels) are wrapped in so-called mu-metal foil.

The carbon loaded foam can be a useful absorber inside the chamber.

For a cost effective functional shielded chamber you should start with Ralph Morrison "Grounding and Shielding" and Henry Ott "Noise Reduction Techniques In Electronic Systems". Ralph specifically addresses shielded chambers and Henry provides a good complementary discussion of shielding.

One of the first requirements is conductive seams, so as others have noted aluminum is out (unless you can weld all the seams). Steel sheet metal is fairly cheap, easily cut, bent and nailed to a wooden frame, and then the seams can be soldered. But the devil is in the details and the details best found in the above books and perhaps a look at shielding gasket suppliers literature.

Only for magnetic interference. All you need is low Z for electrostatic.

The most common are home brewed aluminum screening with aluminum frame. Sometimes steel is used but aluminum is better for the frame. Most I see are for lower frequencies.

greg

Magnetic shielding is just a matter of budget. In a shield can the size of your average water heater, you get about 20 dB per layer of mu metal.

At the OP's size it will likely be about the same. Entire rooms have been made into 3 layer shielded rooms. NIST is making a new one. Smaller areas can be done for budgets that mere mortals' companies can afford.

If you want to do better you can add an active filed bucking system outside the shield can. You use something like a magneto-resistive sensor to sense the field going into the can and a coil to bring that down to zero.

At 60Hz, a layer of aluminum outside the can makes a significant difference. The can looks like the core in a coil raising the inductance and lowering the corner frequency. Many years back NASA made a 6 layer can with 3 of the layers made from about 1/16th copper sheet. It knocks the 60Hz down by something like 80 dB.

On Feb 25, 8:05=A0pm, John Larkin wrote: [...]

Doing the power runs with shielded cable helps a lot. You can use filtered connectors with metal shells to go through the walls. By grounding the body of the connector, you get a layer of shielding basically for free at the connector input. A tube extending from the face by a few feet to move the weakest point a few feet away can also help.

Multiple layers of screen work quite nicely against RF. It is hard to maintain a high mu and let air in but at low frequencies a "duct" leading away to the the air source can be made to keep the lines of force out of the working area.

A nice thing about light is that it can go around corners and through holes that RF can't get through.

Another way to go is to use a DC powered light bulb inside the shield, or to only have the lights on while you set up and then switch off before you measure.

Small things are easier to shield, and pipes/cylinders are easier than other shapes, like boxes with seams and corners. Wrapping a meter-cubed cardboard box with one layer of mu-metal foil would be very expensive and wouldn't do a lot of good. An external field, given the choice, would just as soon shortcut through the big interior as to take the long way around through the high-mu but thin foil... and seams/doors/windows make it worse.

The point being that one should do the math before attempting magnetic shielding to avoid spending a lot of time and money on what may be a small benefit.

Most emi screen rooms are not magnetically shielded.

As you note, thick copper is a decent eddy-current shield at 60 Hz and is practical for shielding small volumes. The copper foil on a pc board has a magnetic penetration time constant in (as I recall) the hundreds of microseconds.

John

I've always wondered how well a 55 gallon steel drum would work for this sort of thing. You'd want to remove the paint where the removable top fits onto the drum... and then the thing would start to rust...sigh. One nice thing is that you could have different tops with different fed throughs.

George H.

A space withing a room is nice. You can build a good sized outer room using aluminum backed drywall with steel studs. I have seen a very large room fitted with 1/4 inch steel plate on the bottom and edges, to reduce magnetic field from a MRI below. I could get a steel nail to move around on a string pretty good while walking above that small MRI unit. Of course and small vibration of electronic devices could induce electrical noise at the vibration frequency.

greg

An active system can be pretty simple and still make a big difference. A friend and I did a few experiments measuring the magnetic field from high tension lines. He built a 3 axis AC magnetometer ( three orthogonal coils and some circuitry to combine and display ). We measured the AC field at my house. Major power transmission lines ran in front of the house. The AC field was actually less than at his house where there were only residential power distribution lines.

Any way we laid out some house wire in a circle about 25 foot diameter. Hooked it up so we had several turns of wire and fed it with a filament transformer feed by a variac. We could reduce the AC field to near zero as I remember. It has been a long time. And concluded that an active field canceling system would work well.

Recommend finding Mil HDBK 419A on the web. It is entitled " Grounding, Bonding, and Shielding for Electronic Equipments and Facilities " . As I remember it has a chapter on building screen rooms.

Dan

Make whatever box you (he) want(s), and remember to fill each seam, during mating, with silver filled, electrically conductive epoxy like what they use to attach IC chip dies to their substrate like: EPOTEK H20E

'They' have chambers big enough to put an Osprey or even a C-17 into. The 'cones' are like 3.5 feet tall, and it is like the area of a couple football fields. NOTHING gets in OR out. Every signal read within there originated in there. And it is nowhere near Vegas. :-)