I would like to build a faraday cage large enough to work in. Can anyone please advise on the following points.
Will aluminium window screen work as well as brass? Are there any more cost-effective materials?
How much does grounding influence the degree of shielding? I would assume grounding is more effective for lower frequencies where reflection is not as much of a factor. Can anyone quantify this effect in Hz?
Now-a-days most aluminium screen is anodized. Does the insulation this adds between each strand diminish its overall effectiveness in grounded applications?
I suspect the main problem with ali is getting a low-impedance join due to the oxide layer. You really need a solid seam join, which typically means welding/soldering/brazing, which is hard to do with ali
If aluminum, take chromatized one. It has a conducting surface. A good faraday has thw shield within each other joined at just one point and at the same point both shields are earthed. Iron is even better, by the way. It is better for lower frequencies.
There was a really good article in Compliance Engineering or one of the ITEM publications a few years back comparing various low-cost approaches to building screened rooms. Galvanized steel actually came out the overall winner in the estimation of the authors, but some of the really low-cost options considered were nearly as good at most frequencies. The resistivity of the steel provides some benefits, IIRC. Paul Mathews
The biggest problem is at the joints. You have to be able to make a good joint all along where the sheets meet.
Copper based materials can be soldered and this saves a lot of labor. Another option would be to put a screw every 1/2 inch but this would take forever.
Grounding is an issue of the common mode voltage. You want all of the grounds of everything inside the shield to be at the same voltage. You also want all the signals and power lines that come into the sheild to pass through the same point and be bypassed there.
The main reason to ground the shield is because it has a large capacitance to the earth. If the shield is at some AC voltage, currents will flow through this capacitance.
If the shielded space is a small fraction (lets say 1%) of the wavelength of a signal, the shield blocks the electrostatic component of the radiation but less so the magnetic. For the magnetic component, you can think of the shield as a turn of an inductor with some resistance.
I would stay away for anodized screen for this reason. You can also get material that is sometimes called "expanded". If you are working below 100MHz, if you go with aluminum, I suggest you look at it.
Even that plastic "bubble wrap" like insulation that has a metalized surface will block a lot of the electrostatic field from 60Hz mains. If you imagine an 8 foot cube with that stuff stapled to the inside. A box like that lowered the electrostatic part of the 60Hz down to about 1mV.
In many cases more layers of less good shielding is better. You most likely want at two layers that are connected to each other only at the ground point. This is much easier to build than 3 layers and not much harder than one. You need a frame to hold all this stuff up. This frame can be nonconductive. Both the inside and outside surfaces can be covered with conductive material. Dry wood works.
Not really. Iron is pretty darn resistive, making it about useless for RFI, and a mere screen isn't thick enough to block much EMI. (For that you need sheer thickness. Hope you have 2x8's for rafters!)
Tim
-- "Librarians are hiding something." - Steven Colbert Website @
Tim, the RF is no problem at all. The poenetration depth is inverse proportional to the conductivity, moreless. So if the 100MHz penetrate 200um instaed of 20um, who cares. And you're having two sheets of 1mm or so. I'm have to look up the numbers.
The skin depth is proportional to 1/sqrt(freq*mu*sigma), where sigma is the conductivity. Iron's high permeability is a help at low frequency, but not by as much as one would think from this calculation.
To think about shielding effectiveness, it's important to remember that the concept of skin depth comes out of a perturbation theory problem. The zero-order solution you start with is a perfectly conducting boundary, i.e. E and H are zero at the metal surface. We often think of RF shielding as a perturbation on the free-space problem, which leads to underestimation of shielding effectiveness, because it assumes that the fields on the metal surface are the same as the unperturbed free space fields, which is far from true.
We also tend to get confused by analogies with light shields, which makes us underestimate the importance of keeping the shield electrically continuous. A truly continuous copper shield of any reasonable thickness will eliminate all electromagnetic radiation from inside.
Iron is mostly important for quasistatic magnetic fields, e.g. from power transformers.
Years ago I came across some material that was being used for a commercial screened room. It was 3/4" plywood with galvanised steel sheet bonded to both surfaces. I don't know how they made electrical connection between the sheets. Boy, was that stuff heavy! I assume the sheeting is available off the shelf somewhere.
Most of the cages and rooms here are aluminum. New buildings were also constructed of aluminum backed drywall. Aluminum frames are used for the supports. I have no idea what question #3 is or means. Please explain. You did not explain the degree of shielding necessary and if there are strong RF fields around. Most cages here are made to surpress 60 Hz static fields, however the new buildings were made of aluminum drywall to help get rid of channel 13 TV across the street.
Some fittings (and sealing flanges on the doors) may be brass, but the classic screen room is bronze wire cloth. Same as used for screen windows on seacost locations (until cheap fiberglass came out).
galv sheet is avalable by the roll (in various widths) from the makers and, available cut to length from resellers, maximum width is probably 2.4m (same as plate),
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