Faraday cages and EM shielding

[mac]

This an easy experiment to demonstrate that a metal box is not always going to stop EM fields.
Go to your kitchen, put your cell phone inside a cookware and cover it... please, do not put it on the fire!
Dial up your cell number from another phone. Yes, it is going to ring... most likely.
You can put a pocket radio inside the cookware and it's very likely you will hear it.
What's going on? Isn't the cookware like a Farday cage?
Yes, but...
The concept of the Faraday cage is that an external EM field rearranges the free charges on the metal, sending the positives charges to the surface near the negative external field. This charges creates an EM* in the other direction cancelling part of the external field. I say part because there will always be rebell electrons that won't move to the surface of the box. In other words, it is impossible that all the energy of the external field can move the exact amount of charges to the surface to create an exact but inverted copy of itself. Think energy conservation and thermodynamics.
Or look it in another way. Wave physics tells us that the wall of the box is a finite energy barrier and that there is a chance that the wave can go to the inside.

There are another issues that are relevant, like circuit gain, layout design, resonance fx with external freq, etc, and intensity and freq of the EM.

For most cases a 125B box is enough to stop RF, or a plastic box with some shielding inside.

http://en.wikipedia.org/wiki/Faraday_cage
http://en.wikipedia.org/wiki/Electromagnetic_shielding

mac

[R.G.]

This an easy experiment to demonstrate that a metal box is not always going to stop EM fields.
Go to your kitchen, put your cell phone inside a cookware and cover it... please, do not put it on the fire!
Dial up your cell number from another phone. Yes, it is going to ring... most likely.
You can put a pocket radio inside the cookware and it's very likely you will hear it.
What's going on? Isn't the cookware like a Farday cage?
Yes, but...

]
Yes, it is something like a Faraday cage, but not quite. Try (a) using a copper pot, not stainless steel; resistivity matters and (b) sealing the junction between the lid and pot with copper tape which makes good ohmic connection to both lid and pot.  The crack between lid and pot lets through all RF with a wavelength less than four times the length of the crack or diameter of the crack, depending on polarization. Sealing the crack seals off this pathway.

Metal boxes do not necessarily stop EM fields. And they do not stop EM fields when you put an effect in them and run long antennas (i.e. guitar cords) out into the RF field. They're pretty good on 60Hz though.

[merlinb]

It is also important to recognise the difference between a magnetic shield and an electric shield (Faraday cage).

To stop low frequency magnetic fields you need a box of high permeability metal, and there must be no cracks or seams which cross the path of the induced flux (cracks along the flux are OK).To stop high frequency magnetic fields you're unlikely to get enough permeability, so you need high conductivity instead, so that currents induced in the shield hopefully generate an opposing field to the one you're trying to stop.

To stop an electric field, the shield must be connected to some part of the circuit you're trying to protect (e.g., the shield must be grounded), otherwise it does nothing. The shielding improves with conductivity. Also, it doesn't have to be a continuous barrier like a magnetic shield, instead it can be a cage or similar, as the bars of the cage concentrate the electric field towards themselves.

[brett]

Hi
I'd be interested in hearing whether a metal pot works as a shield for a phone if it is earthed.
My guess is that it would.  (I am in an analogous situation at work, where there is too much grounded steel re-inforcing in the building for the phones to work)
cheers

[mac]

Yes, it is something like a Faraday cage, but not quite. Try (a) using a copper pot, not stainless steel; resistivity matters and (b) sealing the junction between the lid and pot with copper tape which makes good ohmic connection to both lid and pot.  The crack between lid and pot lets through all RF with a wavelength less than four times the length of the crack or diameter of the crack, depending on polarization. Sealing the crack seals off this pathway.

I tried with those aluminum pressure cookware. Fair good contact and seal between lid and pot, as the lid gets 1 inch inside the pot, with/without grounding.

I had to put one small cookware inside a bigger one, like a russian babushka, to completely isolate my cell from the outside world.

Hi
I'd be interested in hearing whether a metal pot works as a shield for a phone if it is earthed.
My guess is that it would.  (I am in an analogous situation at work, where there is too much grounded steel re-inforcing in the building for the phones to work)
cheers

Brett,
Do you mean the steel mesh used to re-inforce concrete?

mac

[electrosonic]

Sort of on a tangent but...

I was told by a police detective that they have special shielded envelopes which they immediately place blackberrys and iphones in, when they arrest gangster types. They were finding the pdas were being remotely wiped (erased) before they could get any data off them. Is this plausible?

A.

[merlinb]

I was told by a police detective that they have special shielded envelopes which they immediately place blackberrys and iphones in, when they arrest gangster types. They were finding the pdas were being remotely wiped (erased) before they could get any data off them. Is this plausible?

Yes it is; here are some home experiments you can do with Faraday cages:
http://www.juliantrubin.com/bigten/faradaycageexperiments.html

I made an error in my earlier post. A Faraday cage does not need to be grounded if you're trying to protect something inside from a field that is outside. However, if you want to stop a noisy circuit from spewing an electric field into the outside air then it must be enclosed in a  grounded cage.

(At least, I think that's right... )

[mac]

I bet there are some materials, somo non metallic, that are better than copper or aluminum for EM shielding.
The envelope question reminds me the painting of stealth bombers which absorbs radiation.
My guitar has ferrite painting to isolate the circuitry.

I posted about this because I have trouble getting 125 or 1590 enclosures, and I am experimenting with pcb and wood boxes with some kind of isolation, and comparing the results with aluminum boxes.

mac

[PRR]

> the shield must be connected to some part of the circuit you're trying to protect

I was gonna say: imagine a battery power transmitter with a perfectly balanced but short dipole antenna. The perfect cage will sum the two sides of the dipole.

> enclosed in a  grounded cage.

Where is this circuit getting power?

If the power source is inside the cage, a "ground" (whatever that may really be) is not needed.

If the power comes from outside, you probably (not always) get better shielding if the cage is tied to the ambient electric field, generally by tying to power-source ground (wall outlet 3rd pin).

> it can be a cage or similar, as the bars of the cage concentrate the electric field towards themselves.

Somewhat. The openings should be smaller than the waves of interest, AND much smaller than the depth of the cage. Concrete rebar grid might be OK for audio if you stay several yards back of the 4" openings. In practice, copper fly-screen is effective and easier to handle.

> wood boxes with some kind of isolation, and comparing the results with aluminum boxes.

Good hardware store, get METAL (not cloth) Duct-Tape. Thick aluminum foil with stickum. About $7 for a lifetime supply. Overlap generously, bond to circuit ground. This will usually work almost as good as aluminum or zinc boxes. (Ideally you would have an electric bond instead of a gluey overlap; but capacitive coupling is often sufficient.

> I bet there are some materials, somo non metallic, that are better than copper or aluminum for EM shielding.

No.

If Magnetic effects dominate, multiple layers of Iron/Nickel are best. No holes!

Capacitive effects dominate small audio. Here you just want great conductivity at decent price. Multiple layers have a place... but not when you run wires out of the box over/around nasty wires.

> painting of stealth bombers which absorbs radiation.

They do not bounce radar back to the source. They could be "stealth" by sucking all that radiation INto the bomber (but they don't). They (and the radar) use the fact that radar waves are much smaller than airplanes; OTOH 60Hz hum waves are much larger than a stompbox. This is a rather different problem.