Suggestion to electrically disable a Faraday Cage

Question

My plan is to ultimately develop an Autonomous Faraday Cage. The project seems very straight forward; however, I am unsure on how to effectively control the cage (turn on and off).

I have constructed a prototype Faraday Cage at home. From my research on these cages, my first theory was that an un-grounded cage with no external charge should not block the electromagnetic waves emitting from a cell phone. However, placing the phone in the cage, I observed that the cell phone still lost its network capabilities.

Google has provided no suggestions on how to disable the qualities of a Faraday Cage electrically. Obviously, I could arrange a method where the cage opens, allowing the phone to reconnect to the network and Bluetooth. I don't see this as ideal however since my design would involve moving parts (a final design I plan on completing thousands of iterations).

Are there suggestions on how to electrically disable the Faraday Cage?

Extra Note: As I wrote this, I just thought about grounding the inside and outside of the Cage. From my understanding, a Faraday Cage works by creating its own electromagnetic field which cancels external and internal Electromagnetic waves. My reasoning on grounding the inside and outside is that if there is no charge to create the electromagnetic field, then the electromagnetic waves could escape the cage. Is this a reasonable theory?

TL;DR: How may I electrically disable a Faraday Cage?

Answer 1

The following is just a guess. I could be completely wrong.

You can't disable a cage, but you might be able to make a switchable "leak" in the cage.

Imagine this, you have a cage with two antennas. One antenna on the inside, one on the outside. The two antennas are connected with a wire. Where the wire goes through the wall of the cage you place a switch (or relay, or whatever). If the hole in the cage is smaller than 1/4th wavelength, and the switch effectively prevents the signal from penetrating the cage, then you might just have something.

You might need to do some fancy shielding around the hole and switch, to prevent leakage when the switch is off. You might also consider connecting one or both antennas to the wall of the cage when the switch is "off".

It probably won't ever be perfect, but it might do what you want. Unless you are a forigen spy, intent on disabling our security in sensitive military complexes. Then it won't work. For that you need to know the secret combination, which is 1234 by the way.

Answer 2

Your analysis of the cage is not right: it doesn't generate its own field, it acts as an antenna or inductor, absorbing all the emitted RF energy. The energy is dissipated as an eddy current within the cage.

So far as I know you can't electrically disable a Faraday cage.

Answer 3

You cannot electrically disable a faraday cage, unless you basically melt it from the outside.

The meaning of "faraday cage" is: a completely equipotential surrounding of any volume.

It does not matter what potential (ground, isolated, or any) the cage has. Faraday cages are usually grounded because it makes it safe to touch them from the outside. Thats all. Grounding a Faraday cage in no way affects what is going on inside of it.

Say, you have an isolated faraday cage which gets struck with lightning. This would charge the faraday cage. You inside the cage would still be safe, it being the same potential on each surface on the inside. Mind you, charged bodies carry charge only on the outside. But technically, you would be on the same potential as the cage - but it being the same all over the the cage, there is no differential voltage, therefore no current, and therefore no dying.

Now, if you left your charged cage, and set foot on the ground, the charge stored on the cage would flow through you, getting quite dangerous.

Electromagnetic waves may penetrate the faraday cage depending on frequency. This has to do with the thickness of the cage and the depth of the surface eddy currents induced by the waves. Wikipedia explains all that quite nicely. But again, all this has nothing to do with the grounditude of your cage.

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

For instance, your microwave, which is a faraday cage with the waves inside of it, works just as fine if you remove grounding.

Answer 4

Put a window in your cage, for the signal to get through. Test that, to be sure you get signal through the window.

Now string two meshes of wire over the window, one up and down, one back and forth. Keep the meshes insulated from each other. Test that. You want the signal to still get through.

Now put diodes across each cross point. There will be a lot of diodes. To close the window, forward bias the diodes. To open the window, reverse bias the diodes.

Too crazy? Then just put an antenna outside, connect to antenna inside, and again the diode switch across the antenna wires going into the cage. Remember to ground the outer shell of the co-ax antenna wire to the cage where it goes in. Or use SMA bulkhead connectors to get the line into the cage.

Totally untested, of course. But a forward biased diode is a pretty handy way of switching off an RF signal. You may have to reverse bias the diode (to reduce its capacitance) to get it fully open, though. I've only every used this for ordinary RF, not for microwave Ghz signals. For cell phone band, you will need to use PIN diodes rated for Ghz, plain old rectifier diodes will have too much capacitance.

Answer 5

Wrong Problem?

Your question, while interesting, assumes facts not in evidence. There is an unsupported assumption that you need to use a Faraday cage.

Disable Communications, Not Radiation

If your goal is just to selectively disable communications, then all you need to do is use a jammer (broadband high-power emitter) next to your Device-Under-Test (DUT). You can then turn the jammer on and off to disable communication.

Easy. Breezy. Beautiful.

Answer 6

I think creating some kind of electromechanical case, to create an opening/closing window, would be the most effective solution.

Using a "jammer" is interesting but not very plausible because it may disrupt your 'neighbors' reception as well. One addition flaw with using a jammer, may be that those things are super expensive!

I am curious @NickWilliams did you end up implementing a good solution using antennas...