How to un-Faraday a Faraday cage for double-blind research protocols

Dear all,

This is my first post here, and I welcome any answers to the challenge that my research group is up against.  We are essentially trying to "un-Faraday" a Faraday cage by introducing the outside ambient EM radiation into the inside of the cage without doing anything obvious like opening up the door to the enclosure (in order to maintain a double-blind protocol that we have in place).  We were previously told by a couple of advisors on our committee who have EE backgrounds, as well as some experience with RF engineering, and they said to try running a standard copper wire with plastic insulation that would go from the outside of the cage into the inside, with several inches of the termination ends of the wire stripped and exposed to the air.  After trying this, we see no difference in a reduction of the Faraday cage's shielding efficacy using an Icom IC-R20 wide-range radio scanner nor with a smartphone capable of testing for both cellular and Wifi reception.  Is there anything else we could try?

Best regards,

Richard Knowles, Ph.D.   

Hi Richard,

How is your Faraday cage grounded? If it is only grounded at one or a few points all you need to do is unground it. The whole concept of the Faraday cage is that it offers an easy path to GND for EM waves, so they pick that path rather than another. Ungrounded metal will block no EM radiation.

Otherwise you may be able to use repeaters (they make these for repeating cellphone signals inside large concrete buildings or in underground metro stations). But depending on the size of the cage getting good EM waves going inside it will probably prove difficult, because waves will couple with the part of the cage closest to the antena and most likely won't spread well. So I would definitely go for ungrounding if that is possible!

Best of luck!

Hi Felix,

Our Faraday enclosure is a two-layered tent that has a grounding element on the inside layer as well as the outside layer (which will later be connected to each other and then to a grounding rod. It is not true that a Faraday cage needs to be grounding for its shielding capability to be present. You can watch many YouTube videos about this, and also, my personal experience with my own Faraday enclosure is that it works just fine without being grounded.
We are considering using something like a repeater process between two sets of antennas (a pair being on the inside of the cage for TX and a pair being on the outside for RX) with a preamplifier in the middle of each line. If we use standard repeaters, (and please correct me if I'm wrong) then they would only operate within narrow bands and not in a broadband capacity . We want the full spectrum of EM activity on the outiside of our cage being replicated on the inside (well, we've actually decided that 25MHz-6GHz is a "good enough" range for these purposes).
Any other ideas that might be cheaper, and/or more effective than using RX-TX antenna pairings?

What sort of frequency range are you talking about.
Not my area of expertise, but to try to get the same field inside as that outside would be quite difficult I would have thought, you are dealing with a relatively small cavity, and if you re-radiate the signal inside the cavity, then you are dealing with near field radiation and imperfect radiators, the testing of any device within the screen would by highly sensitive to location, any comment from your rf guru's would be useful, since I had to do some antenna testing in a faraday screened room some time ago, and was never happy with the results,
cheers,
Richard

Hi Richard,

I didn't know about ungrounded Faraday enclosures but you're right! In PCB design I have only ever used grounded shielding.

As Mr. Gabric said I think you need to talk to an RF or electromag specialist to get further guidance on this.

Best of luck!

Hi Mr. Gabric,

The frequency ranges of interest are 25MHz-6Ghz. Anything below this range, and it's already going to get through anyway, since ranges that are at the kHz level (such as AM radio) or lower already get through the enclosure due to their low-frequency penetrative effect. And regarding trying to replicate frequncies above this frequency range, well, you have to cut things off at a certain point out of pragmatism (read here, "budget constraints") because there is no way you could ever attempt to replicate the full spectrum. It's potentially infinite in scale on the upper end.

The cavity that we are using is a double-layered Faraday cage custom-made by Holland Shileding for us. It is a tent that has a pyramid shape, with an 11'x11' base and a 7' tall apex. It's spacious enough for one person and some accompanying equipment.

I'm worried about your concern that location within the enclosure would be of importance, since this could create cold and warm spots of reception throughout this inner space that would not represent the true ambient field info well enough to allow for an even modest replication of it. Perhaps if we kept the TX antennas that will be inside the enclosure close together at the center (beneath the apex), this will minimize these kinds of hot and cold spot effects, but I'd like to consult with some experts about all of this as well.

BTW, I was hoping to find some experts on EM radiation and RF engineering on this discusssion board, so this is one of mnay attempts that I've made to relieve myself of my ignorance and find a "good enough" solution to this most vexing problem.

Best regards,
Richard

Hi there.

I once did an extensive study on EPFCGs that involved Faraday cages but I'm not sure I could help much.

They suggested adding exposed wires, I guess hoping to unbalance the charge distribution induced at the cage [did you ensure that all the exposed wire were inside the cage when you did this?]. Also, maybe applying some voltage to those wires would work?

Maybe the most economical fix would be to use highly directional transmitters and receivers. If they don't work, maybe do something to the area perpendicular to the antenna, such as turning the conductor into an insulator for that area only. Is there a way to temporarily make a metal non-conductive [free of electrons] for a fixed time?
The main fuel of the faraday cage principle is the existence of so many free electrons enclosing the target area. Tried applying ac voltage noise to the cage?

Anyway, I wish any of these helped, or at least sparked an idea that could lead to a feasible solution.

Regards,
Justin

Hi Justin,

If by "adding exposed wires", you mean having part of an uninsulated wire sticking outside of the cage with the other part sticking on the inside, then we tried that--with some plastic tubing in the middle of the wire so the wire wouldn't directly touch the cage. This serves as a very poor antenna, and allows some minor EM expressions in the microwave range to leak into the tent. I was noticing 1 bar of GSM cellular streangth, and 1 bar of WiFi strength, but I was still not able to make a call, nor navigate anywhere on the internet with a browser.

Why would you suggest using highly directional antennas since we want to at least transmit in all directions at once on the inside of the cage to help prevent "cold" spots, so omnidirectional seems most appropriate to me. I'm not sure what you're trying to explain with "turning the conductor into an insulator for that area only", besides, I know of no way to turn a conductor into an insulator. A Faraday cage needs no fuel, and is not "powered" by the electrons it traps since it's properties are passive and "built in", though it is true that, without being connected to a grounding source, a Faraday cage can build up a charge which can lead to a shock hazard if you touch the cage. What are you expecting to happen if AC voltage is applied to the cage? How would this help us to replicate the signals from the outside world to the inside of the cage? I have thought of one crazy idea, which is to use the actual Faraday cage as an antenna by connecting the RX antennas to it just after being pre-amped, but I'm pretty sure this would not work.

Thanks for your thoughts, Justin.

Regards,
Richard

Hi Richard,

Sorry, I suggested highly directional antennas because you can just interface it to omnidirectional ones at the outside and inside (connecting them to a repeater first).

Some conductors can become insulators at certain conditions (but I'm not an expert in Materials Sci. so I'd need to do some more intensive research on this, and is partly the reason why my question was existential).

Ok, my metaphor/simile of using "fuel" was wrong. But as long as the charge is distributed, the Faraday phenomenon will persevere.

By applying AC voltage, maybe the even charge distribution will become uneven?

That is an interesting idea. What would a spherical cone shaped antenna's (the faraday cage) footprint be I wonder?

Anyway, this is a very interesting problem. I just sometimes find solutions
when I try the crazy stuff.

Thanks.

Regards,
Justin

Hi Justin,

We aren't going to try to penetrate the shilding with RF by sending a signal from the outside to the inside with highly-directional antennas when a simple cable will suffice. Our Faraday enclosure has a shielded sleeve that runs form the outside to the inside, and can accomodate a cable from the RX antenna on the outside to the TX antenna on the inside.

I don't think that a Faraday cage derives its shielding effects via a distribution of charge on its surface, since you can attach it to a grounding source to dissipate any such charge, and it still works just as well. The fact that such enclosures acccrue free electrons on their surface is just an epiphenomenon of RF radiation being transduced in the electrical energy.

I'm still fascinated with the idea of using the Faraday cage itself as a transceiver. I guess I could use the grounding plate that attaches to the tent as an input channel, and run a cable with an RF frequency to that grounding plate to give it a shot. I'm really not expecting much, but I've been told that you can basically turn any chunk of metal into an antenna as long as it is properly insulated from any other metals that touch it, and use a proper amplifier for it. I don't think I've heard of anyone using an antenna with a hollow cavity in the center, though. Hmm, actually, yes I have, and I forgot I actually have one. It is an extremely-low frequency electrostatic antenna in the shape of a hollow metal sphere. Of course, it's only meant for RX and not TX, My intuition tells me that if you use a hollow antenna as a TX, then it will only be good at transmitting outwardly, but not within the space on the inside.

BTW, I think you were asking about the dimensions off our enclosure, and it is a pyramid shape that is 11x11' at the base, and 7' tall. Incidentally, it is a scaled down model of Cheop's pyramid, but that was just for fun.

Best,
Richard

Just as an update, I wanted to let you all know that we tried our first test of trying to "unFaraday" a Faraday cage, and it actually worked! We only had the lower frequency antenna pair operating from 20MHz up to 2GHz, but using a Minicircuits ZX60-6013E+ amplifier between the Rx and Tx antennas, we were able to pick up FM radio stations on the inside of the enclosure that would normally be blocked out. SUCCESS!! Our next step will be to implement the same process for higher frequencies using a microwave antenna pair that operate from 2-6GHz. I'm looking forward to getting internet and cellular access while *inside* our Faraday cage using this technique with the microwave antennas. Whoo Hoo!!

Hi Richard, I'm really glad it worked.
I hope you don't mind. I'm just interested in understanding more how the cage was unFaradayed. What did you do to the cage and what other things did you do aside from amplifying the signals fed to the Rx and Tx antennas?

Thank you very much. Kudos to a successful endeavor.

Regards,
Justin

Thanks for the Kudos, Justin. We just used two wide-band 20MHz-2GHz discone antennas along with a low-powered (+10-12dB) wide-range amplifier (20Mhz-6GHz). We have a sleave in the Faraday tent that allows you to run cables from the inside to the outside, so that's how we got the antenna cabling from the Rx antenna to the Tx one. The trick on the inside was to move the Tx antenna to the center of the enclosure (beneath the apex of its pyramid.shape). Also, it had to be reting on the ground and not moved up towards the apex in order to work as well. One of the commenters had said that there would be a lot of "dark" and "bright" reception areas on the inside of the tent (maybe due to constructive and destructive nitereference), and perhaps this is why the transmitter had to be placed in the middle--to minimize those effects. Anyway, we were able to pick up all the usual FM radio stations that would normally be blocked out by the shielding of the cage.

Best,
Richard