Rod, I'm not sure that your hypothesis is entirely correct.
In layman's terms a faraday cage works by conducting radiation around the structure thru redistribution. If you took two pieces of copper foil and sandwiched your passport between them it would effectively redistribute the field around the resonant element inside. The problem in a cave isn't so much that it's a faraday cage. It's an issue of power, density, and wavelength. Rock is a poor conductor of low power high frequency EM. As you lower the frequency scale however, from the UHF to VHF to HF and finally down to the LF, SLF, and ULF ranges you suddenly find that the earth is a great transmitter. In fact. Signals, with enough energy, can travel not only miles but hundreds and even thousands of miles. Naval submarines today use SLF (super low frequency) ground stations to communicate. Just like the low frequency sound made by a whale can travel through water so can very low frequency EM waves. -----Original Message----- From: Rod Goke [mailto:[email protected]] Sent: Tuesday, September 15, 2009 3:36 AM To: TM Raines; Rod Goke; TexasCavers Subject: [Texascavers] RFIDs and Cave Radios, Re: [Texascavers] For those of you who are crossing the border... The following discussion was written in response to Terry's question about possible use of aluminum foil to shield an RFID device, but the answer involves an interesting comparison with "cave radios" and an often overlooked weakness in Faraday cage style shielding. Wrapping an RFID equipped card or passport in aluminum foil might shield it, but the devil is in the details. Aluminum foil, or any other material that is a good electrical conductor, tends to reflect radio waves and, hence, be effective as RF shielding. If there are any gaps or holes in the shield, then radio waves may leak through the holes, depending on hole size in comparison with the wavelength of the radio waves. Higher frequency signals have shorter wavelengths and, hence, can leak through smaller gaps or holes in a shield. (Notice that the window in the door of a typical microwave oven has lots of small holes in a conductive shield. The holes allow you to see through, since they are larger than the wavelength of light, but they block the microwaves, since they are smaller than the microwave wavelength.) Shielding effectiveness also can be reduced if part of the metal shield does not make a good electrical connection with the rest of the shield. For example if the metal lid or door of a metal box does not make good electrical contact with the rest of the box, then the effectiveness of the box as a Faraday cage can be compromised. Another potential shielding problem with aluminum foil and with most practical Faraday cage designs is that they don't shield very effectively against magnetic fields. Faraday cages made of aluminum, copper, or other nonferrous metals tend to shield effectively against electrical fields and typical radio signals, but they have very limited value in shielding against magnetic fields. A steady magnetic field, such as that of a permanent magnet, passes easily through this type of shield. A changing magnetic field, such as that produced by an alternating current in a coil of wire, tends to be reduced by a nonferrous metal shield, because the changing magnetic field induces a circular alternating current in the shield, and this induced current produces a magnetic field that tends to cancel out the external magnetic field. This effect would produce very effective shielding against changing magnetic fields if the Faraday cage were made of superconducting material, but it is not nearly as effective when using ordinary metals. With ordinary materials, this shielding effect tends to be most effective for rapidly changing (high frequency) magnetic fields and least effective for slowly changing (low frequency) magnetic fields. In case you are wondering why anyone should care about how well a Faraday cage shields against changing magnetic fields, it is essentially the same issue we face when trying to achieve wireless communication between people in a cave and people on the surface. The earth surrounding a cave acts much like a Faraday cage, effectively blocking most radio signals, but providing only limited shielding against changing magnetic fields. The communication/location devices known as "cave radios" were designed to exploit this weakness in the natural Faraday-cage-like shielding surrounding a cave. A "cave radio" communicates through this natural "Faraday cage" by using a changing magnetic field produced by a coil of wire in the transmitter and detected by a similar coil in the receiver. So what does this have to do with using aluminum foil or some similar Faraday cage package to shield an RFID device? Well, it's essentially the same issue we face in cave-to-surface communications. If the RFID device is designed to communicate using conventional radio signals transmitted and received through a radio style antenna, then a Faraday cage could be expected to provide very effective shielding. If on the other hand, the RFID device is designed to communicate via magnetic induction, like a "cave radio", then the Faraday cage might only weaken the signal and reduce the range of the device instead of totally blocking its communication. Magnetic induction devices tend to be practical only for relatively short range communication, as compared with ordinary radio communication, but they are better suited for transmitting through materials that would reflect or absorb conventional radio signals. I have not investigated the technologies of current RFID devices in much detail, but at least some of the designs I've seen appear to have spiral coils occupying much of the chip area, leading me to suspect that at least some of them are using magnetic induction. The bottom line is that, in principle, relying on a Faraday cage style package to block operation of an RFID device is like relying on a cave environment to block wireless communication. The shielding might be very effective against some, or even most, devices, but it also should be possible to design a device that would transmit through this kind of shield, at least over a short distance. It would be hard to predict how effective a particular shield would be against a particular device without access to either technical details of the device or some way to test its operation. I haven't investigated what does and doesn't work with the various RFID devices currently in use, but you can bet that some people are investigating this and that some of them are disclosing their results and others are not. Rod -----Original Message----- >From: TM Raines <[email protected]> >Sent: Sep 11, 2009 10:48 PM >To: Rod Goke <[email protected]> >Subject: Re: [Texascavers] For those of you who are crossing the border... > >Say Rod, > >Could you wrap your card or passport in aluminum foil and shield it?? > >Curious, Terry > > >On Sep 10, 2009, at 11:56 PM, Rod Goke wrote: > >> The most dangerous aspect of this in my opinion is the use of RFID >> chips in passports and other documents, since these chips can be >> read without your knowledge at any time by anyone within range with >> a suitable RFID reader, even when your documents are stored out of >> sight in your pocket or luggage. In principle, it should be >> possible to store your passport or other RFID equipped documents >> inside a special envelope or case designed to act like a Faraday >> cage, blocking any radio frequency signals traveling to or from the >> RFID chip. I've heard some discussion that envelopes of this type >> might be provided with the new RFID equipped passports, but have >> not checked into this, since I still have a valid old passport with >> no RFID chip. Have any of you had any experience with special >> passport envelopes or other packaging designed to prevent >> surreptitious detection and reading of RFID chips? >> >> The closest experience I've had with a similar issue was during the >> late 1990s with a credit card sized access card that was provided >> to me by my employer to access the building where I worked. The >> access card apparently contained an RFID chip or some similar >> wireless technology, since it did not have to be inserted into a >> reader but could activate the automatic door lock simply by holding >> the access card near the reader. Normally, I carried this card in >> my wallet and used it for after hours access to my office. The >> problem, however, was that this access card also would trigger the >> anti-theft alarm at a local Target store whenever I walked through >> the scanner at their door. The Target security guard didn't suspect >> me of shoplifting, since the alarm went off immediately whenever I >> entered the store, but it was a nuisance for both of us, since we'd >> have to waste time verifying that my access card was the cause >> every time it triggered the alarm. One night the security guard >> gave me a special card designed to eliminate this problem. It was a >> card about the size of a credit card or business card and it >> appeared to be made of thick paper, similar to a business card. I >> suspect, however, that this paper contained some kind of >> electrically conductive material designed to shield against >> whatever electromagnetic signals were used by their reader. All I >> had to do was to place this card next to my access card in my >> wallet, and as long as I carried the 2 cards next to each other, >> there were no more false alarms. >> >> Rod > --------------------------------------------------------------------- Visit our website: http://texascavers.com To unsubscribe, e-mail: [email protected] For additional commands, e-mail: [email protected] --------------------------------------------------------------------- Visit our website: http://texascavers.com To unsubscribe, e-mail: [email protected] For additional commands, e-mail: [email protected]
