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Materials

Radio-Frequency Devices

November 24, 2008 | APPEARED IN VOLUME 86, ISSUE 47

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Several readers have commented on the unwelcome, surreptitious use of radio-frequency infrared device (RFID) tagged badges at conferences (C&EN, Sept. 15, page 4). RFID devices typically employ frequencies in the ranges of 125–134 kHz, 13.56 MHz, 400–930 MHz, 2.45 GHz, and 5.8 GHz. The most commonly used frequencies are around 400 MHz and 900 MHz. The tags can operate at distances up to 300 feet and therein lies the Achilles' heel.

The reader may recall from instruction in physics class that electrical current of such high frequency travels along the surface of a conductor and does not penetrate the conductor; this phenomenon is commonly referred to as the "skin effect." If the RFID device is enclosed within a metal container having no opening or gap larger than one-half wavelength of the impinging radiation, then the induced current will remain on the surface of the container and effectively isolate the RFID contained therein. Such a container is referred to as a "Faraday cage." No grounding wires or electrical supply is needed.

A Faraday cage to shield identity badges may be prepared by folding aluminum foil into the shape of an envelope with a flap closure. The seal need not be hermetic and the conducting surface can have holes in it; even metal window screening or metal plaster lathing can be used, but aluminum foil seems to be most convenient. In other words, you can be somewhat sloppy. It is only necessary that the foil edges make electrical contact and any gaps be less than one-half wavelength of the radiation used.

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A common example is the screen found on the inside of a microwave oven door which, when combined with the metal housing of the oven, completes the cage and shields the room from microwave radiation. In practice, the badge bearing an RFID may be placed in the foil envelope, the flap closed, and out you go. The rest of the day is yours.

Having mastered the rudiments of Faraday shielding, one may—by obvious sequences of shielding or exposing—appear to arrive multiple times or to never leave. At technical meetings, one might also consider handing out aluminum foil envelopes before your competitor speaks.

Albert G. Anderson
Wilmington, Del.

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