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How Are RFID Chips Activated?
And could a reader be set to sound an alarm if it broke a distance perimeter?
Passive RFID transponders are activated if they come within range of an RFID reader. The reader emits energy, and the tag then reflects back a signal. The device could be set to sound an alarm if a tag were no longer readable, and you could adjust the distance at which that would occur, to some degree, by attenuating the signal from the reader antenna.
A passive ultrahigh-frequency (UHF) tag can be interrogated from a distance of approximately 10 to 30 feet, depending on a number of factors, including the type of product it is on. If you wanted the alarm to sound in the event that a tagged object left a 30-foot perimeter, then that would be doable. If you wanted to reduce the perimeter to 15 feet, you could decrease the power being emitted by the reader antenna. A weaker signal would reduce the tag's read range.
The one caveat here is that this approach would not be very precise. If someone were to hold a tagged item under his or her armpit, for example, the water in that person's body would absorb the RF signal from the reader and prevent the tag from being interrogated, and the alarm would thus sound. What's more, metal in the floor—or in nearby shelving—could reflect energy to the reader, and the tag might be read by from 40 feet away in some cases. There are also occasionally null spots (dead zones) within a read field; if a tag were to enter such an area, the alarm might sound.
—Mark Roberti, Founder and Editor, RFID Journal
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