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Playing the Field

To design better-performing tags, we need to understand how tags behave in near and far fields.
By Peter Cole
Aug 15, 2006—By Peter Cole

RFID systems won't be widely adopted until tags are cheap and reliable. The need for low-cost tags inevitably requires them to be passive—that is, to contain no internal power source. Most of the communications applications with which we are familiar (radio, TV, mobile phones) make use of battery or household current at either end of the propagation link.

It would be a mistake to expect passive tags to perform in a manner similar to these well-known applications. When RFID tags are passive, they are normally only just working, or only just not working, as the case may be. There is no room for poor performance, so they must be designed and deployed to operate optimally. To accomplish this, we need to understand electromagnetic fields.

The electromagnetic fields emitted by radios, TVs and cell phones are propagating fields, which carry energy away from the radiating source, and diminish in intensity relatively slowly with distance. But near the transmitting antenna there are also energy storage fields, which exchange energy with parts of the antenna. These fields are stronger near the antenna and diminish in intensity more rapidly with distance than the propagating fields do.

RFID systems can make use of either the propagating fields or the local energy storage fields. An RFID tag does not know and does not care where the field that excites it comes from: If it senses it, it will use it. So it is necessary to study the local, or near, energy storage fields, and the propagating, or far, fields. Of particular interest is the question: Which one will be stronger at the tag position?

At the University of Adelaide's Auto-ID Lab, we have focused our research on how to describe such fields and how to couple to them, particularly when the tags are small, which many applications need. In describing the fields, we note that there are electric fields, which will place forces on charges, and magnetic fields, which will move magnets. The fundamental discoveries of Faraday and Maxwell showed that each, when changing, can produce the other.
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