Playing the Field
To design better-performing tags, we need to understand how tags behave in near and far fields.
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 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.
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