Low-frequency (LF) tags employ inductive coupling to communicate. The coiled antenna in the tag and a coil in the reader antenna form an electromagnetic field, almost like an invisible balloon between the two. As the tag modulates and demodulates its antenna, the reader picks up the signal. This electromagnetic field is not impacted greatly by liquid, allowing LF tags to be read under water.
Battery-assisted passive (BAP) ultrahigh-frequency (UHF) transponders communicate very differently. They use something called backscatter, in which the tag reflects back a signal to the reader, much like a mirror could reflect back light. RF energy, however, is absorbed by water in the UHF spectrum. As a result, very little energy can reach the tag, and the weak signal that the tag reflects back is absorbed by the water, making it virtually impossible to read the tag.
BAP UHF tags are still impacted by the presence of liquids, so a BAP tag under water would have a much shorter read range than one in free air. I would suggest you contact Convergence Systems Ltd. (CSL), which just released a battery-assisted tag designed to work around the human body, which is mostly composed of water. I think CSL could probably offer you good insights into the type of read range that can be achieved underwater using a BAP tag.
—Mark Roberti, Founder and Editor, RFID Journal
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