For example, could a tag inside wood be interrogated?
It depends on the type of passive RFID tag used (LF, HF or UHF) and the density of the wood. If you use a passive UHF transponder, then the penetration distance will also depend on how much moisture is in the wood, as water absorbs RF energy in the UHF spectrum, which causes less energy to reach the tag and be reflected back to the reader antenna. In general, I would say you are probably not going to be able to read a tag more than a few inches deep inside wood.
Other materials, such as most plastics, may be more RF-friendly, and the depth of penetration will thus be greater. The big factors that affect penetration depth are the density of the material and its dialectric constant, which tells you how much electricity the material conducts. Wood has a low dialectric constant (2 to 6) when dry, but it increases to 10 to 30 when wet. Plastics have a low dialectric constant, whereas for metals it is high, making it impossible for RF energy to penetrate them.
I hope this answers your question, and I look forward to seeing you at RFID Journal LIVE! in May.
—Mark Roberti, Founder and Editor, RFID Journal
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