A recent paper given at the ACM Workshop on Hot Topics in Networks describes a system that analyzes the variation in the EM response of UHF RFID tags on containers, due to variation in the chemical composition of the material inside the container. The data is collected with a second antenna and signal processing system.

This question though is more about the primary, standard UHF RFID tags and readers. It looks like putting the same tag on a bottle of water with a very high dielectric constant ε and a container of say popcorn (something with ε closer to that of air) would produce a frequency shift of hundreds of MHz. What are the techniques that the standard UHF RFID system uses to remain sufficiently insensitive to product-dependent frequency shift that could potentially be so large?

UHF RFID from MIT's RFIQ paper

UHF RFID from MIT's RFIQ paper

[UHF RFID from MIT's RFIQ paper

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