Can you please provide a single description that would apply to all passive UHF tags?
I assume you are referring to the radiation pattern of a passive ultrahigh-frequency (UHF) RFID tag. Most tags use a dipole antenna of some kind. The basic pattern of a dipole is the shape of a donut, with little energy reflected in the middle, where the chip is located. Companies have modified the standard dipole antenna to try to improve its ability to read a tag in any direction. The result is essentially a radiation pattern that looks like a fat donut. There are tags that have other types of antennas and thus other radiation patterns, so there is no single description that would apply to all passive UHF RFID tags.
—Mark Roberti, Founder and Editor, RFID Journal
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