With passive ultrahigh-frequency (UHF) tags, the answer is yes, there is. A reader can detect the relative strength of a signal being reflected back from a tag, and can thus determine that one tag is close to another.
This is not an extremely precise determination, however, since the signal might reflect off objects or walls, the floor and the ceiling (this is called multipath), and that might affect a signal’s relative strength, leading the reader to perceive that one tag is closer when it is not. In addition, you might be able to tell which of two tags is close if one is several feet farther from the reader. But you would not be able to discern which is closer to the reader if they are only one centimeter apart.
Active tags use sophisticated software techniques, including relative time of arrival at multiple readers, to pinpoint tag location, so they can tell which is closer, though most active systems operate at long range and can only locate a tagged object to within 10 feet.
With passive high-frequency (HF) systems, I do not believe you can determine which tag is closer to the reader. I could be wrong about this—and, if so, I welcome input from experts in designing and building HF systems—but I believe that the nature of the electromagnetic field created by HF systems enables the reader only to determine that a tag is within the field.
I hope this helps you in designing your system.
—Mark Roberti, Founder and Editor, RFID Journal
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