Why Can’t I Read UHF Tags in Some Spots in the Read Zone?

By RFID Journal

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Ask The ExpertsWhy Can’t I Read UHF Tags in Some Spots in the Read Zone?
RFID Journal Staff asked 14 years ago

Is there a "dark zone" within the zone that an RFID interrogator can read an RFID tag? I'm using a UHF Gen 2 reader to test the tag. When I set the power to 30dBm, I notice that at some distances—for example, 5 meters from the reader antenna—the reader cannot read the tag. But when I move the tag to, say, 6 meters, the reader can read the tag again. And this happens with some other distances as well. At lower power settings, this phenomenon happens less frequently. Is there any scientific explanation for this?

—Name Withheld


Yes, there is. With all UHF interrogators, the read field is not uniform. There are null spots—or holes in the read field—caused by reflections in the waves coming from the reader antenna. These null spots can change as objects in the environment change. They are not a problem in applications where objects are moving through the read field or the reader antenna is moving, because the tag will pass into and out of the null spot and be read. But null spots can be an issue if you are trying to read stationary objects with a fixed reader antenna. If you’re running into this problem, you’ll need to work with a systems integrator who has strong knowledge of RF fields to resolve it.

—Mark Roberti, Editor, RFID Journal

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