The question, I think, refers to passive ultrahigh-frequency (UHF) tags. With active tags, the systems are designed to read tags over long distances—100 feet or more. Passive low-frequency (LF) and high-frequency (HF) tags operate in the near field and can be read consistently at very close range. Most UHF transponders operate in the far field (there are some near-field UHF tags), so when a tag is placed directly in front of the reader, or within a couple of feet to the left or right of it, the tag cannot be read.
With circular polarized reader antennas, the read field is the shape of an American football—it emerges from a point and extends out like a cone, and then, as you get farther out, the read field diminishes. Unless you are creating a tunnel, the reader antenna generally should not be placed too close to (or too far from) where the tags will be read. There is no maximum distance—how far away you can read the tags depends on your environment, the tag antenna design, what the tag is being placed on, whether the reader antenna is circular polarized or linear polarized, and other factors.
A skilled systems integrator should be able to determine the ideal distance to ensure the most accurate reads, or you can find it by trial and error. Place tags on the objects you want to track. Situate them 10 to 12 feet from the reader antenna, and see how frequently the tag is read. Move the device forward one foot, then backward one foot. Then move it forward two feet from the original position, and two feet back from the original location. The point at which the interrogator reads the tag most consistently is the optimal position for the reader antenna.
—Mark Roberti, Editor, RFID Journal
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