How can we register when approximately 30 passive tags are leaving or entering a single field?
Passive tags, both high-frequency (HF) and ultrahigh-frequency (UHF), have anti-collision protocols allowing tags to be read in succession. This happens quickly, so it appears the tags are being read simultaneously. As such, there should be no problem reading 30 tags moving through a single portal. The only challenge would be if the tags were physically shielding one another from the reader's energy.
You can also determine whether tags are moving into or out of a room or facility by setting up two sets of antennas in the doorway. Let's call these reader antennas A and B. If tags are read by the A antennas first, and then by the B antennas, you can tell the tags are moving in one direction. If the tags are read in reverse order, then you know the tagged goods are moving the other way.
—Mark Roberti, Founder and Editor, RFID Journal
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