You can place a tiny high-frequency (HF) tag on each vial of drugs, and place a tiny reader antenna in each slot where a vial is held. An interrogator can determine which of its antennas reads a tag. So by cycling through each antenna, a reader can determine which tag is in which slot. This application requires multiple reader antennas, which can be expensive if you have a lot of area to cover.
What’s more, this doesn’t work as easily over larger distances. Placing an ultrahigh-frequency (UHF) reader antenna under every square floor tile in an office building would tell you within a square foot where an item is located—but it would be far too expensive to be practical.
Ultra-wideband (UWB) technology can provide fairly high levels of accuracy over longer distances. There are also passive phased-array antennas that can provide accuracy to within a few feet. But within inches is very, very difficult. The reason is that it’s not easy to ensure that radio waves came directly from the tagged object. If a wave bounces off different objects, providing inch-level accuracy becomes impossible.
I am unaware of any technology that can provide accuracy to within inches over distances of, say, 20 feet or more. Ultrasound and infrared can provide room-level accuracy or indicate if an item is in a particular drawer or container, but they cannot tell you where an item is located in an open area to within a few inches.
—Mark Roberti, Editor, RFID Journal
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