Can such a tag be interrogated from a long distance?
That would depend on the type of passive RFID tag you are using. Typically, passive low-frequency (LF) and high-frequency (HF) tags have a read range of up to 3 feet, while a simple passive ultrahigh-frequency (UHF) transponder can be read from 15 to 20 feet in free air (that is, with the tag not on an object). There are passive tags, such as Hitachi's µ-chip ("mu-chip"), which have the tag's antenna built into the tiny chip (see Hitachi Unveils Smallest RFID Chip). Because the antenna is so small, the read range is only a few millimeters.
Over the past few years, designers of passive UHF tags have been able to develop special tags that can be read from farther away. I have seen the Omni-ID Ultra tag, designed for use with larger assets, being read from a distance of 80 feet (see Omni-ID Launches New High-Performance UHF Gen 2 Tags). And UHF systems designers have also been working on readers that increase read range. By separating the reader's send and receive function and using a phased-array antenna, Mojix's Star System can read ordinary passive UHF tags from 300 feet away (see The Brightest Star).
So the short answer is that passive tags can be read from 1 millimeter to more than 300 feet away.
—Mark Roberti, Founder and Editor, RFID Journal
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