The distance from which a tag can be read is called its read range. Read range depends on a number of factors, including the frequency of the radio waves uses for tag-reader communication, the size of the tag antenna, the power output of the reader, and whether the tags have a battery to broadcast a signal or gather energy from a reader and merely reflect a weak signal back to the reader. Battery-powered tags typically have a read range of 300 feet (100 meters). These are the kinds of tags used in toll collection systems. High-frequency tags, which are often used in smart cards, have a read range of three feet or less. UHF tags-the kind used on pallets and cases of goods in the supply chain-have a read range of 20 to 30 feet under ideal conditions. If the tags are attached to products with water or metal, the read range can be significantly less. If the size of the UHF antenna is reduced, that will also dramatically reduce the read range. Increasing the power output could increase the range, but most governments restrict the output of readers so that they don&#;t interfere with other RF devices, such as cordless phones.
From how far away would an interrogator be able to read a tag?
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It depends on the type of RFID system being used. A high-frequency (HF) reader based on the ISO air-interface protocol standard, designed for short-range transmission, has a maximum read range of about 18 inches. An active RFID system can read tags from 1,500 feet away or more, as the tags broadcast a signal and the systems are designed for longer-range applications.
Even within one type of RFID, however, there can be a wide array of read ranges. A passive ultrahigh-frequency (UHF) handheld reader has a range of about 10 feet, while a model using a beam-steerable phased-array antenna can interrogate passive tags at a distance of 600 feet or more.
Keep in mind that the reader is only half of the story. A very small passive tag with a small antenna harvests less energy from and reflects less energy back to a reader. It, therefore, has a shorter read range than a tag with a much larger antenna. I have seen small passive UHF tags that can be read by an ordinary fixed reader at a distance of only 2 or 3 feet, and I have seen passive tags that can be interrogated at a range of 80 feet.
&#;Mark Roberti, Founder and Editor, RFID Journal
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