First, let me point out that active tags have a power source (usually, but not always, a battery), whereas passive tags do not. Because of this power source, the tags can broadcast a signal, much like a cell phone. They have a longer read range than passive tags, which simply reflect energy from a reader antenna back to the antenna. Active tags can be picked up from a distance of 1,500 feet away, or more.
Low-frequency (LF) and high-frequency (HF) passive tags are near-field tags, which means they are read within one wavelength of an interrogator. I won’t go into why that is, but their read range of a few millimeters to 3 feet cannot be expanded, by design, to 20 or 30 feet.
Passive ultrahigh-frequency (UHF) tags are far-field transponders, meaning they are typically read from more than one wavelength away, though special UHF tags can be designed to operate in the near field. So the question, then, is from how far away a passive UHF tag can be read.
A typical dipole tag can be read using a conventional UHF reader from 10 feet to 30 feet away, depending on the material to which it is affixed, as well as environmental interference and other factors. There are highly sensitive phased-array antenna systems that can narrowly direct the receiving antenna to detect conventional tags from 60 feet away. Mojix has separated the send and receive functions of the reader antenna. Putting devices that energize a tag closer to that tag enables it to be read from several hundred feet away.
Another factor that determines a tag’s read distance is the antenna’s size and design. A specially designed antenna can harvest more energy from the reader antenna, and thus send a stronger signal back, which can be read from further away. Omni-ID has developed an Ultra tag, designed to compete with some active tags. I have seen this model be interrogated from 80 feet away.
—Mark Roberti, Founder and Editor, RFID Journal
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