What is the maximum range from which a passive RFID tag can be read, and what size antenna would be required?
That's not an easy question to answer. I would say that under ideal conditions, conventional passive ultrahigh-frequency (UHF) transponders can be read from as far away as 50 feet. (By conventional transponder, I mean a tag with a standard dipole antenna of approximately 8 inches.)
I'm not an RF engineer, but it is my understanding that increasing the size of a reader antenna and/or tag antenna would not greatly increase read range. That's because power output from an interrogator is limited by FCC regulations in the United States, and by ETSI regulations in Europe. A tag antenna must be optimized to receive a signal at the frequency being used. For UHF, that means a dipole should be half the length of one wavelength, or approximately eight inches.
New phased-array reader antenna systems have increased read distance considerably, by focusing an antenna in narrow beams or cones that can pick up more faint signals. Mojix, one of the first companies to offer such a system, says its STAR system can pick up UHF tags at a distance of 600 feet.
—Mark Roberti, Editor, RFID Journal
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