Can radio waves travel endlessly through air?
I assume you are asking about the size of a tag antenna and, more specifically, a passive UHF tag antenna. The size of the tag is the critical factor. A smaller antenna would be able to harvest less energy from the reader antenna, and thus would have less energy than a tag with a larger antenna (all other factors being equal) to reflect back a signal to the reader. A weaker signal would be read from a shorter range.
Think of passive UHF RFID tags as sail boats, their antennas as tags, and the speed and distance they can travel as their read range. If your sail boat had a very large sail and mine had a sail that was one-tenth the size, and if we were racing in the same wind, then your boat would travel faster and farther than mine.
In such a scenario, there would be no fundamental or theoretical limit. Radio waves cannot travel endlessly through air (they can through a vacuum), but larger antennas, more efficient chips and more sensitive reader antennas continue to push the boundary of a tag's read range. I have seen a large Omni-ID tag read from a distance of 100 feet (30 meters), and an ordinary passive UHF dipole tag read from 600 feet (183 meters) using a phased-array antenna.
—Mark Roberti, Founder and Editor, RFID Journal
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