When RFID Journal writes about read range, we generally say that passive high-frequency (HF) tags have a read range of about three feet, and passive ultrahigh-frequency (UHF) tags have a read range of about 20 feet. But you are correct. A tag’s read range is highly dependent on the gain (the intensity of the signal in a given direction) and the radiated power (amount of energy emitted by the antenna). There is no standard for how read distance is measured, so naturally, vendors use their own methods to determine read range in a way that makes their tag performance look best. In general, that usually means using the highest gain (most intense beam) and the maximum radiated power allowed by regulations (in the United States, it’s 4 watts of power).
When a company deploys an RFID solution, the application will likely require a broader beam to cover more area (lower gain), and the amount of power will depend on the read distance required. Another big factor in determining read distance in the real world will be the product the tag is placed on. Products with high-water content absorb UHF energy, so if, for example, you’re tagging meat with a standard UHF tag, the meat will absorb energy. The tag will have less energy to reflect back to the reader, so the read distance will be shorter than can be achieved on a more RF-friendly product.
—Mark Roberti, Editor, RFID Journal
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