That depends on the conditions present in your particular environment. If a tag is affixed to RF-friendly material, you can probably get a fairly small ultrahigh-frequency (UHF) tag that would work fine when read from 0.5 meter (1.6 feet) away.
Avery Dennison makes several small tags, including the AD-805, which measures 0.63 inch by 0.63 inch (16.3 millimeters by 16.3 millimeters); the AD-814, which measures 0.866 inch by 0.866 inch (22 millimeters by 22 millimeters); and the AD-815, which measures 0.787 inch by 0.394 inch (20 millimeters by 10 millimeters). Other manufacturers of UHF tags, such as Alien Technology, Invengo and UMP RFID, also offer small tags.
If a tag were placed on an object with a great deal of water in it, such as bottled water or a watermelon, then it might not be feasible to read the tag from that distance, since the water would absorb much of the UHF energy and detune the tag antenna. And if the tag were located on a metallic object, then you would require a special tag. Omni-ID manufactures a tag designed for use on metal—known as the Omni-ID Prox tag—which measures 35 millimeters by 10 millimeters by 4 millimeters (1.4 inches by 0.4 inch by 0.3 inch).
You will likely need to purchase sample tags and test them within your environment, on the particular objects you wish to tag.
—Mark Roberti, Founder and Editor, RFID Journal
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