Why do UHF RFID passive tags not always work on metal?

Generally speaking, passive UHF tags are tuned for mounting and use on dielectric material (non-conductive) such as cardboard, non-carbon plastic and general non-metallic surfaces typical in the supply chain industry.

When used on or in proximity of a metal or metallic dielectric equivalent

surface a passive tag will act in one of 3 ways:

• Conducting surface: Placing a dipole RFID tag flush on a metal surface creates an

effective electrical “short”

• Interferer: Certain distances between a metal surface and dipole RFID tag cause a

backscattered signal to scramble rather than reflect (see next item)

• Reinforcer: If the spacing is just right (around ¼” for UHF between tag and metal surface), tag will properly backscatter