Improving the near-metal performance of UHF RFID tags

It is well-known that UHF RFID tag performance degrades when placed near metal. While the mechanisms for how dipole performance degrades near metal is known, it is generally not known how the parameters of the T-match change in the presence of metal, and what, if anything, can be done to improve near-metal performance. In this paper, we develop a set of expressions that describe the affect of antenna parameters on the input reactance of the antenna near metal, and a set of design principles that can be used to minimize the near-metal impedance mismatch. We conclude by demonstrating these principles with a simple antenna model yields a 12.7 meter free-space read distance, and a read distance of 7 meters when separated by a 3.2 mm HDPE foam dielectric spacer from a large ground plane.