Tuning of on-metal UHF RFID inlay tags loaded with a thin magneto-dielectric slab

The input impedance of UHF radio frequency identification (RFID) inlay tags is analyzed when a thin material slab is placed between the antenna and a metal surface.The two-layer slab is made of a thin magneto-dielectric Emerson&Cuming Eccosorb MCS-U layer with high permeability/permittivity and a thin Rohacell(TM) layer. Two common commercial UHF tag antennas are considered: the Alien ALN-9640 Squiggle® and the ALN-9654 G inlays. The antenna impedance is evaluated both numerically and experimentally, and the power transmission coefficient is calculated in the whole UHF RFID frequency band. It is shown that a tag nonintrinsically suitable for on-metal applications (e.g. inlay tags) can be used on a conductive surface in a low-profile configuration. Indeed, when the above magnetic substrate is used, the antenna input impedance rises up from the very low values induced by the presence of a near conducting surface. Then, with a simple trimming applied to the antenna layout, it is possible to tune the antenna to achieve a conjugate impedance matching with the chip impedance. It is demonstrated how an effective tuning is possible through some simple straight cuts of some antenna parts, without requiring a redesign of a new optimized tag antenna. © 2012 Wiley Periodicals, Inc.