By carving a 'square-smile' slot profile over a folded patch, a miniaturised UHF-RFID tag is obtained, having a convenient two-step tuning mechanism (coarse and fine). This is useful to adapt the same tag to European and US frequencies and to make on-site corrections. The antenna is half the size of a credit card and can be read up to 5m when attached onto the body. The flexible and lightweight EPDM foam substrate makes the tag suited to be integrated in badges, wallets, pockets, plasters, wristbands and various garments.Introduction: Radio frequency identification (RFID) and wireless sensor systems are emerging technologies, attracting remarkable interest in security, healthcare, biomedical applications and even entertainment and social arts. Passive devices are particularly attractive for bodycentric platforms owing to their light weight, low cost and the absence of battery recharging. In particular, tags integrated into clothes, eventually hosting specific sensors, make it possible to remotely monitor human body activity [1,2]. In recent years, several on-body passive dipole and patch tags have been presented [3 -5] for the UHF (866-956 MHz) RFID frequency, but smaller layouts are however needed in some applications in order to simplify the integration with clothes or plasters. Miniaturisation of wearable passive tags generally produces a remarkable degradation of the antenna bandwidth as well as of the radiation efficiency due to the presence of the lossy human body. As a consequence, this can negatively affect the interoperability in different countries and the stability of the performances with respect to the specific body placement.Starting from previous experiments of the same authors with wearable tags in [2], this Letter proposes a new layout over a flexible and low-cost substrate with reduced external dimensions. The geometry provides a two-step tuning mechanism that permits in principle to easily adapt the same tag to any frequency inside the world-wide UHF RFID band (866-956 MHz) and even to make finer corrections for the specific placement. The antenna potentiality is here demonstrated by a parametric analysis and measurements on real prototypes.
A planar "square-smile" like layout is proposed to achieve miniaturized wearable UHF RFID tags. The geometry hosts different slots which contribute to produce size reduction and a two-steps friendly impedance tuning. The resulting antenna surface is half the size of a credit card and permits to be read up to 5m when attached onto the body. Thanks to the flexible and lightweight EPDM foam used as substrate, the tag is suited to be integrated in badges, wallets, pockets, plasters, wristbands and various garments. The particular slotted geometry moreover simplifies the interconnection of chemical or mechanical sensors. An example is given concerning a passive motion detector for the live tracking of human movements and pathological/vital signs in a conventional room.
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