Hindawi Publishing Corporation International Journal of Antennas and Propagation Volume 2013, Article ID 129839, 11 pages http://dx.doi.org/10.1155/2013/129839 Research Article Wearable Quarter-Wave Folded Microstrip Antenna for Passive UHF RFID Applications Thomas Kaufmann, 1,2 Damith C. Ranasinghe, 2 Ming Zhou, 2 and Christophe Fumeaux 1 1 School of Electrical and Electronic Engineering, he University of Adelaide, Adelaide, SA 5005, Australia 2 he Auto-ID Laboratory, he University of Adelaide, Adelaide, SA 5005, Australia Correspondence should be addressed to homas Kaufmann; thomaska@eleceng.adelaide.edu.au Received 10 August 2012; Revised 7 May 2013; Accepted 26 May 2013 Academic Editor: Charles Bunting Copyright © 2013 homas Kaufmann et al. his is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. A wearable low-proile inset-fed quarter-wave folded microstrip patch antenna for noninvasive activity monitoring of elderly is presented. he proposed antenna is embedded with a sensor-enabled passive radio-frequency identiication (RFID) tag operating in the ultra-high frequency (UHF) industrial-scientiic-medical (ISM) band around 900 MHz. he device exhibits a low and narrow proile based on a planar folded quarter-wave length patch structure and is integrated on a lexible substrate to maximise comfort to the wearer. An extended ground plane made from silver fabric successfully minimises the impact of the human body on the antenna performance. Measurements on a prototype demonstrate a relection coeicient ( 11 ) of −30 dB at resonance and a −10 dB bandwidth from 920 MHz to 926 MHz. Simulation results predict a maximum gain of 2.8 dBi. his is conirmed by tag measurements where a 4-meter read range is achieved using a transmit power of 30 dBm, for the case where the passive wearable tag antenna is mounted on a body in a practical setting. his represents an almost 40% increase in read range over an existing dipole antenna placed over a 10 mm isolator layer on a human subject. 1. Introduction he miniaturisation of sensors and wireless systems is gener- ating an explosive growth in body-centric wireless computing applications based on wearable electronic devices, especially in healthcare [1–6]. Consequently, the need for lightweight, low-proile, low cost, and wearable antennas has also grown rapidly during recent years. In this paper we present a suc- cessful design of such a wearable antenna for an emerging class of sensor-enabled passive radio-frequency identiication (RFID) tags for use in healthcare applications. he wireless identiication and sensing platform (WISP) [7] is a passive RFID-based platform suitable for wireless wearable applications. In the present case, it integrates an ac- celerometer and a passive RFID tag into a batteryless system with the capability of being integrated into clothing. he intended application is the monitoring of high falls risk activ- ities for elderly in hospitals and residential care facilities [1, 2]. Typical WISP devices are currently built using a dipole anten- na on a FR4 substrate and as a result exhibit poor per- formance when worn in close proximity to a human body. In this case, the read range of the WISP is less than 2.5 m once attached to clothing worn by a person. his read range is inadequate for monitoring an elderly subject in a care facility. Furthermore, the thin dipole antenna on the WISP is fragile and can pose a signiicant discomfort to the wearer of the device while possible breakages can cause injury. hus, a new antenna design is proposed to replace the dipole structure. A few researchers have investigated wearable RFID tag antennas in the ultra-high frequency (UHF) range [8–11]. Most designs have been based on half-wave length dimen- sions, which correspond to approximately 16 cm in length at 923 MHz. In order to further reduce the physical dimen- sions, approaches including tapering in a bow-tie [9], mean- dering, [10], and meandering with a superstrate [11] have been considered. Further size reductions are achieved in diferent geometries using, for example, Planar Inverted F Antennas (PIFAs) [12], metamaterial superstrates [13], and folded short- ed patches [14]. hese designs usually use coaxial probe feed- ing mechanisms. However, in the context of wearable anten- nas, coaxial line fed antennas are (i) not easy to fabricate;