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International Journal of Antennas and Propagation
Volume 2013 (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

1School of Electrical and Electronic Engineering, The University of Adelaide, Adelaide, SA 5005, Australia
2The Auto-ID Laboratory, The University of Adelaide, Adelaide, SA 5005, Australia

Received 10 August 2012; Revised 7 May 2013; Accepted 26 May 2013

Academic Editor: Charles Bunting

Copyright © 2013 Thomas Kaufmann et al. This 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.

Abstract

A wearable low-profile inset-fed quarter-wave folded microstrip patch antenna for noninvasive activity monitoring of elderly is presented. The proposed antenna is embedded with a sensor-enabled passive radio-frequency identification (RFID) tag operating in the ultra-high frequency (UHF) industrial-scientific-medical (ISM) band around 900 MHz. The device exhibits a low and narrow profile based on a planar folded quarter-wave length patch structure and is integrated on a flexible 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 reflection coefficient () 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. This is confirmed 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. This represents an almost 40% increase in read range over an existing dipole antenna placed over a 10 mm isolator layer on a human subject.