776 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 10, 2011 Mirror-Integrated Transparent Antenna for RFID Application Cristina C. Serra, Carla R. Medeiros, Jorge R. Costa, Senior Member, IEEE, and Carlos A. Fernandes, Senior Member, IEEE Abstract—An optically transparent antenna array is presented for radio frequency identification (RFID) systems, operating in the ultrahigh frequency (UHF) band. The antenna is intended for non- intrusive integration into the mirror surface of fitting rooms in ap- parel stores, keeping with the typical small depth of the mirror structure and low cost. The antenna is used to read RFID tags at- tached to clothes being tried on, so that the associated system can automatically provide the client with interactive information re- garding that specific piece of clothing. The antenna ensures self- confined reading range in front of the mirror, avoiding undesired detection of RFID tags from adjacent fitting rooms. The final an- tenna solution shows a good compromise between transparency and performance, with well-confined RFID detection volume. Index Terms—Printed antenna, radio frequency identification (RFID), reader antenna, tag, transparent antenna. I. INTRODUCTION R ADIO frequency identification (RFID) technology en- ables automatic detection and identification of physical objects through radio waves. A tag with a univocal code is asso- ciated with each object, making identification possible without human intervention. Using frequency bands from high fre- quency (HF) to microwaves, the deployment of RFID systems has increased considerably, being extensively used for automa- tion improvement, inventory control, antitheft, and checkout operations in stores, factories, and warehouses [1]–[3]. Toll systems or electronic passports are just a few of many applica- tions of RFID technology. Passive RFID tags operating in the ultrahigh fre- quency (UHF) band are getting increased attention for mass retail applications because of their very good sensitivity and low cost [4]. This technology presents several opportunities, for instance in clothing stores. One such case would be the possibility to automatically read the tag associated with a piece of clothing being tried on in front of a mirror and, based on Manuscript received June 29, 2011; accepted July 20, 2011. Date of pub- lication July 29, 2011; date of current version August 15, 2011. This work was supported in part by Fundação para a Ciência e Tecnologia under Project RFID-Local PTDC/EEA-TEL/102390/2008. C. C. Serra, C. R. Medeiros, and C. A. Fernandes are with the Instituto de Telecomunicações, Instituto Superior Técnico, Technical University of Lisbon, 1049-001 Lisboa, Portugal (e-mail: cristina.camara.serra@gmail.com; Carla. Medeiros@lx.it.pt; Carlos.Fernandes@lx.it.pt). J. R. Costa is with the Instituto de Telecomunicações, Instituto Superior Técnico, Technical University of Lisbon, 1049-001 Lisboa, Portugal, and also with the Departamento de Ciências e Tecnologias da Informação, Instituto Universitário de Lisboa (ISCTE-IUL), 1649-026 Lisboa, Portugal (e-mail: Jorge.Costa@lx.it.pt). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LAWP.2011.2163053 that information, the associated system would supply the client with added functionalities, such as a list of available sizes and colors of that item or other matching garments. There are currently some solutions based on this idea [5], but none of these address the key challenges: to ensure that the cov- erage of the reader antenna is solely confined to the fitting room (or to the volume in front of the mirror), that the assembled so- lution is shallow and nonintrusive like an ordinary mirror, and most importantly, that it is very low-cost. The solution presented in this letter not only guarantees the desired detection confine- ment without physical barriers, but also ensures invisibility of the antenna and cables. This feature is attractive for aesthetical reasons and to avoid that clients feel uncomfortable with un- usual devices in the fitting room. The antenna cannot be hidden behind the mirror because this is not electromagnetically transparent. Therefore, we propose in this letter to print a transparent patch antenna on the glass sur- face of the mirror and couple energy to it from behind through a slot in the mirroring layer without front cables and soldering points. There are currently several possible materials to create transparent antennas, like spray-on conductive substances [6] or metallic conductive films. However, most of these materials do not present the best optical transparency for the mirror application. Indium tin oxide (ITO) films seem to be the most viable solution so far, for they present the best compromise between optical transparency and electrical conductivity. Even though the associated Ohmic losses reduce the performance of the antenna, there are ways, such as some particular deposition techniques [7], to improve conductivity. This letter is divided into three sections that include the anal- ysis of ITO samples for the transparent antenna, antenna char- acterization, and experimental confirmation of its performance for RFID tag detection. II. ITO FILM CHARACTERIZATION Different types of commercially available ITO sheets were evaluated in order to select the adequate surface resistivity and layer thickness for acceptable antenna radiation efficiency. Used samples of ITO consisted of glass substrate ( , , , and thickness 1 mm) with different ITO coatings. Manufacturers specify the electrical properties of thin conductive films through the sheet resistance in sq and coating thickness [8]. A key point is to correctly model low-conductivity thin films in commercial electro- magnetic (EM) solvers. In order to validate this modeling, the insertion loss of a simple 50- microstrip line (width 1536-1225/$26.00 © 2011 IEEE