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Study of Transparent Antennas for RFID

Serra, C. C. ; Medeiros, C. R. ; Costa, J.R. ; Fernandes, C. A.

Study of Transparent Antennas for RFID, Proc IEEE AP-S/URSI International Symp., Spokane, United States, Vol. -, pp. - - -, July, 2011.

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This work presents a new concept of optically transparent antenna for an RFID (Radio Frequency Identification) system, operating in the UHF (Ultra-High-Frequency) band (project FCT PTDC/EEA-TEL/102390/2008). The objective is to incorporate a shallow low cost antenna into the surface of fitting-room mirrors in apparel stores in order to allow the client to interact in a simple way with the
product database from the store. The antenna is intended to read the RFID tags attached to the clothes being tried on in front of the mirror and transmit the data to an associated management system which automatically provides the client in real time with information related to that specific piece of clothing, alternative sizes, colors or models.

A few antenna design challenges are raised by this application. The main one is to guarantee that the associated RFID reading range is self-confined to the fittingroom and centered with the mirror, to avoid interference with adjacent fittingroom RFID readers. This circumvents the need for physical barriers that increase cost and deter retrofitting of existing installations. Also, the antenna must be
shallow and its feeding cables must be invisible not only for aesthetical reasons, but also so that the client doesn’t feel uncomfortable with unusual devices inside the fitting-room. The antenna cannot be just hidden behind the mirror since this is not electromagnetically transparent. The idea in this work is to print an optically transparent array of patch antennas on the glass surface of the mirror and use the mirroring layer as the antenna ground plane. RF power is coupled from behind the mirror, through appropriately designed slots.

After a detailed analysis of transparent conductive materials, ITO (Indium- Titanium Oxide) with a pre-determined surface resistance was selected for this application. An appropriate antenna configuration was optimized to cope with the above specifications. A prototype of the array was fabricated using a conventional mirror and evaluated in terms of input reflection and radiation pattern.

Prototype RFID detection performance was also evaluated using commercial RFID reader and tags. The final mirror-integrated antenna solution shows good compromise between radiation efficiency and optical transparency, good reading range confined to the volume of a fitting-room and good tolerance to tag orientation due to the use of circular polarization. The prototype looks very much a typical shallow mirror from fitting-rooms and it is easy to fabricate without requiring especially skilled professionals or difficult fine tuning after fabrication. Even though there are some aspects that need improvement in order that this solution is ready for industrialization, it fully demonstrates the concept chosen to solve the presented problem.