Design of Paper-Substrate Dipole Antennas Magnetically Coupled to UHF RFID Silicon Chips F. Alimenti, G. Orecchini, M. Virili, V. Palazzari, P. Mezzanotte, L. Roselli University of Perugia, Dept. of Electronic and Information Engineering, via G. Duranti 93, 06125 Perugia, ITALY Abstract—This work investigates the design of a paper- substrate dipole antenna and its magnetic coupling to a UHF RFID chip. The magnetic coupling is realized by means of a heterogeneous transformer, the primary winding of which is implemented on the same paper-substrate of the antenna. The secondary winding, instead, is directly fabricated on the Si RFID chip, thus does not require for galvanic contacts between chip and antenna. The transformer insertion loss can be reduced to the device Maximum Available Gain (MAG) if a proper impedance termination of the primary (Z S,opt ) and secondary (Z L,opt ) windings is adopted. For the considered heterogeneous transformer the MAG is quite low and around −0.6 dB. Following this idea, a bow-tie dipole antenna is designed to meet Z S,opt at the operating frequency of 868 MHz. The antenna size is reduced by exploiting a meander line. As a result, the designed dipole features an overall length of about 40 mm. To the authors knowledge this is the first bow-tie antenna the design of which has been optimized for the magnetic RFID coupling concept. Index Terms—Dipole antennas, RFID, heterogeneous integra- tion, flexible electronics, ink-jet paper printed circuits. I. I NTRODUCTION RFID tags working in the UHF frequency range rely on low power CMOS circuits and flexible substrate antennas. In recent papers an ultra-low cost assembly process for chip and antennas has been demonstrated, promising a significant break-trough in RFID technology. To this purpose the CMOS chip is magnetically coupled to the antenna realized on a paper substrate, thus eliminating all the galvanic contacts between the chip and the antenna itself [1], [2]. The magnetic coupling is established, as in Fig. 1, by a heterogeneous planar transformer, the primary and secondary windings of which are implemented on paper substrate and Si chip respectively. As a result the RFID chip can be mounted by mere placing and gluing process steps. In particular the chip will be pad-less and completely passivated, the pad-ring being substituted by the secondary coil of the transformer. Considering a typical 1 mm 2 RFID chip area, and assuming a secondary winding similar to the on-chip coil reported in [3], several heterogeneous transformer geometries have been developed. The insertion loss of these devices is quite low in the UHF frequency range (less than 1 dB) and can be minimized in order to achieve the Maximum Available Gain (MAG), that holds when optimum terminations are provided at both primary (i.e. the antenna side) and secondary (i.e. the chip side) windings. Fig. 1. RFID chip magnetically coupled to a dipole antenna. Particular of the coupling transformer (top): the primary winding is fabricated on paper whereas the secondary winding is on-chip. This paper deals with the design of a bow-tie dipole antenna on paper substrate. A meander-line matching network has been embedded within the antenna. This way, without the need for additional components, the antenna impedance can meet the optimum value required by the heterogeneous transformer. As the final step, the primary winding of the heterogeneous transformer has been integrated with the bow-tie antenna and the overall structure is validated by means of electromagnetic simulations. The obtained layout can be simply produced by exploiting the inkjet printing process with conductive silver ink. To the authors knowledge this is the first bow-tie antenna the design of which has been optimized according to the magnetic RFID coupling concept. II. TRANSFORMER STRUCTURES The heterogeneous transformer is constituted by a primary winding on the paper substrate and by a secondary winding on the silicon chip. On the paper substrate the coil is assumed to be printed by means of an inkjet technology. The silver ink conductivity is around 2.5 × 10 7 S/m as in [4]. The ink thickness, instead, is around 2 μm as pointed-out in [5]. The minimum metal track width and spacing is 50 μm, correspond- ing to the maximum spatial resolution of the printer. 2011 IEEE International Conference on RFID-Technologies and Applications 978-1-4577-0027-9/11/$26.00 ©2011 IEEE 219