Optimum antenna coil structure for inductive powering of passive rfid tags

Radio frequency inductive links are becoming extensively used in wireless powering of embedded devices such as RFID systems and sensors. The design of RF coils for such links is often empirical and non optimal. Based on loop antenna theory we present a novel analytical model for near field inductive coupling incorporating misalignment of the RF coil system. Formulae are derived for the magnetic field at the tag coil when it is laterally and angularly misaligned from the reader and a new power transfer function presented. For the first time a near-field power transfer formula is suggested incorporating coil characteristics and misalignment factors. This novel power transfer function allows a comparison between different coil structures such as short solenoids, with air or ferromagnetic core, planar and printed spirals with respect to power delivered at the tag and its relative position to the reader. It is also possible to define the maximum degree of misalignment permissible in a given application. This analysis allows a formal design procedure to be established in order to identify the optimum coil geometry for RFID tags with respect to read range, angular, lateral displacement and operating frequency of the system.

[1]  G. Temple.  Static and Dynamic Electricity , 1940, Nature.

[2]  Mani Soma,et al.  Radio-Frequency Coils in Implantable Devices: Misalignment Analysis and Design Procedure , 1987, IEEE Transactions on Biomedical Engineering.

[3]  Valerie M. Thomas,et al.  Optimization of inductive RFID technology , 2001, Proceedings of the 2001 IEEE International Symposium on Electronics and the Environment. 2001 IEEE ISEE (Cat. No.01CH37190).

[4]  Joshua R. Smith,et al.  Battery-free wireless identification and sensing , 2005, IEEE Pervasive Computing.

[5]  David C. Yates,et al.  Optimal transmission frequency for ultralow-power short-range radio links , 2004, IEEE Transactions on Circuits and Systems I: Regular Papers.

[6]  S. Ramo,et al.  Fields and Waves in Communication Electronics , 1966 .

[7]  David Ki-Wai Cheng,et al.  Design and analysis of planar inductors , 1995, IAS '95. Conference Record of the 1995 IEEE Industry Applications Conference Thirtieth IAS Annual Meeting.

[8]  E. Snelling.  Soft ferrites: properties and applications , 1969 .

[9]  R. Good.  Elliptic integrals, the forgotten functions , 2001 .

[10]  Ron Weinstein,et al.  RFID: a technical overview and its application to the enterprise , 2005, IT Professional.

[11]  P. Nikitin,et al.  Antenna design for UHF RFID tags: a review and a practical application , 2005, IEEE Transactions on Antennas and Propagation.

[12]  Sérgio Francisco Pichorim,et al.  Design of coils for millimeter- and submillimeter-sized biotelemetry , 2004, IEEE Transactions on Biomedical Engineering.

[13]  Klaus Finkenzeller,et al.  RFID Handbook: Radio-Frequency Identification Fundamentals and Applications , 2000 .

[14]  H. Wheeler,et al.  Small antennas , 1975 .

[15]  C.M. Zierhofer,et al.  Geometric approach for coupling enhancement of magnetically coupled coils , 1996, IEEE Transactions on Biomedical Engineering.