1. Design and Miniaturization of an RFID Tag Using a Simple Rectangular Patch Antenna for Metallic Object Identification Mun Leng Ng Auto-ID Lab @ Adelaide School of Electrical & Electronic Engineering University of Adelaide Australia mng@eleceng.adelaide.edu.au

2. RFID System Basic components:

3. Tagging Metallic Objects (1) Effects of metallic surfaces on RFID tag antennas: • Insufficient interrogation fields

4. Tagging Metallic Objects (2) • Detuning of resonant frequency • Impedance mismatch • Change in directivity and radiation pattern

5. Tagging Metallic Objects (3) Possible solutions: • Use antennas that require a ground plane to operate • Use antennas that utilizes the EM fields present near the metallic surface to operate • Leaving a gap between tag antenna and metallic object

6. Objectives • Design a simple tag for metallic objects that uses a basic rectangular patch antenna with a very simple impedance matching method • Analyze the effect of size reduction of the tag antenna above towards the read range performance

7. The Tag Design • The tag consists of: • Designed to operate in the Australia UHF RFID band (920 MHz – 926 MHz) • Target frequency used in design calculations and simulations is 923 MHz Impedance matching Tag antenna Tag chip

8. The Tag Antenna • A regular rectangular patch antenna is used • Material: FR4 (double-sided copper clad) thickness h = 1.6mm relative dielectric permittivity ɛr = 4.4 • Dimensions:Patch length Lpatch = 77mmPatch width Wpatch = 99mm

9. RFID tag chip and equivalent circuit: Rchip = 1kΩ and Cchip = 1.2pFEquivalent impedance Zchip = 20 – j141 Ω(at 923MHz) The Tag Chip

10. To obtain maximum power transfer Use inset feed methodTransform the antenna impedance at the inset using a microstrip line The combination of the inset distance and microstrip line length gives a total impedance equals to the conjugate of Zchip Used microstrip line with characteristic impedance 50 Ω Impedance Matching

11. Simulation using Ansoft HFSS Inset feed distance and microstrip line length determined through simulations A small square area (3 mm x3 mm) connected to the ground plane through a via located at the end of the microstrip line Tag Design Simulation (1)

12. Simulation results:Total impedance = 17+j144 Ω at 923 MHz Tag antenna structure also simulated on a 1.5λ x 1.5λaluminium metallic plane Tag Design Simulation (2)

13. RFID reader (Model ALR-9780-EA) suitable for operation in Australia is used in the measurement Total radiated power from the antenna is 4 W EIRP Tag is placed on a 1.5λ × 1.5λ aluminium metallic plane and with the reader antenna radiating at normal incidence to the metallic plane Read range measured = 1.44 m. Read Range Measurement

14. Aim is to reduce the size of the tag antenna to find the smallest possible size while still:- offering acceptable read range performance- maintaining a low tag cost Tag size reduction done by:- reducing the patch width Wpatchof the tag antenna- Patch length Lpatchremained the same- Material remained the same: Low-cost FR4 Wpatchreduced at steps of 10 mm, from 99 mm (original full size) to 19 mm. Effect of Wpatch reduction on read range performance analyzed Tag Size Reduction

15. Same simulation methods used From simulations, found that as Wpatch is reduced:- Antenna impedance increased- Resonant frequency of the tag antenna has also increased slightly Hence, total impedance of the tag antenna structure changed To compensate for the impedance change, inset feed distance and microstrip line length adjusted slightly for each case Tag antenna structures also simulated on a 1.5λ x 1.5λaluminium metallic plane Tags Simulation (1)

16. Simulation results for Wpatch = 19, 49 and 99 mm shown: Tags Simulation (2) Power loss ratio curves Wpatch = 49 mm Wpatch = 99 mm (original size) Wpatch = 19 mm

17. Tags Simulation (3) Radiation pattern Wpatch = 99 mm (original size) Wpatch = 49 mm Wpatch = 19 mm

18. Effect on Read Range (1) • Fabricated tags:Smallest tag has patch width 19 mmLargest tag (original size) has patch width 99 mm

19. Effect on Read Range (2) • Read range measurement results:

20. Observations:A pattern in the reduction of read range when Wpatchis reducedRead range of the smallest size tag (with Wpatch= 19 mm) is about half that of the full size tag (with Wpatch= 99 mm) Read range for the smallest tag is still acceptable considering the amount of tag size reduction compared to the full size tag Effect on Read Range (3)

21. A simple tag for metallic objects presented Tag has satisfactory read range performance An analysis of the reduction of the tag antenna size (reduction of patch width) and the effect on the read range performance is also presented. There is a trade-of between the antenna size and the read range performance If read range requirement is lower, a smaller tag will be beneficial in terms of cost and the ease of attaching the tag to smaller metallic objects Conclusion

22. Thank you ! Questions can be directed to: Mun Leng Ng mng@eleceng.adelaide.edu.au