Design and Miniaturization of an RFID Tag Using

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

RFID System Basic components:

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

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

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

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

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

The Tag Antenna • A regular rectangular patch antenna is used • Material: FR 4 (double-sided copper clad) thickness h = 1. 6 mm relative dielectric permittivity ɛr = 4. 4 • Dimensions: Patch length Lpatch = 77 mm Patch width Wpatch = 99 mm

The Tag Chip • RFID tag chip and equivalent circuit: • Rchip = 1 kΩ and Cchip = 1. 2 p. F Equivalent impedance Zchip = 20 – j 141 Ω (at 923 MHz)

Impedance Matching • To obtain maximum power transfer • Use inset feed method Transform 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

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

Tag Design Simulation (2) • Simulation results: Total impedance = 17+j 144 Ω at 923 MHz • Tag antenna structure also simulated on a 1. 5λ x 1. 5λ aluminium metallic plane

Read Range Measurement • 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.

Tag Size Reduction • 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 Wpatch of the tag antenna - Patch length Lpatch remained the same - Material remained the same: Low-cost FR 4 • Wpatch reduced at steps of 10 mm, from 99 mm (original full size) to 19 mm. • Effect of Wpatch reduction on read range performance analyzed

Tags Simulation (1) • 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. 5λ aluminium metallic plane

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

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

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

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

Effect on Read Range (3) • Observations: A pattern in the reduction of read range when Wpatch is reduced Read 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

Conclusion • 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

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