Design and Miniaturization of an RFID Tag Using a Simple Rectangular Patch Antenna for Metallic Object Identification Mun Leng Ng Auto-ID Adelaide School of Electrical & Electronic Engineering University of Adelaide Australia

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

The Tag Antenna A regular rectangular patch antenna is used Material:FR4 (double-sided copper clad) thickness h = 1.6 mm relative dielectric permittivity ɛ r = 4.4 Dimensions: Patch length L patch = 77 mm Patch width W patch = 99 mm

The Tag Chip RFID tag chip and equivalent circuit: R chip = 1 kΩ and C chip = 1.2 pF Equivalent impedance Z chip = 20 – j141 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 Z chip 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+j144 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 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 W patch of the tag antenna - Patch length L patch remained the same - Material remained the same: Low-cost FR4 W patch reduced at steps of 10 mm, from 99 mm (original full size) to 19 mm. Effect of W patch reduction on read range performance analyzed

Tags Simulation (1) Same simulation methods used From simulations, found that as W patch 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 W patch = 19, 49 and 99 mm shown: W patch = 19 mm W patch = 99 mm (original size) Power loss ratio curves W patch = 49 mm

Tags Simulation (3) W patch = 19 mm Radiation pattern W patch = 99 mm (original size) W patch = 49 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 W patch is reduced Read range of the smallest size tag (with W patch = 19 mm) is about half that of the full size tag (with W patch = 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