A SMALL PASSIVE UHF RFID TAG FOR METALLIC ITEM IDENTIFICATION Mun Leng Ng Auto-ID Adelaide School of Electrical & Electronic Engineering University of Adelaide Australia

Introduction on RFID What is RFID? RFID basic components:

Metallic Environment Can be the surrounding or the item (object) to be identified. - Surrounding: Warehouses full of metallic shelves Industrial area with heavy machineries - Object to be identified: Can food Metallic mechanical parts Metallic beer kegs Focus is on metallic object. Challenge: To get sufficient fields to reach RFID tag antenna near metal.

Common Tag for Metallic Objects Conventional planar passive UHF RFID tags not suitable for metallic item identification. Existing RFID tags for metallic objects: –Example: Uses patch antenna, printed inverted-F antenna –Normally big in area. –To be small, need high dielectric constant substrate which may be expensive.

Design Concept Small in size Exploits the theory of boundary conditions for better performance

Theory and Design Expressions used for antenna parameters calculations:

Theory and Design Tag chip used: R chip = 3 k  and C chip = 1.15 pF (in parallel) Z chip = 7 – j150  (at 915 MHz)

Theory and Design Physical dimension of antenna adjusted to provide sufficient inductance to be tuned with by capacitance of the tag chip. Radiation resistance of antenna is expected to be low since antenna size is kept small. Impedance matching network not added to maintain the simplicity and low cost of design. Calculated antenna dimension: H rec = 10 mm, L rec = 25 mm, W rec = 5 mm

Simulations Using Ansoft HFSS Two cases considered: - RFID tag in free space - RFID tag near metallic surface

Simulations Radiation pattern (yz-plane): Free space Above metallic plane Z ant = j150  Peak directivity = 1.3 dB Z ant = j151  Peak directivity = 6.4 dB

Implementation and Fine-Tuning Fine tuning using a small un-tuned loop Small un-tuned loop To network analyser

Testing of Tag Set up: RFID reader with output peak power 250 mW. Use 8dBi gain circularly polarised reader antenna. Total transmit power = 1.6 W EIRP Attach RFID tag approximately 3 mm above metallic plane. Reader antenna radiate at normal incidence to the metallic plane.

Testing of Tag Read range measured over 900 MHz – 940 MHz:

Testing of Tag Tag designed for operation in the USA RFID band (which also spans the Australian RFID band). Measurement shows that tag able to perform well in RFID band of interest. For 4W EIRP transmit power, read range is expected to increase by 1.6 times.

Conclusion A new RFID tag design for tagging metallic objects. The tag shown in this paper is designed for metallic beer kegs but can be redesigned for other metallic surfaces, with minimal changes. It is small, simple to manufacture and low in cost.