A SMALL PASSIVE UHF RFID TAG FOR METALLIC ITEM IDENTIFICATION

Mun Leng Ng

Auto-ID Lab @ Adelaide

School of Electrical & Electronic Engineering

University of Adelaide

Australia

mng@eleceng.adelaide.edu.au

Introduction on RFID

• What is RFID?• RFID basic components:

HO STCO M PUTER

RFID REA DER RA DIO W AV ES RFID TAG SO N O BJE CTS

RE ADERAN TEN NA

Metallic Environment

• Can be the surrounding or the item (object) to be identified.- Surrounding:

Warehouses full of metallic shelvesIndustrial area with heavy machineries

- Object to be identified:Can foodMetallic mechanical partsMetallic 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:

• Rchip = 3 k and Cchip = 1.15 pF (in parallel)

Zchip = 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:Hrec = 10 mm, Lrec = 25 mm, Wrec = 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 Abovemetallic plane

Zant = 0.25 + j150

Peak directivity = 1.3 dB

Zant = 0.54 + j151

Peak directivity = 6.4 dB

Implementation and Fine-Tuning

• Fine tuning using a small un-tuned loop

Small un-tuned loop

To networkanalyser

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.