1. Dual-frequency Antenna Design for RFID Application
Kin Seong Leong
Auto-ID Laboratory, School of Electrical and Electronic Engineering, The University of Adelaide

2. Introduction Radio Frequency Identification (RFID) Item level tagging
Enable supply chain automation.
Item level tagging
Each and every item has it own tag with unique ID.
Tag is usually passive.

3. Frequency Bands in RFID
LF (<135 kHz)
HF (13.56 MHz)
UHF (860 – 960 MHz)
Microwave (2.45 GHz)

4.   Frequency Band in RFID LF (<135 kHz) HF (13.56 MHz)
UHF (860 – 960 MHz)
Microwave (2.45 GHz)  

5. HF vs UHF

6. Proposal Formulation Merge HF and UHF Dual Frequency Antenna
(With frequency ratio ≈ 70)
Hence, the natural way of thinking is to combing them so that a tag will have all the advantages offered by HF and UHF tag.

7. Current Technology Microstrip patch antenna Common aperture antenna
Too low frequency ratio (< 5).
Common aperture antenna
Dual feed point

8. Brain Storming Merging a HF antenna and an UHF antenna. Idea:
A HF multi-turn coil antenna.
A UHF planar dipole.
A transmission line to separate both the above antennas.
The 3rd way is to merge two working antenna together, a working HF with a working UHF dipole antenna. They are merged in a way that both of them are not affecting each other. Of course the challenge includes matching network design and the single feed constraint.

9. Design Aim (1)
Antenna impedance equals to the complement of the input impedance of the RFID chip at UHF operation
Design frequency: 960 MHz
Chip impedance: 17 - j150Ω
Design aim: 17 + j150Ω
A resonance point at HF.
Parallel resonance.
Zero reactance and infinite resistance.

10. Design Aim (2) A single feed antenna.
Avoid modification on existing chip
Reasonable antenna size and cost.
Not the focus of this paper.
The final design must not be larger than mm square.

11. A Simple HF RFID Antenna
A multi-turn planar spiral antenna.

12. A Simple UHF RFID Antenna
A dipole with matching network.
RFID chip is usually capacitive. The matching network is to transform the antenna into inductive to enable conjugate matching.

13. An Initial Picture
Feed point chosen to be at B.

14. Final Design

15. Final Design (1)
Transmission line to transfer the HF coil antenna impedance to very high value (ideally open circuit).
Chip

16. Final Design (2)
Overlapping loops to provide high capacitance.
Chip

17. Final Design (3)
A gap to prevent the UHF antenna shorting the HF antenna. A patch on the bottom provides path for UHF operation.
Chip

18. Final Design (4)
DC path for rectifier circuit (some type).
Chip

19. Simulation Using Ansoft HFSS Simulated impedance (at 960 MHz):
24 + j143Ω
Very near to the target of 17 + j150Ω
Resonance near Mz

20. Fabrication
On double-sided FR4

21. Measurement Setup
SMA Connector
(At the chip
location)

22. HF Testing
Transmission measurement: Resonance at HF.

23. UHF Testing (1)
Impedance measurement: Matching impedance with respect to RFID chip.

24. UHF Testing (2) At 960 MHz: Balance to unbalance problem
50 + j135Ω
Balance to unbalance problem
BALUN needed.
Pattern in good agreement

25. Future Work Miniaturization. Actual testing with RFID chips.
To fit in small objects.
Actual testing with RFID chips.
To obtain performance (read range) measurement.

26. Conclusion
a detailed design for a high frequency ratio dual-frequency antenna.