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

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.

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

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

HF vs UHF

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.

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

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.

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.

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 14400 mm square.

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

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.

An Initial Picture Feed point chosen to be at B.

Final Design

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

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

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

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

Simulation Using Ansoft HFSS Simulated impedance (at 960 MHz): 24 + j143Ω Very near to the target of 17 + j150Ω Resonance near 13.56 Mz

Fabrication On double-sided FR4

Measurement Setup SMA Connector (At the chip location)

HF Testing Transmission measurement: Resonance at HF.

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

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

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

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