Dual-frequency Antenna Design for RFID Application

Kin Seong LeongAuto-ID Laboratory, School of Electrical and

Electronic Engineering, The University of Adelaide

Introduction

• Radio Frequency Identification (RFID)– 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)

Current Technology

• Microstrip patch 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.

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

Chip

Final Design (1)

• Transmission line to transfer the HF coil antenna impedance to very high value (ideally open circuit).

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:– 50 + j135Ω

• Balance to unbalance problem– BALUN needed.

• Pattern in good agreement

Future Work

• Miniaturization.– 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.