Dualfrequency Antenna Design for RFID Application

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)
• 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 (lt135 kHz)
• HF (13.56 MHz)
• UHF (860 960 MHz)
• Microwave (2.45 GHz)

4
Frequency Band in RFID

• LF (lt135 kHz)
• HF (13.56 MHz)
• UHF (860 960 MHz)
• Microwave (2.45 GHz)

?
?
5
HF vs UHF
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Proposal Formulation

• Merge HF and UHF
• Dual Frequency Antenna
• (With frequency ratio 70)

7
Current Technology

• Microstrip patch antenna
• Too low frequency ratio (lt 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.

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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 - j150O
• Design aim 17 j150O
• A resonance point at HF.
• Parallel resonance.
• Zero reactance and infinite resistance.

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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.

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A Simple HF RFID Antenna

• A multi-turn planar spiral antenna.

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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.

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Final Design
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Final Design (1)

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

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Final Design (2)

• Overlapping loops to provide high capacitance.

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Final Design (3)

• A gap to prevent the UHF antenna shorting the HF
  antenna. A patch on the bottom provides path for
  UHF operation.

18
Final Design (4)

• DC path for rectifier circuit (some type).

19
Simulation

• Using Ansoft HFSS
• Simulated impedance (at 960 MHz)
• 24 j143O
• Very near to the target of 17 j150O
• Resonance near 13.56 Mz

20
Fabrication

• On double-sided FR4

21
Measurement Setup
SMA Connector (At the chip location)
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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
• 50 j135O
• Balance to unbalance problem
• BALUN needed.
• Pattern in good agreement

25
Future Work

• Miniaturization.
• 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.