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Design and Miniaturization of an RFID Tag Using a Simple Rectangular Patch Antenna for Metallic Object Identification. Mun Leng Ng Auto-ID Lab @ Adelaide School of Electrical & Electronic Engineering University of Adelaide Australia [email protected] RFID System.

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Design and Miniaturization of an RFID Tag Using a Simple Rectangular Patch Antenna for Metallic Object Identification

Mun Leng Ng

Auto-ID Lab @ Adelaide

School of Electrical & Electronic Engineering

University of Adelaide

Australia

[email protected]

rfid system
RFID System

Basic components:

tagging metallic objects 1
Tagging Metallic Objects (1)

Effects of metallic surfaces on RFID tag antennas:

  • Insufficient interrogation fields
tagging metallic objects 2
Tagging Metallic Objects (2)
  • Detuning of resonant frequency
  • Impedance mismatch
  • Change in directivity and radiation pattern
tagging metallic objects 3
Tagging Metallic Objects (3)

Possible solutions:

  • Use antennas that require a ground plane to operate
  • Use antennas that utilizes the EM fields present near the metallic surface to operate
  • Leaving a gap between tag antenna and metallic object
objectives
Objectives
  • Design a simple tag for metallic objects that uses a basic rectangular patch antenna with a very simple impedance matching method
  • Analyze the effect of size reduction of the tag antenna above towards the read range performance
the tag design
The Tag Design
  • The tag consists of:
  • Designed to operate in the Australia UHF RFID band (920 MHz – 926 MHz)
  • Target frequency used in design calculations and simulations is 923 MHz

Impedance matching

Tag antenna

Tag chip

the tag antenna
The Tag Antenna
  • A regular rectangular patch antenna is used
  • Material: FR4 (double-sided copper clad) thickness h = 1.6mm relative dielectric permittivity ɛr = 4.4
  • Dimensions:Patch length Lpatch = 77mmPatch width Wpatch = 99mm
the tag chip
RFID tag chip and equivalent circuit:

Rchip = 1kΩ and Cchip = 1.2pFEquivalent impedance Zchip = 20 – j141 Ω(at 923MHz)

The Tag Chip
impedance matching
To obtain maximum power transfer

Use inset feed methodTransform the antenna impedance at the inset using a microstrip line

The combination of the inset distance and microstrip line length gives a total impedance equals to the conjugate of Zchip

Used microstrip line with characteristic impedance 50 Ω

Impedance Matching
tag design simulation 1
Simulation using Ansoft HFSS

Inset feed distance and microstrip line length determined through simulations

A small square area (3 mm x3 mm) connected to the ground plane through a via located at the end of the microstrip line

Tag Design Simulation (1)
tag design simulation 2
Simulation results:Total impedance = 17+j144 Ω at 923 MHz

Tag antenna structure also simulated on a 1.5λ x 1.5λaluminium metallic plane

Tag Design Simulation (2)
read range measurement
RFID reader (Model ALR-9780-EA) suitable for operation in Australia is used in the measurement

Total radiated power from the antenna is 4 W EIRP

Tag is placed on a 1.5λ × 1.5λ aluminium metallic plane and with the reader antenna radiating at normal incidence to the metallic plane

Read range measured = 1.44 m.

Read Range Measurement
tag size reduction
Aim is to reduce the size of the tag antenna to find the smallest possible size while still:- offering acceptable read range performance- maintaining a low tag cost

Tag size reduction done by:- reducing the patch width Wpatchof the tag antenna- Patch length Lpatchremained the same- Material remained the same: Low-cost FR4

Wpatchreduced at steps of 10 mm, from 99 mm (original full size) to 19 mm.

Effect of Wpatch reduction on read range performance analyzed

Tag Size Reduction
tags simulation 1
Same simulation methods used

From simulations, found that as Wpatch is reduced:- Antenna impedance increased- Resonant frequency of the tag antenna has also increased slightly

Hence, total impedance of the tag antenna structure changed

To compensate for the impedance change, inset feed distance and microstrip line length adjusted slightly for each case

Tag antenna structures also simulated on a 1.5λ x 1.5λaluminium metallic plane

Tags Simulation (1)
tags simulation 2
Simulation results for Wpatch = 19, 49 and 99 mm shown:Tags Simulation (2)

Power loss ratio curves

Wpatch = 49 mm

Wpatch = 99 mm

(original size)

Wpatch = 19 mm

tags simulation 3
Tags Simulation (3)

Radiation pattern

Wpatch = 99 mm

(original size)

Wpatch = 49 mm

Wpatch = 19 mm

effect on read range 1
Effect on Read Range (1)
  • Fabricated tags:Smallest tag has patch width 19 mmLargest tag (original size) has patch width 99 mm
effect on read range 2
Effect on Read Range (2)
  • Read range measurement results:
effect on read range 3
Observations:A pattern in the reduction of read range when Wpatchis reducedRead range of the smallest size tag (with Wpatch= 19 mm) is about half that of the full size tag (with Wpatch= 99 mm)

Read range for the smallest tag is still acceptable considering the amount of tag size reduction compared to the full size tag

Effect on Read Range (3)
conclusion
A simple tag for metallic objects presented

Tag has satisfactory read range performance

An analysis of the reduction of the tag antenna size (reduction of patch width) and the effect on the read range performance is also presented.

There is a trade-of between the antenna size and the read range performance

If read range requirement is lower, a smaller tag will be beneficial in terms of cost and the ease of attaching the tag to smaller metallic objects

Conclusion
thank you

Thank you !

Questions can be directed to:

Mun Leng Ng

[email protected]

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