design and miniaturization of an rfid tag using a simple rectangular patch antenna for metallic...
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Design and Miniaturization of an RFID Tag Using a Simple Rectangular Patch Antenna
for Metallic Object Identification
Mun Leng NgAuto-ID Lab @ Adelaide
School of Electrical & Electronic EngineeringUniversity of Adelaide
RFID System
Basic components:
HO STCO M PUTER
RFID REA DER RA DIO W AV ES RFID TAG SO N O BJE CTS
RE ADERAN TEN NA
Tagging Metallic Objects (1)
Effects of metallic surfaces on RFID tag antennas:
• Insufficient interrogation fields
charge
current
perfect electric conductor perfect electric conductor
electricfield
magneticfield
Tagging Metallic Objects (2)
• Detuning of resonant frequency
• Impedance mismatch
• Change in directivity and radiation pattern
LC2π
1rf
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
• 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 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
Tag antennaImpedance matching
Tag chip
The Tag Antenna
• A regular rectangular patch antenna is used• Material: FR4 (double-sided copper clad)
thickness h = 1.6 mmrelative dielectric permittivity ɛr =
4.4• Dimensions:
Patch length Lpatch = 77 mmPatch width Wpatch = 99 mm
The Tag Chip
• RFID tag chip and equivalent circuit:
• Rchip = 1 kΩ and Cchip = 1.2 pFEquivalent impedance Zchip = 20 – j141 Ω(at 923 MHz)
Rchip Cchip
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
Tag Design Simulation (1)
• Simulation using Ansoft HFSS
• Inset feed distance and microstrip line length determined through simulations
• A small square area (3 mm x 3 mm) connected to the ground plane through a via located at the end of the microstrip line xin
abw
Wpatch
Lpa
tch
W + 12hpatch
L +
12h
patc
h
lline
via
patchsubstrate
chip
Tag Design Simulation (2)
• Simulation results:Total impedance = 17+j144 Ω at 923 MHz
• Tag antenna structure also simulated on a 1.5λ x 1.5λ aluminium metallic plane
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.
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 Wpatch of the tag antenna- Patch length Lpatch remained the same- Material remained the same: Low-cost FR4
• Wpatch reduced at steps of 10 mm, from 99 mm (original full size) to 19 mm.
• Effect of Wpatch reduction on read range performance analyzed
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 1.5λ aluminium metallic plane
Tags Simulation (2)• Simulation results for Wpatch = 19, 49 and 99 mm shown:
Wpatch = 19 mm
Wpatch = 99 mm
(original size)
Power loss ratio curves
Wpatch = 49 mm
Tags Simulation (3)
Wpatch = 19 mm
Radiation pattern
Wpatch = 99 mm
(original size) Wpatch = 49 mm
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)
• Read range measurement results:
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
0 20 40 60 80 100 120
Patch width, Wpatch (mm)
Rea
d ra
nge
(m)
Free spaceOn metal plane
Effect on Read Range (3)
• Observations:
A pattern in the reduction of read range when Wpatch is reduced
Read 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
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
