References: 1. Hsieh et al., Key Factors Affecting the Performance of RFID Tag Antennas, Current Trends and Challenges in RFID, Chapter 8, 151-170, InTech (2011)
2. N. D. Reynolds, Long range Ultra-High Frequnecy (UHF) Radio-frequency Identification (RFID) Antenna Desgn, MSc Thesis, Purdue University (2005)
3. Rao et al., Impedance Matching Concepts in RFID Transponder Design, Fourth IEEE Workshop on Automatic Identification Advanced Technologies
(2005)
RESULTS Validation: From Figure 4, it can be seen the model trends gave reasonable
& realistic results, and followed the physical test results, for both read range
& power transmission coefficient. However, the model predicted marginally
higher values for the tag’s resonance frequency, the read range & the power
transmission coefficient at resonance. Where, the percentage increase in the
tag’s resonance frequency, read range & power transmission coefficient were
found to be 1.34%, 2.43% & 4.77% above the physical test data,
respectively. These small variations were deemed minor & the results from
the model acceptable.
Optimization: Overall it took a total 42h 23m of simulation time to find the an
optimized antenna design as illustrated in Figure 5, using both the BOBYQA
& Monte Carlo methods [7]. The optimized solution’s objective value () was
found to be 0.676, & gave a read range of 2.38m. Variations in read range for
different reader settings are presented in Table 1.
INTRODUCTION RFID tags are ever increasing in their use, from the tracking of products, to
touch-less technologies, seen in today’s payment cards. With this there has
been an increasing need to reduce their size, & the power required to
activate the tag, while maximizing their operating range, or read range. In
order to maximize a tag’s read range, it is important to ideally match the
impedance of the tag’s antenna with the chip (Figure 1) [1-3].
AIM To developed a numerical model to quantify a RFID tags read range,
validate the model against physical test data & use it to optimize a tag’s
antenna design to maximize the read range for an example store card.
Impedance Matching of RFID Tags to Maximize
Read Range & Optimization of Antenna Design Mark S Yeoman1, Mark A O’Neill2 1. Continuum Blue Ltd., Tredomen Innovation & Technology Park, CF82 7FQ, United Kingdom
2. Tumbling Dice Ltd., 39 Delaval Terrace, Newcastle upon Tyne, NE3 4RT, United Kingdom
DISCUSSION & CONCLUSION An RFID tag model was developed & validated. The model was found to
marginally over-estimate the tag’s response. This is possibly due to
variations in geometric & material properties compared to the physical
samples used in [3]. The model was used to find an optimal tag antenna
design, where geometric & manufacturing constraints were implemented.
These designs are to be manufactured & tested for further model validation.
ACKNOWLEDGEMENTS The authors acknowledge the support of the TSB in
providing funding for this research
Table 1. Optimisation runs and changes in Power Transmission Coefficient ()
METHOD (1) A numerical model of an RFID tag’s antenna & chip was developed in
COMSOL (Figure 2), using the RF Module’s Electromagnetic Waves,
Frequency Domain node to describe the physics of the RFID tag’s circuit
(Figure 1ii). In order to maximize the read range of an RFID system, it is
important to ideally match the impedance of the tag’s antenna with the chip.
The power transmission coefficient (), which relates the power absorbed by
the chip (Pc) to the maximum power from the antenna (Pa), describes the
impedance match between chip and antenna, where as 1 the better the
match. The power received by the tag’s antenna can be calculated using
Friis’ free-space transmission equation, from which one is able to formulate
the read range (r) for a particular RFID tag design & reader. The model
developed was then validated against physical test data from literature[3].
Stage# Optimisation
Solver Design Start
Point Run time
Objective Value
1 BOBYQA Initial design 2h 13m 0.498
2 Monte Carlo Solution above
2h 13m 0.900
3 BOBYQA Solution above
2h 13m 0.900
Initial (start) design objective value 0.303
Description Units Reader System
Reader Power W 1 2 1 2
Reader Antenna
Gain dBi 9 9 11 11
Read Range m 2.38 3.36 2.99 4.23
Equations. Power Transmission Coefficient (), Friis Free-Space Transmission &
Read Range (r) Pc : Power absorbed by chip
Pa : Maximum power from antenna
Rc : Chip resistance Ra : Antenna resistance Zc : Chip impedance Za : Antenna impedance : Wavelength Pr : Reader transmitted power Gr : Reader antenna gain Ga : Tag antenna gain Pth : Chip minimum threshold power
𝜏 =4𝑅𝑐𝑅𝑎𝑍𝑐 + 𝑍𝑎
2
𝑃𝑎 = 𝑃𝑟𝐺𝑟𝐺𝑎
4𝜋𝑑
2
𝑃𝑐 = 𝑃𝑎𝜏
𝑟 =
4𝜋
𝑃𝑟𝐺𝑟𝐺𝑎𝜏
𝑃𝑡ℎ
Figure 1. (i) Illustration of RFID System & (ii) Equivalent Circuit of RFID Tag
Figure 2. COMSOL model of RFID tag, including substrate, antenna & chip
4. Murata Magicstrap® Technical Data Sheet, Murata Manufacturing Co., Ltd., Kyoto, Japan, www.murata.com
5. OBID® UHF Long Range Reader LRU1002 Product Data Sheet, FEIG Electronic GmbH, Lange Strasse 4, D-35781 Weilburg, Hessen, Germany,
www.feig.de
6. OBID i-scan® UHF Antenna series Product Data Sheet, FEIG Electronic GmbH, Lange Strasse 4, D-35781 Weilburg, Hessen, Germany, www.feig.de
7. COMSOL Multiphysics® Version 4.4, Optimization Module Documentation, COMSOL AB (2014)
Figure 3. Tag antenna start design & geometric variables for optimization
Figure 4. Comparisons of (i) read range & (ii) power transmission coefficient
obtained from COMSOL model vs. physical test data from Rao et al. [3]
Table 1. Read ranges for different reader settings Figure 5. Optimized tag antenna design
METHOD (2) An optimisation node was added to the model to optimize the geometric
parameters of a store card antenna (Figure 3), to maximise the read range for
a specific chip and reader system. The geometric parameters of the antenna
were constrained to a specified design region. Additionally, manufacturing
constraints were added to ensure that the chip mounting & antenna
manufacture were possible.
METHOD (3) The optimization objective function was set to maximize the power
transmission coefficient (), for the following chip & reader system:
• Chip frequency / impedance [4]: 866.5 MHz /15-45j
• Reader Power [5] / Antenna Gain [6]: 1W / 9dBi Za
Zc
Va
Tag Antenna Tag Chip
(ii) Equivalent Circuit of RFID Tag
Reader
Reader
antenna
Reader interrogating
electromagnetic field
(i) Illustration of RFID System
Tag induced
electromagnetic field RFID tag
Tag Antenna optimisation start
design
Chip (Murata MagicStrap®)
75mm
15
mm
45
mm
71.2mm
L1/t
1
Chip (Murata MagicStrap®)
Sym
met
ry li
ne
L2/t2
L3/t
3
L4/t4
L5/t
5 L6/t6
L8/t8
L10/t10 L14/t14
L12/t12 L16/t16
L7/t
7
L9/t
9
L11
/t1
1
L13
/t1
3
L15
/t1
5
L17
/t1
7
l# : section No. # length
t# : section No. # width KEY
RFID Tag Chip (Murata MagicStrap®)
Tag antenna design
Substrate
Air domain & perfectly matched layer (PML)
(i) Read Range Data (ii) Power Transmission Coefficient Data
1.0
1.5
2.0
2.5
3.0
3.5
4.0
875 900 925 950 975 1000 1025 1050
Re
ad R
ange
(m
)
Frequency (MHz)
Physical Data (Rao et al. 2005)
COMSOL Equivalent Modal Data
0.0
0.1
0.2
0.3
0.4
0.5
0.6
875 900 925 950 975 1000 1025 1050
Po
we
r Tr
ansm
issi
on
Co
effi
cie
nt
Frequency (MHz)
Physical Data (Rao et al. 2005)
COMSOL Equivalent Modal Data
Optimized antenna design
Chip (Murata MagicStrap®)
75mm
40
.2m
m
45
mm
Excerpt from the Proceedings of the 2014 COMSOL Conference in Cambridge