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