DESIGN OF A COMPACT UHF RFID TAGANTENNA USING AN INDUCTIVELYCOUPLED PARASITIC ELEMENT

Taeik Kim, Uisheon Kim, and Jaehoon ChoiDepartment of Electronics and Computer Engineering 17,Haengdang-Dong, Seongdong-Gu, Seoul, 139-791, Republic ofKorea; Corresponding author: choijh@hanyang.ac.kr

Received 24 April 2010

ABSTRACT: In this article, a compact tag antenna for UHF RFID

systems is proposed. The proposed antenna consists of a T-matchingnetwork, meandered dipole, and an inverted-U shaped parasitic

element. The tag antenna is miniaturized by utilizing inductive couplingbetween the meandered dipole and the parasitic element. The overalldimension of the proposed tag antenna is 17.96 � 35.6 mm2. It has a

gain of 1.52 dBi and a maximum reading distance of 6.3 m at 915MHz. The size of the tag antenna is about 33% of that of other RFIDtag antennas, such as a meander line antenna and a coplanar

inverted-F antenna. In spite of its small size, the proposed tag antennayields superior performance. VC 2010 Wiley Periodicals, Inc. Microwave

Opt Technol Lett 53:239–242, 2011; View this article online at

wileyonlinelibrary.com. DOI 10.1002/mop.25753

Key words: RFID; tag antenna; inductive coupling; parasitic element

1. INTRODUCTION

Radio-frequency identification (RFID) systems have been inves-

tigated extensively as a key technology for future communica-

tions. Recently, research has been conducted on various applica-

tions of RFID systems, such as electronic toll collection, asset

identification, retail item management, animal tracking, and ve-

hicle security. Compared with a barcode system, an RFID sys-

tem has advantages of identification capability, semipermanent

durability, rapid identification, long reading range, and flexibil-

ity. For these reasons, logistics companies and distribution mar-

kets have replaced traditional barcode systems by RFID systems

to improve efficiency and convenience. As most UHF RFID

tags have to be attached to small objects, the antenna geometry

must be miniaturized without serious degradation of the radia-

tion efficiency. Many studies have focused on reducing the size

of tag antennas using various configurations, such as ‘‘Meander-

ing’’ and ‘‘Inverted-F’’ [1]. However, compact antennas using

meandering lines have low gain and narrow bandwidth. Even

though antennas using an inverted-F structure have high gain,

the height or size of the antenna becomes large because of the

additional ground plane.

In this article, we proposed a compact UHF RFID tag

antenna using a parasitic element. To reduce the size of a tag

antenna, inductive coupling between a meandered dipole and a

parasitic element is utilized. Critical parametric analysis has

been conducted, and the performance of the proposed tag

antenna is compared with that of existing small tag antennas,

such as a meander-line antenna (MLA) and a coplanar inverted-

F antenna (IFA).

2. ANTENNA DESIGN

Figure 1 shows the configuration of the proposed tag antenna.

The proposed antenna consists of a T-matching network, mean-

dered dipole, and an inverted-U shaped parasitic element. The

tag antenna was fabricated on FR4 (er ¼ 4.4, tan d ¼ 0.02,

thickness ¼ 0.6 mm). The tag chip used in this study is an Alien

Higgs-2 microchip with an impedance of 16-j131 X at

915 MHz. The proposed tag antenna was designed and analyzed

using the Ansoft high-frequency structure simulator (HFSS

Ver.10) [2].

The T-matching network is often used for impedance match-

ing of RFID tag antennas. The impedance of an antenna can be

easily controlled by varying the length and width of the short

Figure 1 The configuration of the proposed tag antenna

Figure 2 Input impedance characteristic of the proposed tag antenna

versus frequency for various values of G1

DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 53, No. 2, February 2011 239

transmission line and the gap distance between the short line

and the antenna [3]. In Figure 1, L1 is the length of the short

line, and G1 is the gap distance between the short line and

antenna. The width of the meandered dipole is 1 mm, and that

of the short line is 0.5 mm. Figures 2 and 3 show the input im-

pedance characteristics of the proposed tag antenna as a function

of frequency for various values of L1 and G1. As L1 or G1

increases, the input impedance of the antenna becomes higher,

and the impedance changes rapidly as frequency varies. For im-

pedance matching between the tag antenna and tag IC, L1 and

G1 are chosen to be 13.1 and 0.5 mm, respectively. Both the

input resistance and reactance of a tag antenna increase as the

total length of the trace increases.

A meandered dipole is connected to the T-matching network

to reduce the length of the dipole. The meandering structure

makes the antenna compact and provides near-omnidirectional

performance in the plane perpendicular to the axis of the mean-

der line. The spacing between lines of meandering structure is

uniform and has a value of 2.99 mm. The horizontal length of

the meander line is 10.65 mm, and the outside line length L2 is

chosen such that L2 ¼ 0.5 þ G1 þ 5 � 1 þ 4 � 2.99.

The inverted U-shaped parasitic element is inserted between

meander lines. The parasitic element is 0.3 mm apart from the

top line of a dipole and separated from meander lines by G2.

The width of the parasitic element is 1 mm and the height of

the parasitic element is adjusted to match with the bottom line

of the meandered structure so that the effect of coupling

between the parasitic element and meander lines can be maxi-

mized. Figure 4 shows the current distribution of the proposed

tag antenna. The current exiting at the feeding point generates a

strong current loop consisting of a top portion of the meandered

dipole and T-matching network and weak current flows on the

remaining part of the meandered line. The secondary current is

induced on the parasitic element. Figure 5 shows the equivalent

circuit model of the proposed tag antenna. The mutual induct-

ance, Map, is caused by the mutual interaction between the

meandered line and the parasitic element. By this inductive cou-

pling, the input impedance of the tag antenna is changed, so that

the conjugate matching between the antenna and the tag IC can

be accomplished. Figure 6 shows a comparison of input imped-

ance characteristics with and without a parasitic element. Figure

7 shows the resonant frequencies of the tag antenna for various

values of G2. As G2 decreases, the value of mutual inductance

increases. Thus, the resonant frequency of the tag shifts toward

a lower frequency. However, the gain of the antenna is

decreased, because the direction of current flowing on the para-

sitic element is opposite that of the meander line. To achieve a

gain over 1.5 dBi, G2 is chosen to be 0.5 mm. When the

antenna was matched at 915 MHz by increasing the size of the

meandered dipole without using a parasitic element, the gain of

the antenna was 0.5 dBi.

3. RESULTS

The design parameters of the tag antenna are listed in Table 1.

The performances of the proposed tag antenna are compared

with those of other small antennas in Ref. 1. Table 2 shows a

comparison of the performances of three antennas. The perform-

ance factors for the proposed tag antenna, such as size and gain,

are better than those of the conventional small antennas. The

area (0.055k � 0.109k) of the proposed antenna is only 30% of

Figure 3 Input impedance characteristic of the proposed tag antenna

versus frequency for various values of L1

Figure 4 Currents distribution on the meandered dipole and nearby parasitic element

240 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 53, No. 2, February 2011 DOI 10.1002/mop

MLA [1] and IFA [1]. The simulated and measured S11 charac-

teristics of the proposed tag antenna are illustrated in Figure 8.

The result shows that the proposed tag antenna is conjugately

matched with the tag IC at 915 MHz and has a 10-dB return

Figure 5 Equivalent circuit model for the proposed tag antenna

Figure 6 Comparison of input impedance characteristics of the pro-

posed tag antenna with and without parasitic element

Figure 7 Resonant frequencies of the tag antenna for various values

of G2

TABLE 1 The Proposed Design Parameters

Parameter G1 G2 L1 L2

Unit (mm) 0.5 0.5 13.1 17.96

TABLE 2 Comparison of Performances of the Antennas withDifferent Design Scheme

MLA [1] IFA [1] Proposed

fo 870 MHz 870 MHz 915 MHz

fmin 856 MHz 852 MHz 900 MHz

fmax 885 MHz 887 MHz 936 MHz

Df 29 MHz 35 MHz 36 MHz

Bandwidth 3.3% 4.0% 3.9%

Gain 1.46 dBi 1.33 dBi 1.52 dBi

Size 0.14 k � 0.14 k 0.14 k � 0.14 k 0.055 k � 0.109 k

Figure 8 Simulated and measured S11 characteristics of the proposed

tag antenna

Figure 9 Measured wake-up sensitivity and recognition distance ver-

sus frequency

DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 53, No. 2, February 2011 241

loss bandwidth of 9 MHz (910�919 MHz). The recognition dis-

tance for the tag is obtained from the Friis transmission formula

[4] as follows:

R ¼ k4p

1ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiPtag

EIRP�Gtag�ð1�jCj2Þ�c

q (1)

where EIRP (effective isotropic-radiated power) is the maximum

power available for an RFID reader and is standardized as 36

dBm in many countries. The polarization mismatch loss, c, hasa value of �3dB for the proposed tag antenna with linear polar-

ization. Power sensitivity, Ptag, is proportional to the sensitivity

of the tag IC. Gtag is the antenna gain and 1 � | C |2 represents

the impedance mismatch loss between the tag antenna and the

tag IC. The recognition distance and wake-up sensitivity were

measured within an anechoic chamber. The measurement system

is comprised of a computer, an RFID reader (Mercury4, TM-

M4/W-NA-02), a reader antenna (EMW antenna, FSDC-07),

and variable attenuators. The wake-up sensitivity of the tag

antenna was �15.48 dBm at 915 MHz, and the measured maxi-

mum recognition distance was 6.3 m, as shown in Figure 9. The

simulated and measured radiation patterns in Figure 10 were

similar to those of a typical dipole antenna. The simulated and

measured peak gains of the tag antenna were 1.52 and 1.48 dBi

at 915 MHz, respectively.

4. CONCLUSION

We proposed a compact UHF RFID tag antenna using a para-

sitic element. The proposed tag antenna is miniaturized by uti-

lizing inductive coupling between the meandered dipole and the

parasitic element. The area (0.055k � 0.109k) of the proposed

antenna is only 30% of MLA [1] and IFA [1]. The peak gain of

the antenna is 1.52 dBi, and the measured maximum reading

distance is 6.3 m at 915 MHz. The performance factors for the

proposed tag antenna, such as size and gain, are better than

those of other small tag antennas [5–7].

ACKNOWLEDGMENT

This work was supported by the Seoul R&BD program (10848),

Republic of Korea.

REFERENCES

1. G. Marrocco, The art of UHF RFID antenna design: Impedance-

matching and size-reduction techniques, IEEE Antennas Propag

Mag 50, 66–79, Jan 2008.

2. Ansoft High Frequency Structure Simulator (HFSS), Ver.10.0,

Ansoft Corporation.

3. C.A. Balanis, Antenna Theory, 3rd ed., Wiley, New York, 2005.

4. Y. Choi, U. Kim, J. Kim, and J. Choi, Design of a modified folded dipole

antenna for UHF RFID tag, IEE Electron Lett 45 (2009), 387–389.

5. K. Finkenzeller, RFID handbook, Wiley, New York, 2000.

6. S.W. Bae, W.S. Lee, K.H. Chang, S.W. Kwon, and Y.J. Yoon, A

small RFID tag antenna with bandwidth-enhanced characteristics

and a simple feeding structure, Microwave Opt Technol Lett 50

(2008), 2027–2031.

7. G. Gonzalez, Microwave Transistor Amplifiers, 2nd ed., New

jersey, Prentice Hall, 1997.

VC 2010 Wiley Periodicals, Inc.

DOUBLE-CAVITY FIBER FABRY-PEROTTUNABLE FILTER

Yi Jiang,1 Caijie Tang,1 and Chong-Wen Wang2

1 School of Optoelectronic, Beijing Institute of Technology, Beijing100081, China; Corresponding author: bitjy@bit.edu.cn2 School of Software, Beijing Institute of Technology, Beijing,10081, China

Received 24 April 2010

ABSTRACT: A double-cavity fiber Fabry-Perot tunable filter isproposed and experimentally demonstrated. There are two cavities in thetunable filter. The two cavities have close properties, and are driven by

same piezoelectric transducers. One cavity is used to scan wavelength,and another one is used to calibrate the wavelength. The double-cavityfilter is anticipated to be used to interrogate not only the spectra of

passive components, but also that of active light sources. VC 2010 Wiley

Periodicals, Inc. Microwave Opt Technol Lett 53:242–245, 2011; View

this article online at wileyonlinelibrary.com. DOI 10.1002/mop.25752

Key words: fiber Fabry-Perot filter; fiber optic sensors

Figure 10 Simulated and measured radiation patterns of the proposed

tag antenna at 915MHz

242 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 53, No. 2, February 2011 DOI 10.1002/mop