Progress In Electromagnetics Research Symposium, Beijing, China, March 23–27, 2009 1257

A Miniature Chip Antenna Design for a Passive UHF RFID Tag toBe Built in a Portable Device

Yu-Shu Lin, Hsien-Wen Liu, Kuo-Hsien Wu, and Chang-Fa YangDepartment of Electrical Engineering

National Taiwan University of Science and TechnologyTaipei, Taiwan

Abstract— In this paper, a miniature antenna for a passive UHF RFID tag is designed, whichmay be built in a portable device. Matching techniques for a Gen2 tag IC are employed to enhancethe readable distance of the tag for long-range reading purposes, where quasi-lumped and lumpedelements are used to match the chip antenna to the tag IC having complex input impedance.A commercial simulator, HFSSTM is used to analyze the performance of the antenna. Also,measurements in an anechoic chamber of the RFID education and research center at NationalTaiwan University of Science and Technology are performed to evaluate the readable range ofthe chip tag, which is more than 5 m for a reader with an EIRP equal to 4W. The chip antennaoperating in the 900 MHz RFID band proposed here has dimensions of only 10× 9.5× 0.8mm3.Thus, this miniature tag may be flexibly built in a portable device to allow long-range reading.

1. INTRODUCTION

In recent years, Radio Frequency Identification (RFID) has been developed for many applications,such as supply chain managements, retail store applications, etc. For UHF passive RFID systems,the passive tags need to have good impedance matching to achieve efficient power transfer betweenthe tag IC and antenna. Usually, tag antennas in ordinary half-wave dipole forms were designed [1–6]. In this paper, a miniature chip antenna having a size of only 10× 9.5 × 0.8mm3 for the UHFRFID passive tag is presented, which can be easily built in a portable device. An impedancematching approach by using quasi-lumped components is also investigated and is compared withthat by applying lumped elements. A commercial simulator, HFSSTM [7] has been employed toanalyze the electrical characteristics of the tag antenna and quasi-lumped elements.

2. RESULTS

As shown in Fig. 1, a miniature chip tag operating in the RFID UHF band is proposed here, whichhas antenna dimensions of only 10 × 9.5 × 0.8 mm3. To have a low cost design, this tag antennais fabricated on an FR4 substrate with a relative permittivity εr = 4.4 and loss tangent δ = 0.02.Performance of the proposed tag antenna having an input impedance of 26+j3Ω is demonstrated inFig. 2, where a 100×48mm2 test board is used here. The input impedance of the Monza Gen2 tagIC we have used is equivalent to that of an RC series circuit with R = 33 ohm and C = 1.58 pF. Toachieve a conjugated match with this tag IC, as shown in Fig. 3(a), a lumped inductance of 18 nHis serially connected between the proposed antenna and tag IC. Also, another matching circuit,composed of two compact quasi-lumped elements, is applied to obtain the conjugated match, asshown in Fig. 3(b). This quasi-lumped circuit consists of a series inductance and a shunt inductance,where the equivalent values of the inductances are extracted to be 12.3 nH and 12.5 nH, respectively.

Figure 1: Geometry of the proposed miniature chip tag antenna.

1258 PIERS Proceedings, Beijing, China, March 23–27, 2009

Fig. 4 shows the measured input impedances and reflections of the lumped and quasi-lumped designsfor the proposed chip antenna. The quasi-lumped design has a wider bandwidth in the RFID UHFband.

(a) (b)

Figure 2: The performance of the proposed passive tag antenna. (a) Return loss; (b) Simulated far-fieldradiation pattern at 922MHz.

(a) (b)

Figure 3: Configurations of the lumped and quasi-lumped matching circuits. (a) Lumped circuit withL1 = 8mm, L2 = 11.75mm, L3 = 1.95mm, L4 = 2.25mm, L5 = 7.2mm and L6 = 4 mm; (b) Quasi-lumpedcircuit with L7 = 4mm and L8 = 4 mm.

The readable range of the tag was tested in an antenna measurement anechoic chamber, asshown in Fig. 5. A transmitting horn antenna with a gain Gt was fed by an RFID reader via aconnecting cable with loss Lc. The proposed tag antenna, attached on a Styrofoam, was orientedto obtain the maximum power from the reader antenna. Since RF charging of the passive tag bythe reader is the critical part for a success reading, the maximum readable range for a given EIRPcan be determined by the following formula [6]:

rmax = d

√EIRP

PminGtLc(1)

where d is the distance between the tag and the transmitting antenna, Pmin is the minimum powerof the reader to be able to read the tag, Gt = 5.7 dBi and Lc = −1.5 dB at 922MHz for thechamber system. In those measurements, d was fixed at 3.6m, and the measured Pmin for lumpedand quasi-lumped designs were individually obtained to be 29 dBm and 28 dBm. Therefore, if theEIRP of the reader is set to 4W, the maximum readable ranges with the lumped and quasi-lumped

Progress In Electromagnetics Research Symposium, Beijing, China, March 23–27, 2009 1259

designs will be 5 m and 5.5 m, respectively. Besides, read-range experiments were also performedin a corridor to compare with those from chamber measurements, as listed in Table 1. Due tomulti-path contributions, the distances measured in the corridor were longer in this test case.

Table 1: Experimental results of readable range measurements.

Readable range

Matching circuit

Maximum range in

the corridor

Maximum range in

the anechoic chamber

Lumped elements 5.8 m 5 m

Quasi-lumped elements 6 m 5.5 m

(a) (b)

Figure 4: Measured input impedances and return losses of the proposed tag antenna with the lumped andquasi-lumped matching circuits. (a) Input impedance; (b) Return loss.

(a) (b)

Figure 5: The readable range measurements in an anechoic chamber. (a) Measurement setup; (b) Chamberenvironment.

3. CONCLUSIONS

In this paper, we have proposed and analyzed a miniature chip antenna with lumped and quasi-lumped matching circuits for passive UHF RFID tag applications. The performance of the proposedtag antenna was examined through the readable range measurements in an anechoic chamber. The

1260 PIERS Proceedings, Beijing, China, March 23–27, 2009

compact chip tag with either quasi-lumped or lumped matching circuits can have a readable rangemore than 5 m with a 4 W EIRP reader.

REFERENCES

1. Rao, K. V. S., P. V. Nikitin, and S. F. Lam, “Antenna design for UHF RFID tags: A reviewand a practical application,” IEEE Trans. Antennas Propag., Vol. 53, No. 12, 3870–3876, Dec.2005.

2. X. Qing and N. yang, “A folded dipole antenna for RFID,” Proc. IEEE AP-S Int. Symp. Dig.,97–100, Jun. 2004.

3. Ahn, J., H. Jang, H. Moon, J.-W. Lee, and B. Lee, “Inductively coupled compact RFID tagantenna at 910 MHz with near-isotropic radar cross-section (RCS) patterns,” IEEE Ante. andWireless Propag. Lett., Vol. 6, 518–520, Jun. 2007.

4. Son, H.-W. and C.-S. Pyo, “Design of RFID tag antennas using an inductively coupled feed,”Electronics Lett., Vol. 41, No. 18, 994–996, Sept. 2005.

5. Liu, Y.-W., S.-Y. Chen, and P. Hsu, “Short-ended coplanar strip antenna for UHF RFID tags,”Proc. IEEE AP-S Int. Symp. Dig., 1773–1776, Jun. 2007.

6. Kurokawa, K., “Power waves and the scattering matrix,” IEEE Trans. Microw. Theory Tech.,Vol. 13, No. 3, 194–202, Mar. 1965.

7. HFSS, User’s guide 9.2, Ansoft Corp., USA.