1180 PIERS Proceedings, Moscow, Russia, August 1821, 2009

Design of a Miniaturized Broadband Tag Antenna for UHF RFIDSystem

Xingyu Zhang and Anping ZhaoAdvanced Systems Engineering, Nokia Research Center, Beijing 100176, China

Abstract In order to achieve the characteristics of miniature, low cost and broad bandwidthfor RFID tag antennas, a novel passive UHF planar tag antenna is proposed in this paper.The proposed antenna is designed in the form of inductively coupled and comprised of a longfolded dipole and a modified double T-matching network. It was constructed with a thin copperlayer printed on a 0.24mm-thick PET substrate for low cost production. The presented antennaprovides a fairly wide bandwidth, which is much larger than those of the existing antennaswith similar structures and satisfies the bandwidth requirements of the worldwide UHF RFIDsystems. Besides, the main radiation pattern of the reported antenna tends to the orientation thatis perpendicular to the antenna surface which helps to identify the target objects. Furthermore,the gain of the antenna meets the demands of UHF RFID systems. All the features above makethe proposed antenna applicable in use for UHF RFID systems.

1. INTRODUCTION

Radio frequency identification (RFID) that just began its explosive development in the last decadeis actually with a long history of more than half a century [1]. It has lately attracted more andmore attention for use in efficiently tracking and identifying objects in various supply chains fromsecurity and control point of view. An RFID system basically consists of a transponder (a tag),a reader antenna and a computer connected to the reader. Data is transferred between the tagand the read/write device by means of electromagnetic waves at the allocated frequency bands of125 kHz, 13.56MHz, 840845MHz, 869MHz, 902928MHz, 955MHz, 2.45GHz and 5.8GHz [2].Antennas are fundamental elements in RFID communication systems. Also, the design of RFIDtag antenna becomes more complicated and critical when the operating frequency rises to themicrowave band region. A tag antenna should be small size, low profile and simple structure forlow cost in production and convenient in use. Various kinds of RFID tag antennas have beenreported in the open literatures. A compact slotted PIFA-type RFID tag antenna was studiedin [3]. A tag antenna using the cavity for long reading range was presented in [4]. Two inductivelycoupled RFID antennas in two different structures, namely arc-shape and dual-body configurations,were designed in [5]. A dipole-type printed RFID antenna operating at UHF band (from 868 to965MHz) was reported in [6, 7]. However, these tag antennas are neither small in size and low inprofile nor broad in bandwidth for practical applications.

In this paper, a compact planar RFID tag antenna with low profile and fairly wide bandwidth isproposed. The antenna is inductively coupled in order to easily implement the impedance conjugate-matching between the tag antenna and the microchip, for the reactance of the microchip is ratherlarge because of the production process. The presented antenna is comprised of a folded dipole anda modified double T-matching network. It was fabricated by printing a thin copper layer on a PETsubstrate the profile of which is 0.24mm. The designed antenna with a volume of 40 50 0.28mm3is smaller than those antennas mentioned above. Besides, the operational bandwidth of the designedantenna when conjugate-matched to the microchip satisfies the bandwidth requirements of theRFID systems in all UHF bands, which helps to the world-wide circulation of the RFID merchandise.

2. ANTENNA DESIGN AND RESULTS ANALYSIS

The configuration of the proposed RFID tag antenna is shown in Fig. 1. The antenna has a simpleand symmetrical structure by printing it on one side of a PET (dielectric constant r = 3.6 and losstan = 0.003) substrate with size of 40 50mm2 and thickness of 0.24mm. The antenna consistsof a long folded dipole that is used to reduce the size of the antenna. A double T-matching networkis used to adjust the power transmission coefficient between the tag antenna and the microchip (i.e.,to implement the impedance conjugate-matching between the tag antenna and the microchip). Theuniform width of the meandered stripline is 2mm and a 2mm length slot in the center is reservedfor the feeding position (i.e., the place for attaching the microchip).

Progress In Electromagnetics Research Symposium Proceedings, Moscow, Russia, August 1821, 2009 1181

Figure 1: Geometry of the proposed RFID tag an-tenna.

Figure 2: Impedance characteristics of the proposedantenna.

Figure 3: Equivalent model of the feeding structure of the tag antenna.

Figure 4: Impedance variations of the studiedRFID tag antenna.

Figure 5: Return loss of the presented antenna whenconjugate-matched to the microchip impedance.

Figure 2 depicts the simulated impedance characteristics of the proposed antenna. It can beclearly seen from Fig. 2 that the impedance of the designed antenna is 11.2+j132 at 915MHz. Theinput impedance of the microchip (ALN-9338-R) specified in this study is 6.2 j127 at 915MHzwhich indicates that this antenna design meets the requirement of the impedance conjugate-matching between the tag antenna and the microchip. Besides, the feeding structure of the tagantenna in the center can be clearly explained by the transmission line theory. Fig. 3 shows theequivalent model of the feeding structure. The input impedance of the transmission line in idealsituation can be described as formula (1):

Z(x) = jZ0 tan(2pix

)(1)

when x satisfies the condition of 0 < x < /4, tan(2pix/) > 0 can be achieved. In this case,Z(x) acts as an inductor and the reactance becomes larger with the increase of x. While x satisfiesthe condition of x > /4, tan(2pix/) < 0 can be observed. Z(x) then acts as a capacitor andthe reactance becomes smaller with the increase of x. Hence, the impedance of the tag antennacan be simply adjusted by changing the distance (described by d in Fig. 1) between the feedingand the shorting striplines. Meanwhile, the impedance of the tag antenna can also be changedby adjusting the stripline width of the dipole and double T-matching network, and the separation

1182 PIERS Proceedings, Moscow, Russia, August 1821, 2009

between the meandered strplines of the dipole (which is fixed at 6mm in this study). Fig. 4 portraysthe impedance characteristics of the studied RFID tag antenna with variation of the parameter d.It can be seen from Fig. 4 that the impedance conjugate-matching is optimized while d is 17mm.

Figure 5 denotes the computed return loss of the presented antenna when conjugate-matched tothe impedance of the microchip. One can see from Fig. 5 that the return loss of the antenna is lowerthan 10 dB when the frequency ranges from 0.8 to 1GHz, which satisfies the bandwidth need ofthe worldwide RFID system in UHF bands. In particular, compared to the existing antennas withthe same structures, much bigger operational bandwidth is achieved for the proposed antenna. Thisalso helps to the circulation and practical applications of the RFID merchandise in the world.

The far-field radiation patterns of the designed tag antenna at 915MHz are plotted in Fig. 6. Itcan be noted that, due to the symmetrical structure, the main radiation direction of the presentedantenna tends to the orientation that is perpendicular to the antenna surface, which helps to theidentification of the target objects for the RFID tag antenna.

The calculated gain of the proposed RFID tag antenna at the operational frequency band isexhibited in Fig. 7. One can conclude from Fig. 7 that the gain is between 0.5 and 2.11 dBi whenthe operating frequency ranges from 0.8 to 1GHz. It can also be illustrated that the gain of theantenna doesnt decrease much although fold and coupling of the long dipole exist in the antennadesign.

(a) E-plane (b) H-plane

Figure 6: Radiation patterns of the proposed tag antenna at 915MHz.

Figure 7: Gain of the proposed RFID tag antenna at the operational frequency band.

Progress In Electromagnetics Research Symposium Proceedings, Moscow, Russia, August 1821, 2009 1183

3. CONCLUSIONS

A compact and low profile passive RFID tag antenna with broadband characteristics has beendesigned in this paper. It realizes a fairly broad bandwidth (S11 < 10 dB between 0.8GHz and1GHz) and achievable gain (from 0.5 to 2.11 dBi) with acceptable radiation patterns, although itsvolume is only 40 50 0.28mm3 based on a long folded dipole. The antenna can be fabricated bysimply printing a copper layer on one side of the PET substrate. The impedance of the tag antennacan be simply adjusted by changing the width and distance of the copper stripline that helps to theimpedance conjugate matching between the tag antenna and the microchip. Moreover, this antennacan be easily mounted and has good compatibility with other microwave circuit components.

REFERENCES

1. Landt, J., The history of RFID, IEEE Potentials, Vol. 24, No. 4, 811, Oct.Nov. 2005.2. Keskiammi, M. and M. Kivikoski, Using text as a meander line for RFID transponder anten-

nas, IEEE Antennas and Wireless Propagation Letters, Vol. 3, 372374, 2004.3. Kwon, H. and B. Lee, Compact slotted planar inverted-F RFID tag mountable on metallic

objects, Electronics Letters, Vol. 41, No. 24, 13081310, 2005.4. Kyoung, H. L., L. Jin-Seong, K. Goojo, and H. M. Byung, Design of UHF RFID metal tag

with long reading range using cavity, Asia-Pacific Microwave Conference (Invited Paper),Dec. 1620, 2008.

5. Yang, L., B. S. Serkan, and M. M. Tentzeris, Design and development of novel inductivelycoupled RFID antennas, IEEE International Symposium on Antennas and Propagation, 10351038, Jul. 2006.

6. Ahmed, I., T. Vuong, G. Anthony, and T. Smail, New design antenna for RFID UHF tags,IEEE International Symposium on Antennas and Propagation, 13551358, Jul. 2006.

7. Cho, C., H. Choo, and I. Park, Broadband RFID tag antenna with quasi-isotropic radiationpattern, Electronics Letters, Vol. 41, No. 20, 10911092, 2005.