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2011 International Conference on Advanced Technologies for Communications (ATC 2011)

Improving Radiation Characteristics of UHF RFIDAntennas by Zigzag Structures

Van Hieu Nguyenl, Hong Phuong Phan, Member,

EEE, Manh Ha Hoangl1. Faculty of Electrical and Electronic Engineering

HCMC University of Technology

Ho Chi Minh Ciy, Vienam

phphuongcmut edu vn

Astact- antenna structure for UF RFID tag matchedto ST Microelectronics XG2 passive tag Ie is proposed in thispaper. The design is realized on PET (olyethyleneTerephthalate) substrate that has the overall dimension of 82.5mm x 45.29 mm. The simulation shows that the proposed zigzagshaped antenna has the return loss at the resonant frequency of-39 dB and omnidirectional radiation characteristics with themaximum gain of about 1.97 dBi that is much better than that ofthe initial meander dipole. The prototype antenna was tested byRohde Shwarz ZVB Vector Network Analyzer and showedgood t with the simulation results.

Kywods antnna dsign; ; tag; and lnantnna;antnna adiation antnna fcincy

I. NTOCTON

FID - Radio Frequency Identication is a technology ofautomatic identication of objects by radio equency transmission which was rstly found in 1930s and had become

more popular in 1960s and 1970s [1]. Today FID technologyis widely used in dierent mode and sophisticated ways withdifferent standards: LF (125-134 kHz), HF (13.56 MHz), UHF(860-960 MHz), etc.

It is known, the antenna is the most important part of anFID tag and its dimensions aect signicantly the dimensionsof the tag. Therefore, the antenna design is in huge need inorder to have good radiation characteristics such as omnidirectional radiation pattes, high efciency, good impedancematching, as well as decrease the antenna dimensions in order to reduce the fabrication cost and the price of tag products.Dipole antenna has been in fact studied previously with variedstructures and shapes [1] and each conguration had suitableadvantages for its application.

Furthermore, strong development of FID technologyresults in more professional ad exible antenna design tomeet technical and social requirements. Thus, the problem ofFID antennas has been attracting the attention of manyexperts, researchers, engineers as well as students that has beenshown in different references [2][3][4]. In [2], a special antennastructure with a 90-degree bent patch has been proposed to

Dat Son Nguyen2, Mau Chien Dang2, Thuat

Nguyen-Tran2' Nhan Ai Tran2, Anh Hoang2, EricFribourg-Banc3

2. Laborator for Nanotechnology -NationalUniversit Ho Chi Minh City, Communit 6, Thu

Duc disric, Ho Chi Minh ciy, Vienam

3. CEA-LETI-MINATEC, 17, rue des Martyrs, 38054Grenoble Cedex 9, France

(*) ntthuat@vnuhcm edu vn

 have half-eld radiation of 4.72 dBi ad a bandwidth of about25 MHz. However, this atenna has a big size (200mm x55mm) and 3-dB beam width of 150 degrees shows its low

directivity. In order to mount to a tag chip with a largeinductive reactance, authors in [3] proposed a dipole antennaenclosed by a short stub to increase its inductive reactance forconjugate matching. However, the antenna eciency has notbeen mentioned. In [4], the authors have proposed a specialantena form on a exible square Liquid Crystal Polymer(LCP) substrate in order to obtain a high efciency. As it canbe seen, the meandering shape of a dipole antena can reduceits size, but also has some weanesses. Hence, it is necessary tond a new design of FID antennas with high gain, largebandwidth and omni-directional radiation patterns in order toincrease reading range of a whole system.

This paper will focus on enhancing the directivity and gainof an FID dipole antenna with a special zigzag-shape. Then,starting with this structure, the authors will study someapproaches to build a UHF FID dipole antenna with a betterbadwidth while its omni-directional radiation characteristicsae guaateed. The paper is further organized as follows:Section II shows the design problems and the method of theirsolution to design a UHF RFID tag antenna to match with a STMicroelectronics XRAG2 passive tag I. Next, Section IIIillustrates some simulation results obtained by using CSTMicrowave Studio. A prototpe has been made and tested by the Rohde & Shwarz ZVB Vector Network Analyzer andnally, the conclusions are given in Section IV. A descriptionof the research platform for FID in our institution can befound in [5].

II. TH SN NTNNA ONATON

For the design, STMicroelectronics XRAG2 passive tag ICcompatible with the EPC Global Class- Generation-2 UHFRFID specication was used. The impedance of the IC is 10 j245 Ohms. However, the impedance for the design is chosen to be 20j250 Ohms in order to compensate some parasiticresistance and capacitance of the contact beteen the antennaand the chip. The substrate PET (86mm x 54mm) with

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 thickness of h=0.08 mm, relative dielectric permittivity of =3and tan=0.02 is chosen to realize the design idea.

We need to design an antena for conjugate matching with the tag IC, i.e. the input impedance of the antena should be20+j250 Ohm at 868 MHz. The initial design was based on a ha-wavelength straight dipole that the beam patterns are known as omni-directional (Fig. 1).

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Figure I The radiation pae of a ha-wavelength straight dipole antenna

Although these characteristics are good, the antenna is still too big and its input impedance does not match with the chip.Therefore, we need to increase impedance by reference om[1], an inductive reactance was created by a meander line that

 reduces the dimension at the same time. As a result, theantena radiation patterns are no longer omni-directional. Theconguration of this antenna and its simulated radiation patterns and gain obtained by using CST Microwave Studio areshown in Fig. 2 and Fig.3.

Figure 2 The meander line antenna structure

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Figure 3 The radiation patterns of the meander line dipole antenna

However, as mentioned in [6], the current in neighboringarms ows in opposite directions and does not radiate so that the radiation efciency decreases resulting in reduction of theantena gain. To avoid this effect, a zigzag shape taen om the idea of a zigzag wire antenna [7] is applied, ad 90-degreebent line is used in the antena structure onl to adust its inutimedance Fi.4). Radiation pattern returned as omnidirectional (Fig.5).

From the relativel ood roerties of the imressiveziza structure, we could al it to desin more effectiveRID antenna. In order to improve the bandwidth, a triangularame has been added [8]. The conguration of the proposedantena using a zigzag-shaped dipole is shown in Fig. 6. It is

103

constructed on a PET substrate (=3, tan=0.02) with thicknessof 0.08 mm.

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Figure 4 Cancelled current effect reduction [I]

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Figure 5 The radiation pattern of dipole antenna with zigzag shape.

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Figure 6 The structure of proposed RDUF antenna

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Figure 7 The real and imaginary pars of impedance for dierent triangle frame open angles

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Fig. 7 shows the real and imagin pats of the inputimpedance for dierent triangle frames. Aer some simpleadjustments for impedance matching, by changing size of thezigzag angles and the open-agle of triangle ame, based on the results of analysis to obtain the desired impedance (20 + j250 Ohms) to match with the complex conjugate of the chips,

nally, we obtained the antenna structure with overalldimension of 82.5 mm x 45.29 mm.

III. SMULATON AND XPERMENTAL RSUTS

The retu loss of the proposed antenna at the resonatequency is -39 dB aer the impedance matching with the chip(Fig. 8). The radiation patterns of the proposed antenna arequite similar to that of a straight dipole antenna and themaximum gain is about 1.97 dBi as shown in Fig. 9 which wasconsiderably improved as compaed to that of the initialmeander diole.

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Figure 8 The return loss coecient before and aer matching with chip

The bandwidth of the proposed antenna design was34MHz, covering the equency range om 866 to 868 MHz(which is allowed in Vietnam for UHF FID systems)(Fig.), and the obtained input impedance is 49+j248.5 Ohm(Fig.13 a).

The 3-dB beamwidth is 87.3 degrees (Fig.11) resulting in gooddirectivity of the antena which could increase the readingdistance of the UHF FID system.

In order to test the simulation results, a prototpe has beensuccessfully fabricated at Laborator for Nanotechnology(LNT) - Ho Chi Minh Cit National Universit by usingphotolithography followed by wet etching on sputtered copperlayer [9]. On this prototpe, a thickness of m was fabricatedin order to rapidly compare the experimental results withsimulation. Further experimental works will be carried out to thicken the copper layer by means of low cost batch productionelectrodeposition procedures so that 10 m of thickness can be

obtained. The prototpe is tested by using Rohde & ShwarzZVB Vector Network Analyzer. Since PET substrate canotstand high temperature, the measurement was done using abroadband quarter-wavelength balun [10] designed andfabricated by the authors to characterize various broadbanddipole antennas (Fig. 12 a)).

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Figure 9 The 3D radiation pae of the proposed antenna

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Figure 10 The 34 MHz bandwidth at -IOdB of the proposed antenna

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Figure II The 3-dB beamwidth of the proposed antenna

The measurement was set up as shown in Fig.12 b), where the antenna was connected to the balun followed by a cable toRohde & Shwarz ZVB Vector Network Analyzer.

The simulation and measured input impedance of theprototype ae shown in Fig. 13 a) and b). As it can be seen, themeasured impedace is 76 + j256 Ohms that is greater than thesimulation value by 26+j8 Ohms. It can be explained by the

fact that the copper layer thickness of the fabricated antenna isonly 0.001 mm while the value used in simulation is 0.01 mmas the fabrication target.

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a)

b)

Figure 12 Measurement of the protope a). The broadband balun used for themeasurement; b). The measurement scheme

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Figure 13 The resulted input impedance of antenna (a) simulation and (b)measured

105

Besides, the errors ca be caused by other factors such asnon-perfect measurement environment, SMA connectors and cable eects [11] that the authors are trying to solve. At themoment, due to some limited facilities, only the antennaimpedance can be measured. The authors are carring out themeasurement of other radiation characteristics of the antenna, then mounting the chip on it to test the reading distace of thewhole system.

IV. CONLUSIONS

The proposed zigzag-shaped antena has a return loss at theresonant equency of -39 dB and omni-directional radiationcharacteristics with the mimum gain of about l.97 dBi. Thestructure is easy to adjust for conjugate matching to variouscommercial ID tag chips. Based on the rst proposedantena, some other RFID antenna structures can be developedin order to have desired characteristics to adapt to variousapplications. Apart om measuring the antenna itself, theauthors are going to mount the chip on it and test the readingdistance of the whole tag.

KNOWLEDGMENT

This work is supported by the Ministry of Sciences andTechnology, Vietnam.

EFEENES

[] Gaetano Marrocco "The art of UHF FD antenna design: impedancematching and size -reducing technequis Published in IEEE antennasand propagation magazine, Vo.50, N., Jan. 2008.

[2] Amit Rawal, Nemai . Karmakar, "A Novel LShaped FD Tag Antenna, Proceedings of the 10th European onference on WirelessTechnology 2007 pp. 245248.

[3] Youngbaek hoi Uisheon Kim, Jaehoon hoi "Design of a Dipole Tag Antenna Enclosed by a ShortStub for UHF FD Appl ication, IEEE Antennas and Propagation Societ International Symposium, 2008, pp.14.

[4] S. Basat S. Bhaachara, A. Rida S. Johnston L. Yang M.M.Tentzeris, J. Laskar "Fabrication and Assembly of a Novel HighEfciency UHF D Tag on Flexible LP Substrate, Electronicomponents and Technology onference 2006 pp. 13521355.

[5] Ai Nhan Tran, Son at Nguyen, Duc Hoang Thai Pham,Thuat NguyenTran, hien Mau Dang and Eric FribourgBlanc "A new platform forFID research in Vietnam Adv. Nat. Sci.: Nanosci. Nanotechnol. (2010) 045015

[6] Daniel Dobkin, The in ID - Passive UHF D in Practice.Elsevier Inc., 2008

[7] ZainudDeen, S.H.; Awadalla, K.H.; Bahnacy, A.I.; Sharshar, H.A."Performance analysis of zigzag antenna for portable radio equipment, Antennas and Propagation Societ International Symposium, 1997, pp.442447.

[8] Stefanie Alki Delichatsios, Pro. Neil Gershenfeld "GA Optimizationfor FD Broadband Antenna Application -MAS.862, May 22, 2006.

[9] Luong Hong Dung, Tran Huy Nam, Tran Nhan Ai, NguyenTran Thuat,Dang Mau hien, Eric FribourgBlanc "Stdy of FD antennafabricaton. Proceedings of the 2nd International Workshop on Nanotechnology and Application (A 2009), pp. 622625

[10] www.rf ic.co.uk" Balun Design -lesson 3a, sheet 16.

[11] Kin Seong Leong, Mun Leng Ng, ole, P.H "Investigation of Fable Eect on FD Tag Antenna Impedance Measurement, IEEE Antennas and Propagation Societ International Symposium, 2007, pp.573576.