A Compact Split-Ring Resonator Using Spiral

ERC-89

4th International Conference on Advanced Technologies For Signal and Image Processing – ATSIP’ 2018 March 21-24, 2018 – Sousse, Tunisia

A Compact Split-Ring Resonator Using Spiral Technique For UHF RFID Tag Mahdi Abdelkarim1, Gerard Zamora2, Ferran Paredes2, Jordi Bonache2, Ferran Martín2 and Ali Gharsallah1 1

Research Unit for High Frequency Electronic Circuits and Systems, Physics Department, 2 CIMITEC, Departament d’Enginyeria Electrònica, 1 Faculty of Science, University of ElManar, Tunis, Tunisia 2 Universitat Autònoma de Barcelona, Barcelona, Spain [email protected]

Abstract—In this paper, the development of a compact tag antenna based on Split Ring Resonator with spiral technique printed on flexible Arlon CuClad 250LX substrate is proposed to operate at North-American UHF-RFID band (902–928 MHz). The spiral technique is used to reduce the passive SRR tag antenna, providing proper impedance matching with the RFID chip. The antenna presents compact size with total dimensions of 16.17mm x 16.17mm (λ/20 x λ/20). The antenna was designed and fabricated on Arlon CuClad 250LX substrate. The RFID IC chip Alien Higgs 3 was soldered directly to the antenna and the read range of the prototype was measured, reaching 4 m. The measurements are in good agreement with simulated results. Keywords; Compact size antenna ; flexible antenna; passive RFID tag ; Spiral split ring resonator.

I.

Introduction

Nowadays Radio Frequency Identification (RFID) system is one of the best communication technologies which use electromagnetic waves for automatic identification of objects. RFID systems have a lot of advantages such as easy read and write with high date rate between the reader and tag. among others. RFID technology has been increasingly widespread in many applications in various area such as service industries, distribution logistics, libraries, passport, smart parking. In these applications, the requirements of small object identifications are drastically increased, therefore the design of a compact tag antenna become compulsory and represents an important challenge to the RFID researcher. For this purpose, several papers have been reported on reduction size of passive UHF RFID tag antenna [1-5]. In [6], a compact size tag antenna (30 mm x 8 mm) using RFID IC chip Alien Higgs 3 with a maximum reading range of up to 0.5 m was specifically designed for UHF RFID applications. In [7] the authors proposed a dipole antenna (35mm x 18mm) suitable for use as UHF RFID tag antenna. The measured read range is about of 1.9m. In [8], a wearable passive tag (80 mm x 80 mm) based on a split ring antenna using RFID IC chip NXP UCODEG2iL with simulated attainable read ranges of 4m and it was proposed to be used in the close proximity of the human body. In [9], a very small size tag antenna (λ0/11 x λ0/11) based on a square-shaped split ring resonator (SRR) antenna using RFID IC chip Alien Higgs 3 with maximum read range reaches 10 m was also proposed. Moreover, a design of a miniaturized meander line tag antenna (43 mm × 10 mm) using Higgs 4 IC chip with value of reading

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range of 0.8m is reported in [10] for wireless tracking of the precious small size for goods/packages in transport. In [11], small UHF-RFID circular-shaped split ring resonator tag antenna (30 mm × 30 mm) using RFID IC chip Alien Higgs 3 with measured read range reaching 9.3 m at working frequency. In [12], the authors proposed a compact (λ0/7 × λ0/7) tag based on complementary split-ring resonators (CSRR) using Alien Higgs 3 chip with a maximum read range of 6.8 m, which is able to operate over metal surfaces with a non-significant performance degradation. In[13] 2-turn spiral resonator antenna (35 mm x 40 mm) with a maximum reading range of 6.7m and good radiation efficiency reaching 75% is proposed for passive RFID tags to operate at the Europe Band (859 MHz 872 MHz). Most of these tags used split-ring resonators for its manufacturing, low cost, easy to match with RFID chips and also due to their very small dimension in term of wavelength. in spite of this type of antenna is limited by such dimensions which are generally about λ/12 at UHF RFID band. In this work the main objective is to compact the design of SRR tag antenna by considering the spiral technique. This technique presents not only a very small size, but also an acceptable read range for used at UHF RFID applications. II.

ANTENNA DESIGN

Figure.1 Topology of the (a) Double SRR form (b) Proposed Spiral-SRR form The design of the proposed antenna is inspired from the classic square split-ring resonator form (Fig. 1a) [9]. It can be seen that the presented antenna (Fig. 1b) is composed by two spiral elements connected to the external ring of split ring resonator through the Gapout looking for reduction the size of classic SRR. The external ring element plays the role of a

radiating body of the antenna and spiral elements play the role of inductively coupled feeding where we placed the RFID IC chip. The dimension of the unit antenna is illustrated in fig1.b. The tag antenna is mounted on flexible Arlon CuClad 250LX substrate with a relative permittivity of εr =2.42, with an thickness of h = 0.49 mm. The size of the antenna is 16.2 mm x 16.2 mm. The designed antenna was matched to the RFID IC chip Alien Higgs 3 with an input impedance of ZC = 25 −j190 Ω and minimum power sensitivity equal to −15 dBm at the center frequency 915 MHz. In effort to minimize the tag size, keeping good impedance matching, we need the antenna impedance to be the same as the input impedance of the chip RC = 25 Ω, without using any external matching network. The transmission coefficient between the antenna and the chip input impedance is evaluated from the power reflection coefficient S11. At the working frequency, the power reflection coefficient S11 depends on the ratio between Rin and Rc. The port position was initially placed at the center of the resonator, where the maximum current appeared. However, at this position, the input impedance of the resonator is the order of few ohms and conjugate matching cannot be achieved. Therefore, in order to obtain good matching, the port is moved through the edge of the SRR arms, since the electric currents decrease and, as a consequence, the impedance increase. The port was finally set at the position where the input impedance is the same of that of the input impedance of the chip Rin ≈ Rc. III RESULT AND DISCUSSION

both rings. Contrariwise in case of Spiral SRR, the small dimension of external ring and the maximizing of the runways force the direction of the current density to be opposite directional and different at both spiral lines which can reduce the tag antenna size.

(a)

(b) Figure.3 Simulation results of the surface current in (a) classic SRR antenna at 1.98GHz (b) Proposed Spiral SRR antenna at 915MHz) 90 0

0

120

E-plane Classic-SRR antenna H-plane Classic-SRR antenna 60

-2 -4

Proposed Spiral-SRR antenna

-10

30

150

-6

S11(dB)

-8 -10

-20

180

0

-8 -6

-30

-150

-4

-30

-2

-40

-50 0,80

-120

0

0,85

0,90

0,95

-60 -90

(a)

1,00

frequency(GHz) 90 0

Figure 2 Simulated power reflection coefficient S11 of the proposed tag In order to better illustrate the influence of the tag antenna in term of miniaturization, a simulation of the return loss is carried out in Figure 2. It can be observed that the presence of two spiral elements in the external ring of split ring resonator has a resonance frequency of 915MHz, whereas the SRR with the same dimensions has an operation frequency of almost 2 GHz. In Fig. 2, it can be also seen the power reflection coefficient S11 of -37 dB with a narrow frequency bandwidth of 8 MHz. Fig. 3 shows the current distribution in the classic SRR antenna and proposed miniaturized antenna, it can be seen that the currents density direction in classic SRR are the same at

120

E-Plane Spiral-SRR antenna H-plane Spiral-SRR antenna 60

-2 -4

150

30

-6 -8 -10

180

0

-8 -6 -4

-150

-30

-2 0

-120

-60 -90

(b) Figure.4 Simulation results of the radiation pattern of (a) classic SRR (b) Proposed Spiral SRR in the H and E-planes at the tag resonance

The comparison of simulated radiation patterns between our proposed tag antenna at 915MHz and classic SRR at 1.98GHz are shown in Figure 4. It can be observed that the Classic SRR has a maximum gain of -0.1dB with radiation efficiency ηrad = 74 %. By adding the two spiral elements into the external ring of split ring resonator, The antenna gain of Spiral SRR becomes low -1.6dB and the efficiency is decreased to 43.5 %. This is due that the size of the Spiral SRR antenna is smaller than the SRR, which explains that the miniaturization of the antenna dimension involves unavoidable maximum achievable gain degradation. Figure (5) presents the simulated input impedance of the Spiral SRR antenna. It can be seen that the antenna impedance is suitable for the chip impedance ZC = 25 − j190 Ω at the frequency of interest. As mentioned, it involves good conjugate matching, equivalent to -37 dB.

get an understanding about the practical performance of the tag antenna. Figure 7 shows the photograph of RFID antenna read range measurement setup available in the laboratory. The tag is placed inside the TEM Cell (Wavecontrol WaveCell) which is connected to a vector signal generator (Agilent N5182A) and signal analyzer (Agilent N9020A) by means of the circulator and 50 Ω coaxial cable. At each frequency, the signal generator sends a power Pt to TEM Cell in effort to activate the tag which needs a minimum threshold power Pth for chip activation. Once the tag is activated, it sends to Agilent N9020A a backscattered response as can be seen in fig.8. Now, after measurement, we can determine the antenna read range by solving the following equation [11]:

r= 200

Real Spiral-SRR antenna Imag Spiral-SRR antenna

Impedance(Ohm)

150

100

50

0 900

905

910

915

920

λ 4π

EIRP ⋅ Gr ⋅τ Pchip

where λ: wavelength Gr: gain of the tag antenna, Pchip: Power of chip τ: Power transmission coefficient. EIRP: Equivalent isotropic radiated power Figure 9 proves that the proposed tag antenna read range can reach 4m at working frequency. Good agreement between the simulation and measurement results has been achieved, although a slight shift frequency occurred.

Frequency(MHz)

Figure 5 Simulated input impedance of the Spiral SRR antenna

Figure.7 photo of the antenna read rang measurement setup using TEM Cell.

Figure.6 Photos of the compact tag antenna prototype In order to evaluate the performance of our proposed SRR antenna, the prototype is fabricated and measured in terms of read range because it is the most interesting characteristic to

Figure 8 Photo of the tag antenna response at working frequency f0=915MHz

V CONCLUSION In this paper, a simple flexible SRR-based RFID antenna for North America (902-928 MHz) RFID system by using the RFID IC chip Alien Higgs 3 has been successfully studied. Based on work results, a proposed antenna was designed and fabricated with a dimension slightly lower than 16.2 mm x 16.2 mm (λ/20 x λ/20). The read range of the tag prototype was measured, reaching up to 4m and demonstrating that this tag is a perfect candidate for RFID applications where a compact size of the antenna is required. References [1]

Figure. 9 Simulated and Measured tag read range.

[2]

TABLE I: Comparison of the proposed tag with previous works Ref

Antenna dimension

(mm)

[3]

Dielectric

RFID

Chip

RFID Frequency Band

Maximum Read Range (m)

[4]

Substrate

[5]

Ref[6]

30 x 8

polyimide

Alien Higgs-3

NorthAmerican

0.5

Ref[7]

35 x 18

FR4

Alien Higgs-3

NorthAmerican

1.9

[6]

Ref[9]

32x 32

Arlon CuClad 250LX

Alien Higgs-3

NorthAmerican

11

Ref[10]

43 x 10

FR4

Alien Higgs-4

Europe

0.8

Ref[11]

30 × 30

Arlon CuClad 250LX

Alien Higgs-3

NorthAmerican

9.3

[8]

Ref[13]

35 x 40

Rogers RO3010

European

6.7

[9]

16.17x 16.17

Arlon CuClad 250LX

NXP SL3S10 01FTT Alien Higgs-3

NorthAmerican

4.2

Proposed Tag

[7]

In effort to evaluate our tag in terms of miniaturization and read range, a comparison of the proposed design to some works that appear in literature with their dimensions and performance parameters has been tabulated as shown in table I. It can be seen that the overall proposed antenna size (16.2mm x 16.2mm) using RFID IC chip Alien Higgs 3 in the presented work is the smallest among the tags which are made on flexible Arlon CuClad 250LX substrate [9], [11] with an approximated size reduction of 50%. In addition, the proposed tag can achieve a reading range of 4 m, which is longer than that tags presented in [6], [7], and [10], with antenna dimension more compact. Finally, we can conclude that the proposed tag not only occupies a compact volume, but also offers a good read range.

[10]

[11]

[12]

[13]

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