MODIFIED OPEN STUB MULTI-RESONATOR BASED CHIPLESS RFID TAG

Dinesh R*1, Anila P V,2Nijas C M3, Sumi M4 and P Mohanan5

1 2 3 4 5Centre for Research in Electromagnetics and Antennas (CREMA), Department of Electronics, Cochin University of Science and Technology, Cochin-22, Kerala, India

dinsh84@gmail.com1, anilapv@gmail.com2, nijasmhmmd@gmail.com3,sumi.harikrishnan@gmail.com4, drmohan@gmail.com5

Abstract

A planar, fully printable, frequency signature based multi-resonator circuit for chipless RFID transponder system and swipe card is presented. The amplitude and/or group delay of the spectral signature are utilized for the chipless tag or swipe card multi-resonator circuit, providing a 1: 1correspondence of data bits. It comprises of a bifurcated microstrip transmission line meeting at the far end within which the open circuited shunt stub multi-resonators are positioned. The developed structure can be used effectively as a swipe card. The application of the proposed circuit together with its cross-polarized transmitting and receiving microstrip ultra-wideband disc loaded monopole antennas for RFID transponder system is also presented. The experimental and simulated results in support of the potentials of the chipless tag for low cost item tagging such as bank notes and secured documents are explained in this paper. The circuit is having high quality factor and has a very small footprint of 24x10mm2.

Index Terms:—RFID,chipless Tag, Multi-Resonator

1. Introduction

Radio Frequency Identification (RFID) is a powerful enabling technology with ever widening application. The primary potential of the most promising chipless RFID tag is that they could be eventually printed as a miniaturized circuit on a low loss substrate which can replace trillions of barcodes yearly with something far more versatile and reliable. A new idea of using identical arrays of capacitively tuned microstrip dipoles for different frequency for a passive low cost RFID tag is explained in [1]. Engheta et.al in [2] demonstrates the development of RFID tag by placing the space-filling curve inclusions in an array and scaling each element within the array such that each element has its own separate resonant frequency. A spectral signature based tag using spiral multi-resonator circuit for short range application is presented in [3, 5]. The paper [4] describes the use of microstrip open stub resonators for compact chipless RFID tag.

The paper explains a simple planar multi-resonator circuit for chipless RFID tag based on spectral signature encoding technique and its application as swipe card and RFID transponder. It encodes data into the spectrum using the resonant structures and hence having a unique ID. The spectral signatures are created by the multi-resonating circuit which act as multi stop band filters. The proposed circuit is having high quality factor and is very compact compared to other circuits, having only a footprint of 24x10mm2 dimension. The number of data bits can be increased by adding more number of resonators without any considerable change in overall dimension of the circuit.

2. Geometry

The structure comprises of a bifurcated microstrip transmission line meeting at the far end within which the open circuited shunt open stub multi-resonators are positioned. The open circuited shunt stub resonator is very attractive for band notch action due to its high Q-factor and simple structure. The multi-resonator encodes data in their resonant frequencies. The geometry of the proposed multi-resonator fabricated on a substrate of dielectric constant 3.7 and loss tangent 0.003 is given in Fig.1.

Considering each resonator, it will work as an open stub shunt resonator. The equivalent circuit of an open stub resonator is a parallel L-C tank circuit as shown in Fig.2, which offers high impedance at its resonance, the resonance is known as anti-resonance. Each resonance is due to the corresponding open stub resonator, which is connected to the microstrip line at one end and the resonant frequencies are determined by the length of each stub. Each resonator is independently resonating at its quarter wavelength frequency (λg/4) where λg is the guided wavelength. The resonator exhibits high quality factor due to high inductive reactance which avoids the merging of nearest resonant frequencies.

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Fig.1. Geometry of Multi-Resonator circuit (L=24mm, W=10mm, h=1.6mm, εr=3.7, tanδ=0.003)

Fig.2. Geometry of a single resonator with its equivalent circuit (L=24mm,W=10mm)

3. Results and Discussion

The resonant frequencies of the prototype resonators (having total physical dimension 2.4cmX1cm when fabricated on a substrate of permittivity 3.7, loss tangent 0.003 and height 1.6mm) corresponding to each resonator is given in Table.1 and the physical lengths corresponding to each resonator are also given in the same table.

Table 1: Resonant frequencies of Resonators

Resonator Physical

dimension(mm)

Resonant Frequency(GHz)

1 18 2.443 2 17.5 2.730 3 16 3.003 4 15 3.200 5 14 3.392 6 13 3.520 7 12 3.860 8 11 4.300 Spacing between each resonator =0.5mm

Width of each resonator element=0.3mm

The tag encodes its spectral signature into the interrogation signal spectrum using the above multi-resonating

circuit which is a multi-stop band filter. The stop band resonances introduce magnitude attenuation and phase jumps to the transmitted interrogation signal at their resonant frequencies which are detected as abrupt amplitude attenuations and phase jumps. The equivalent model of the multi-resonator circuit with its equivalent parameters is shown in Fig.3. The simulated and measured transmission parameter of the multi-resonator along with its group delay is shown in Fig.4 and

Fig.5. All the measurements are done using PNA E8362B Network Analyzer and simulations are done using Ansoft HFSS. The input impedance is also plotted and is shown in Fig.6.

Fig.3 Equivalent circuit model of multi-resonators

Fig.4. Measured Transmission Parameter and Group Delay Fig.5. Simulated S21(dB)

Fig.6. Impedance of the Multi-resonator

For a chipless RFID tag, a disc monopole UWB antenna has been used as the receiving and retransmitting antenna.The geometry of the monopole antenna along with the design parameters and its reflection characteristics is depicted in Fig.7. The bistatic RFID measurement set up with the UWB monopole antenna in the anacheoic chamber and the measured transmission parameter and group delay are presented in Fig.8.

Fig.7. Structure and reflection characteristic of the UWB monopole antenna (R = 15mm, W3 = 3mm, Lg = 0.6mm, Lg1 = 40mm and Lg2 = 20mm)

Fig.4. Measurement Set up and measured Transmission Parameter and Group Delay

4. Conclusion

A novel fully printable multi-resonator circuit for chipless RFID tag is fabricated and its characteristics are analyzed. The structure is simple, compact and easy to fabricate and is based on open stub multi resonating structure. The multi-resonator circuit can be extended for any number of bits by incorporating additional resonators. Replacing the disc monopole antenna with other compact UWB antenna we can make RFID tag even smaller. The proposed RFID tag is a promising candidate for the newly developed near field identification, tracking and security applications.

5. References

[1]. I. Jalaly and I. D. Robertson, “Capacitively Tuned Split Microstrip Resonators for RFID Barcodes," 2005 European Microwave Conference, France, vol. 2, pp. 5-6, Oct. 2005.

[2]. J. McVay, A. Hoorfar, and N. Engetha, “Space flling curve RFID tags,"2006 IEEE Radio and Wireless Symposium, San Diego, USA, vol. 2,pp. 199-202, Jan. 2006.

[3]. S. Preradovic, S. Member, I. Balbin, N. C. Karmakar, S. Member, and G. F. Swiegers, “Multiresonator-Based Chipless RFID System for Low-Cost Item Tracking," IEEE Transactions on Microwave Theroy and Techniques, vol. 57, no. 5, pp. 1411-1419, May 2009.

[4]. C. M. Nijas, R. Dinesh, U. Deepak, A. Rasheed, S. Mridula, K. Vasudevan, and P. Mohanan, “Chipless RFID Tag Using Multiple Microstrip Open Stub Resonators," IEEE Transactions on Antennas and Propagation, vol. 60, no. 9, pp. 4429-4432, Sep. 2012.

[5]. Stevan Preradovic, Isaac Balbin, Nemai C. Karmakar and Gerry Swiegers,” A Novel Chipless RFID System Based on Planar Multiresonators for Barcode Replacement”, Procee. of IEEE international Conference on RFID, pp. 289-296, April 2008

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