Design and Measurements of Antennas for RFID), made by Conductive Inkon Plastics

Delphine Bechevet, student IEEE Member, Tan-Phu Vuong, IEEE Member andSmail Tedjini, senior IEEE Member

LCIS-INPGrenoble, ESISAR, 50, rue de Laffemas, 26902 Valence, France.E-Mail: Delphine.Bechevet(esisar.inpg.fr

Embedded communicating systems are composed with digital, analogue and RFparts with specific caracteristics for each one. Antennas are fundamental elementsin communication systems. The objective of our work is the design of low-costintegrated antennas in the ISM band, and more specifically for RFID applications.This implies material selection. Plastics and conductive ink seem to be goodcandidates as respectively antenna's substrates and radiating patch. However, thedielectric and conductive properties of these low-cost materials are quasi-unknown in the ISM band. This communication addresses the experimentaldetermination and the use of these parameters.

Introduction

Traceability and tracking solutions based on Radio Frequency Identification(RFID) are topics of great interest. Several R&D groups are studying on solutionsto exploit the 2.45 GHz band [1]. To obtain low-cost RFID tags, the developmentof planar antennas lies on three steps [2]. The first one is the study and thedetermination of the electromagnetic properties of low cost substrates. Thus, wedeveloped a cavity technique based on the small perturbations theory to measurethe permittivity of plastics. The second step is to take into account of theconductive part of the planar antenna. Indeed, some conductive materials, putdown on substrate, cost less than noble materials such as copper, silver or gold.Let us note, the deposit method of low-cost conductive material should remaincheap. Knowing substrates properties, the design follows, and antennas realized.Finally, the third and last step is the design optimization and antennasmeasurements in free space.

Determination of Substrates Electromagnetic Parameters

In order to measure substrates electromagnetic parameters, we chose a methodbased on resonant frequencies in finite waveguide: small perturbations method.This last one consists in a rectangular waveguide cavity, brass made, excited bySMA based miniature antennas forming current loops, and with effectivedimensions corresponding to resonance frequencies (empty cavity) in GHz-band(1.5GHz-capture frequency). A VNA measures the resonant frequencies and Sparameters (reflection and transmission coefficients). These are determined for

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empty and partially plastic filled cavity. A perturbation technique is used toexploit the measured data (resonant frequencies and transmission coefficients)and determine the permittivity and loss factor for the considered material. Thistechnique is valid under the assumption of very small plastic volume compared tothe empty cavity volume [3]. Typical cavity transmission results, measured, arereported on Fig. 1.

Frequency (GHz)Fig. 1: Comparison of different materials transmission coefficients vs. frequency

One notices the shift of the resonant frequencies corresponding to three differentcases. The empty case corresponds to the higher frequency for each mode.These experimental results are then used to deduce the permittivity and loss factorof the considered material [4]. It is important to notice the necessity to calibratethe cavity with a sample for which electromagnetic parameters are well-knownbetween 2 and 3 GHz. Under these conditions, we calculate relative permittivityand dissipation factor for nine different polymers (Table 1).

Materials e' i, 2.45GHz tan8 (a 2.45GHz xlo11 2.16 1

3 2.64 1.8

5 2.47 2.4

7 2.81 2.9

9 2.38 6

Table 1: Permittivity and dissipation factor for 9 polymers at 2.45GHz.

Using these properties, rectangular patch antenna can be designed [6,7] at 2.45GHz. Let us note, the rectangular patch antenna is the simpliest planar antenna butits area is the biggest. So, if we demonstrate the faisability of this antenna, we areallowed to extand the faisability for other planar antennas such as slot antennas,knowing that the other antennas areas will be always smaller.

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Consideration of Conductive Parameters

Once, the electromagnetic paramaters held, we proposed a design for realization.Conductive ink has been deposited, by silk-screen printing. This deposit methodis one of industrial ones and so costs little compared to laboratory deposit method,for instance. More than that, it permits to deposit enough conductive material toexceed skin-depth.Before measuring antennas realized, height and conductivity have been measured,with respectively a profilometer and four-tips method.Knowing resistance and height h, we calculate the resistivity p [5] andconductivity e for each antenna realized. From these last parameters we calculatedthe skin depth 6, at 2.45 GHz. We compare it to deposited ink thickness andconfirm that this last one is greater or equal to ( (Table 2). Sample #3 (Table 2) is acase apart, because during the drying phase, the ink has been stressed so that thedeposit obtained is not homogenous. Thus the antenna is not acceptable(confirmed by measurement).

Sample 1 2 3 4 5 6

e x104 (S.cm-') 2.17 5.42 0.20 1.69 2.81 3.196 6>0 7.04; '4 31.36 8.43;0 649 5.94h (>m) 15 14 14 18 16 16

Table 2: Comparison between deposited ink height and calculated skin depth

All of parameters presented above are required to design patch antenna:permittivity c, dielectric losses factor tan6, for substrate parameters andconductivity a and height h, for conductive ink parameters.

Design and Measurements

The first step of design is geometric calculations [6,7], while the second part is theuse of electromagnetic software like CST Microwave Studio, divided itself inmany steps like geometric part (Fig. 2), meshing, limit conditions definition part orresults part. Many parameters limit the reflection coefficient, such as materialconductivity crand dielectric losses factor tan&

Fig. 2: Geometric design and farfield in 3D of rectangular patch antenna, with plastic substrate

Once design optimized, we compare measurements parameters [7] of the realantennas (Fig. 3) to those simulated. We will present in the entire communication,

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performances of antennas, created by conductive ink on two different plasticsubstrates. It is important to notice; we purchased measurements in realenvironment (with full radiated objects) and not in anechoic chamber.

Fig. 3: Pictures of rectangular patch antenna, on substrates #3 and #8 (Table 1).

The comparison will lie on SI , S21 parameters in dB, and on impedance Z.

Conclusion

Antenna design on non conventional substrates and electrodes is a main topic formany wireless applications like RFID at UHF bands, for low-cost prospecting.This communication addresses a method which includes experimentalcharacterization of materials, design of antennas operating around 2.45 GHz. Theobtained measurements results are very attractive, and we will present them in theentire communication.

References:

[1] A.Bensky, Short-range Wireless Communication-Fundamentals of RF System Design andApplication,LLH Technology Publishing, 2000.[2] S.Tedjini, T.P.Vuong, V.Beroulle, P.Marcel. "Radio-Frequency Identification Systems fromAntenna Characterization to System Validation ", APCM, 2004.[3] Robert E.Collin, Field Theory ofGuides Waves, IEEE Press, second edition, 1991.[4] E. Jao Jules. "Couplages entre proprnites thermiques, reactivite chimique et viscosite desmateriaux composites thermodurcissables en relation avec leur condition de leur elaboration fondlesur I 'hysteresis di0lectriques, Ecole Nationale Superieure des Arts et M6tiers, 2001.[5] D. L. Smith, Thin film Deposition - Principles & Practice, Mc Graw-Hill International Editions,1995.[6] R. Garg, P. Bhartia, I. Bahl, A. Ittipiboon, Microsrrip Antenna Design Handbook, Artech House,Inc, 2001.[7] A. Balanis, Antenna theory, WILEY, second edition, 1997.

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