3D Patch Antenna using a Cardbord Substrate for RFID Reader Applications

Giuseppina Monti, Luca Catarinucci, Claudia Vasanelli, Luciano Tarricone Department of Innovation for Engineering

University of Salento Lecce, Italy

{giuseppina.monti, luca.catarinucci, luciano.tarricone}@unisalento.it, claudia.vasanelli@libero.it

Abstract— This paper presents a novel Radio Frequency Identification (RFID) reader antenna guaranteeing an almost omnidirectional irradiation and a circular polarization. More specifically, the antenna here presented consists of four patch antennas placed on the lateral faces of a cardboard cube. This way, a high performance antenna with a very low environmental impact is obtained. Numerical and experimental results are reported and discussed demonstrating that the antenna here presented is an optimum candidate to be used as reader antenna of ultra high frequency RFID systems.

Keywords-patch antennas, eco-friendly substrate, cardbord, circular polarization, reader antenna, RFID systems, adhesive copper.

I. INTRODUCTION In the last years a great interest in green electronics has

raised; many studies focused the attention on the use of biopolymer in electronics, which can be used for example as electrolyte in batteries or as a substrate for fabricating PCBs [1�3].

At the same time, there was a growing use of Radio Frequency Identification (RFID) systems [4-13], and in the next few years, an even more capillary distribution of such a technology is expected. As a consequence, the use of paper as substrate is very appealing, as paper is fully recyclable, cheap, and its properties (roughness, electric parameters, thickness and so on) can be ad-hoc customized.

Moreover, paper is easy to use in making prototypes, gives the possibility to fabricate electronic circuits on disposable products and packaging and can be made hydrophobic or fire retardant with the application of appropriate coating, which can enable the utilization of inkjet technology to print electronics [12].

This paper proposes a fully-3D Ultra-High Frequency (UHF) reader antenna on cheap and eco-friendly substrates. Compared to standard reader antennas, the solution here presented has several advantages: it is cheap and environmentally friendly, it has compact dimensions and it allows the integration of electronic equipment thanks to its cubic structure. Furthermore, it exhibits an almost omnidirectional irradiation, very useful in some specific

applications, and a circular polarization that ensures efficient reading of tags. To avoid any problem of electromagnetic compatibility between radiating elements on the surface of the cubic structure and the electronic devices potentially located in the cube’s hollow, microstrip technology is used, so that the ground-plane on the back of the antenna works as electromagnetic shield for the inner volume of the cube.

The paper is structured as follows. In sect. II the geometry of the proposed antenna is briefly described and some numerical and experimental results are given and discussed. Finally, conclusions are drawn in sect. III.

Figure 1. Geometry of the proposed antenna

II. PROPOSED ANTENNA: NUMERICAL AND EXPERIMENTAL RESULTS

The geometry of the proposed antenna is illustrated in Fig. 1. It consists of four patch antennas placed on the lateral faces of a cube. As for the substrate used for the antenna, that is the material of the cube, it is a cardboard with a thickness of 3 mm. The cube antenna design process started by determining the constitutive parameters of the cardboard

978-2-87487-029-3 © 2012 EuMA 31 Oct - 2 Nov 2012, Amsterdam, The Netherlands

Proceedings of the 9th European Radar Conference

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material in the frequency range of interest, that is the frequency range of European UHF RFID systems (i.e., [860 MHz, 870 MHz]).

Figure 2. Reflection coefficent of the antenna illustrated in Fig. 1 calculated by means of full-wave simulation results.

Figure 3. Radiation pattern of the proposed antenna calculated by means of full-wave simulations.

This has been done by realizing some simple patches using the selected cardboard as substrate; then, by means of a comparison between full-wave simulations and experimental data, we calculated the following values for the cardboard constitutive parameters:

� � 1.2�� re � ; � � 075.0tan �� , @ f=866.6 MHz. (1)

Where, � �re �� and � ��tan are the real part of the electric permittivity and the loss tangent, respectively.

A. Numerical Results The values given in (1) were adopted in designing the cube

antenna; the full-wave simulator CST Microwave Studio has been used for simulations and optimizations. Corresponding results are given in Figs. 1-3. In particular, Fig. 1 illustrates the geometry of the antenna; dimensions are reported in Table I.

TABLE I: DIMENSIONS OF THE ANTENNA ILLUSTRATED IN FIG. 1.

Type of patch antenna Dimensions

W L L1 W1 D

Standard patch antenna 90 mm

119.8 mm

39,85 mm

5 mm

171 mm

The antenna has only one RF input port and a network

consisting of four microstrip lines has been used to feed each single patch. From Fig. 1, it can be seen the presence of a loading open-ended stub for each microstrip line; this stub was used to achieve the matching with respect to a 50 coaxial cable. Both the feeding lines and the loading stubs were realized on the bottom face of the cube so to reduce the introduction of spurious radiations. Figs. 2 and 3 show the full-wave simulation results obtained for the reflection coefficient and the radiation pattern, respectively. It is evident that an almost omnidirectional pattern was obtained in the xy plane.

TABLE II: DESIGN STRATEGIES INVESTIGATED FOR MINIATURIZATION.

Layout of the patch antenna

D 126 mm 110 mm 104 mm

Volume reduction with respect to the use

of a standard patch -59% -73% -76%

The possibility of reducing the volume occupied by the

antenna was also investigated, by using for each patch the design strategies illustrated in Table II. In particular, we exploited the use of a fractal geometry in designing the side of the patch [13-15], of a horizontal slot located in the middle of the side of the patch, and of a diagonal slot. From Tables II and III, it can be seen that the largest size reduction was obtained by designing each patch with a diagonal slot. In particular, this strategy allows to obtain a reduction of the volume occupied by the cube equal to 76%; the corresponding geometry is illustrated in Fig. 4. Furthermore, from the radiation pattern point of view, the use of a diagonal slot on

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the patches results in a circular polarization while remaining almost ominidirectional in the xy plane.

TABLE III: DIMENSIONS OF THE ANTENNAS ILLUSTRATED IN

TABLE II.

Type of patch antenna Dimensions

W L L1 W1 D

Patch antenna – Koch curve 88 mm

91.2 mm

43,85 mm

5 mm

126 mm

Patch antenna – Slot on radiating edge

78 mm 78 mm 45,85

mm 5

mm 110 mm

Patch antena – Diagonal slot 74.3 mm

74.3 mm

45,85 mm

5 mm

104 mm

Figure 4. Geometry of the cube antenna using patches with a slot on the diagonal.

Figure 5. Reflection coefficient calculated by means of full-wave simulations for the antenna illustrated in Fig. 4.

B. Experimental Results In order to verify the reliability of numerical results, a

prototype was realized for both the antenna of Fig. 1 and the one of Fig. 4. Photographs are given in Figs. 6 and 7. The patch antennas and the feeding network were realized by using adhesive copper. The measured reflection coefficients are

given in Figs. 8 and 9; it can be seen that they are in a good agreement with numerical data shown in the same figures.

Figure 6. Photograph of the realized prototype.

Figure 7. Photograph of the prototype realized for the compact antenna using patches with diagonal slots; corresponding dimensions are given in Table III. The slot dimensions are 4,95 mm x 83,6 mm.

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Figure 8. Comparison between experimental data and full-wave simulations for the reflection coefficient of the prototype shown in Fig. 6.

Figure 9. Comparison between experimental data and full-wave simulations for the reflection coefficient of the prototype shown in Fig. 7.

III. CONCLUSIONS A novel reader antenna for radio frequency identification

applications has been presented. The main features of the proposed antenna are the use of a cardboard substrate, resulting in a low impact on the environment of the overall antenna, combined with compact dimensions, circular polarization and an almost omnidirectional radiation pattern. Experimental and numerical results demonstrating the effectiveness of the proposed design approach have been presented and discussed.

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