Proceedings of Asia-Pacific Microwave Conference 2010 FRIG-20

Circular Polarized Small Antenna based Metamaterial CoplanarWaveguide (CPW) Transmission Line

Jaehyurk Choi #1, Sungjoon Lim #2

# School of Electrical and Electronics Engineering, College of Engineering Chung-An? University, Seoul, Republic of Korea

jaehyurk@gmail.com 2 sungj oon@cau . ac . kr

Abstract - In this paper, a circular polarized small antenna is proposed by way of a metamaterial coplarnar-waveguide (CPW) transmission line (TL). The zeroth-order resonance of the metamaterial TL is introduced to minimize the antenna size. Its bandwidth is expanded due to the design flexibility of the CPW configuration. The circular polarization (CP) is achieved by the L-shaped stub whose current distribution is orthogonal to the original current distribution from the zeroth-order resonance. Details of the design procedure are described. The proposed antenna's performance is demonstrated by numerical and experimental results.

Index Terms - Circular polarization, metamaterial, small antenna, CPW antenna

I. INTRODUCTION

Recently, metamaterial-based antennas such as zeroth-order resonant (ZOR) antennas and leaky-wave antennas have been presented [1]-[3]. Especially, many researchers put their efforts on developing ZOR antennas because the zeroth-order property enables to design a small efficient resonant antenna. The circular polarized antennas are also gaining more attention in recent years because they have good performance in the multipath fading environment [4]. Until now, only few studies of the circular polarized metamaterial antennas have been investigated [5]-[6]. However, they have some drawbacks such as dual feed, via fabrication process, and 90 phase shifter.

In this paper, a novel circular polarized small antenna is proposed. The proposed antenna design is based on the metamaterial concept in order to reduce the antenna size. Furthermore, the CPW technology is applied to the configuration of antenna. [7]. The CPW-fed metamaterial structure has many advantages, such as a low profile, light weight, via-free structure, easy integration, and wider impedance bandwidth. For the ZOR antenna, one of the most enhanced properties by using the CPW -fed structure instead of adapting microstrip line structure is wider impedance bandwidth. However, its far-field propagation characteristic is fundamentally linear polarization. In order to achieve a CP radiation pattern, two orthogonal electric field vectors from the antenna aperture must have equal amplitude and 90 degree phase difference. For the proposed antenna, these requirements are accomplished by adjusting a geometrical topology of the antenna. By employing an L-shaped stub between the patch and ground plane, another electric field

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Fig. 1. Schematic configuration of the microstripline-fed CRLH based ZOR antenna with magnetic current densities.

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Fig. 2. Magnetic current densities of CPW-fed ZOR antenna.

vector is generated on the antenna. It is orthogonal to the primary vector which makes the linear polarized radiation pattern. Thus, by optimizing geometrical parameters of the stub, the requirements of the CP radiation pattern are satisfied.

Copyright 2010 JEICE 2044

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Fig. 4. Simulated and measured reflection coefficients against frequency for the array.

polarization. 16 r---------------------,

II. THEORY AND ANTENNA DESIGN

The radiation field pattern from conventional microstrip line (MSL)-fed ZOR antenna is determined by the magnetic current densities along the perimeter of patch [I] as described in Fig. l. From duality, an equivalent magnetic loop antenna is operates as an ideal electric dipole antenna. Thus, the MSL-fed ZOR antenna is polarized in the elevation-direction. Similarly, for the CPW-fed ZOR antenna, the magnetic loop current density is generated as shown in Fig. 2; because the picture describes a CPW-fed ZOR antenna which consists of two unit cells, there are two equivalent magnetic loops. However, the far-field radiation of the CPW-fed ZOR antenna is mainly due to the magnetic current vector (M!) between the patch and matching stub. Other vectors such as M2 and M4 are slightly effects on the far-field radiation. Because the gap between the patch and ground plane is much wider than that of between the patch and matching stub, the magnitude of vector M2 as well as is small and ignorable. The other vector (M3) is cancelled out at the far-field (Fraunhofer) region due to the out-of-phase relation with the vector (Ms) from the second unit cell. In conclusion, the CPW-fed ZOR antenna is polarized in the y-direction.

As mentioned previously, the ZOR antennas have inherently linear polarized (LP) radiation property. In order to generate CP radiation, conventionally, the designer should fabricate and connect an extra circuit such as 90 phase shifter on the ZOR antennas. At the same time, the ZOR antennas with CP radiation should be excited by two feeding structure. As a result, the sizes of those antennas are

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Frequency (GHz)

Fig. 5. Simulated AR against frequency in the broadside direction.

increased. Those large sizes of antenna is disadvantageous for portable devices.

This paper proposes the novel small antenna with circular polarized radiation pattern. For the small sized antenna, the design of the proposed antenna is based on the composite right/left-handed (CRLH) TL concept. Moreover, the CPW technology is used for the enhanced bandwidth. The antenna configuration is illustrated in Fig. 3. The antenna is based on the CPW-fed CRLH metamaterial structure which consists of meander lines, top patches, and bottom patch. Additionally, in order to generate the circular polarized radiation pattern, the L-shaped stub is applied to the given structure. Finally,

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Fig. 6. Simulated radiation patterns of the antenna. (a) 3.83 GHz Y-Z Plane. (b) 3.83 GHz X-Z Plane.

the y-directional magnetic current occurs between the stub and ground plane, while the x-directional magnetic current is exist. However, for the CP radiation pattern, two orthogonal

field vectors should not only be existed, but also have phase difference of 90 degree as well as same amplitude. In the proposed geometry, the phases of two radiating vectors are adjusted by modifying the length of L-shaped stub. After investigation of characteristic parameters such as input impedance and axial ratio (AR) of the CP against varied Lshaped stub dimensions, the proposed small antenna based on the CPW-fed metamaterial structure is optimally designed and fabricated to exhibit its CP radiation pattern.

III. RESULTS

Fig. 4 and Fig. 5 show the simulated and measured results of the impedance bandwidth and CP bandwidth of the proposed antenna. The simulated and measured impedance bandwidths (lOdB reflection coefficient) are about 6.5% (3.7 - 3.95 GHz) and 4.8 % (3.735 - 3.92 GHz), respectively. The simulated CP bandwidth (AR < 3dB at broadside direction) of the antenna is about 6.1 % (3.645-3.876 GHz).

Fig. 6 shows the simulated radiation patterns at the 3.83 GHz plotted in the x-z and y-z planes. The obtained copolarization of the radiation pattern at 3.83 GHz is similar to a planar monopole antenna. It is observed that the crosspolarization of the radiation pattern is large because of the orthogonal magnetic current density between the L-shaped stub and ground plane. Therefore, it proves that the CP radiation property of the antenna occurs at the broadside direction.

IV. CONCLUSION

The circular polarized small antenna is proposed. The configuration of antenna is based on the CPW-fed metamaterial structure in order to reduce the antenna size as well as improve its impedance bandwidth. Furthermore, to generate the CP radiation pattern, the L-shaped stub is added on the CPW-fed metamaterial structure. Since the L-shaped stub enables antenna to generate two orthogonal field vectors, proposed antenna have the CP radiation pattern without extra circuits such as a 90 phase shifter. The proposed design was simulated using a commercial 3D EM simulator. The result shows that the proposed design satisfies CP radiation requirements at the broadside direction.

ACKNOWLEDGEMENT

This work was supported by the IT R&D program of MKEIKEIT. [KI002084, A Study on Mobile Communication Systems for Next-Generation Vehicles with Internal Antenna Array]

REFERENCES

[1] A. Lai, K. M. K. H. Leong, and T. Itoh, "Infinite wavelength resonant antennas with monopolar radiation pattem based on

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periodic structures," IEEE Trans. Antennas Propag., pp. 868-876, Mar. 2007.

[2] T. Jang, S. Lim, "A Novel Broadband Co-Planar Waveguide (CPW) Zeroth Order Resonant Antenna," in Proc. Asia-Pacific Microwave Conf., Singapore, pp. 52 - 55, Dec. 2009.

[3] S. Lim, C. Caloz, and T. Itoh, "Metamaterial-based electronically controlled transmission-line structure as a novel leaky-wave antenna with tunable radiation angle and beamwidth," IEEE Trans. Microwave Theory Tech., vol. 53, pp. 161-173, Jan. 2005.

[4] B. Y. Toh et al., "Understanding and measuring circular polarization," IEEE Trans. Educ., vol. 46, no. 3, pp. 313-318, 2003.

[5] C. J. Lee, K.M.K.H. Leong, and T. Itoh, "Composite Right/Left-Handed Transmission Line Based Compact

Resonant Antennas for RF Module Integration," IEEE Transaction on Antennas and Propagation, Vol. 54, No.8, pp. 2283-2291,2006.

[6] A.Y.F. Yang and A.Z. Elsherbeni, "A dual band circularly polarized ring antenna based on composite right and left handed metamaterials," Progress In Electromagnetics Research, Vol. 78, pp. 73-81, 2008.

[7] T. Jang, S. Lim, "A Compact Zeroth-Order Resonant Antenna on Vialess CPW Single Layer," in ETRI Journal., Jun. 2010.

[8] A. Sanada, C. Caloz, and T. Itoh, "Zeroth order resonance in composite rightlleft-handed transmission line resonators," in Proc. Asia-Pacific Microwave Conf, Seoul, Korea, vol. 3, pp. 1588-1592, Nov. 2003.

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