Fractal GPS Antenna Design on Piezoelectric Substrate

Tzu-Chun Tang #1, Cheng-Han Tsai #2, Ken-Huang Lin #3, Yu-Tung Huang *4, Chi-Yun Chen *5 #Department of Electrical Engineering, National Sun Yat-San University, Taiwan

1a565656kimo@pcs.ee.nsysu.edu.tw

2tsaichp@hotmail.com.tw 3khlin@mail.nsysu.edu.tw

*TAI-SAW Technology CO., LTD. 4ythuang@mail.taisaw.com 5francischen@mail.taisaw.com

Abstract — This paper presents a study of fractal GPS

antenna design on piezoelectric substrate. The antenna size is reduced by using piezoelectric materials with high dielectric constant and the fractal structure in antenna design. The desired high effective dielectric constant can be obtained by the special arrangement of the ground. The Koch fractal structure is employed in antenna design. With the corner-truncated structure, the right-handed circular polarization (RHCP) is obtained. The radiation efficiency of the antenna is considered, and then the modified ground is introduced to further improve the radiation efficiency of the antenna. The radiation efficiency can be significantly improved when the modified ground is designed on the system board. The effect of the GPS module connections to the antenna is also investigated, and then the matching circuit is added to obtain better matching.

Index Terms — Fractal antenna, piezoelectric material, modified ground, matching circuit.

I. INTRODUCTION

Low-profile, compact antennas are currently under consideration for use in modern wireless communication devices. For the convenient assembly, the antenna was designed to be cubic-like such as surface mounted device [1] or chip antenna [2] so that it is easily soldered with the circuit. Furthermore, the antenna can be integrated with the active or passive devices that share the same concept as SIP (system in package). Integrated passive device (IPD) is also called when the antenna is integrated only with the passive components. Such antennas may use high dielectric constant material as substrate to minimize the size of the antenna. LTCC antenna is one of the examples [3-4].

Except for using high dielectric constant as the substrate, the dimension of the antenna can also be reduced by the structure of antenna itself. Antenna designed with fractal structure, which meanders the current, will resonate at the lower frequency or see a size reduction. Additionally, its space-filling and self-similarity [5] can provide the antenna that operates in multiband. Four types of fractal structure are frequently seen: Sierpinski, Koch, Tree, and Hilbert Curve. Occasionally, Sierpinski [6] and Koch [7] were used in planar structure design. The structure of the Sierpinski is the crossed-slot or the squared-slot on the radiating element. The structure of the Koch is the truncated squared-slot along the

edge of the radiating element. Due to the RHCP requirement, the Koch is preferred in GPS band because the corner-truncated structure would have phase difference between two currents. In addition, the structure is suitable for coplanar feeding techniques and integrated with the passive devices or front-end circuit. SAW filters are included in some front-end circuits. Because of this, the effect of piezoelectric substrate needs to be considered.

This paper is organized as follows. Section II introduces the properties of the piezoelectric material. Section III describes the fractal antenna design, ground design modification to improve the radiation efficiency, and the impacts of the GPS module on antenna. We draw the conclusion in Section IV.

II. PIEZOELECTRIC MATERIAL

Piezoelectric material is used as the substrate of SAW devices. SAW devices employ the negative property of piezoelectric and inter-digital structure on piezoelectric material which transfers the time-varying electromagnetic wave energy into mechanical energy. The time-varying electromagnetic wave excites the elastic mechanical wave on the surface of piezoelectric material. Contrarily, the positive property of piezoelectric and the inter-digital structure will transfer the mechanical energy into electrical energy. Exploiting the spacing between fingers in inter-digital structure, the number of electrodes, the wavelength and the propagation velocity of the surface wave can be determined, and then the frequency is obtained. The SAW filer can be designed by the procedures discussed in [8].

Due to the crystal nature of piezoelectric material, the permittivity is anisotropic with high dielectric constant. For example, the permittivity of LiTaO3 can be expressed as (1)

where 1ε =53.6, and 2ε =43.4. If the electric field is z-directed polarization, then 2ε is the dominant parameter in design. In this paper, we will use LiTaO3 as the substrate.

1

1

2 ,

0 00 0 ( 1 )0 0

r

εε ε

ε

⎡ ⎤⎢ ⎥= ⎢ ⎥⎢ ⎥⎣ ⎦

Proceedings of Asia-Pacific Microwave Conference 2010

Copyright 2010 IEICE

TH4D-2

991

III. FRACTAL GPS ANTENNA DESIGN

In this section, we will discuss the antenna design approach in three parts. First, we reduce the antenna size by fractal structure, which meanders the current path, then the antenna resonates at the lower frequency or sees a size reduction. Second, we introduce the modified ground design, which improves the radiation efficiency of the antenna that is placed on the system board. Finally, we simulate the effects of the GPS module which is connected to the antenna.

A. Fractal Structure

In this paper, we employ two techniques for the reduction of the antenna size, piezoelectric materials and fractal structure. There are two reasons for using the piezoelectric material as the substrate. First, piezoelectric materials have high permittivity which can be used to minimize the size of the antenna. Second, it can be integrated with the SAW filter. We now proceed to the design of the fractal antenna. As shown in Fig.1, The dimension of the antenna is 18×18×1.15 mm³. The antenna is the Koch type fractal structure of the first stage which is designed on the piezoelectric material. Because the antenna is used for GPS application, RHCP is required. The fractal structure is thus designed to have two orthogonal current paths and then excite two orthogonal electric fields. The corner-truncated structure at the two opposite sides would contribute to 90゜phase difference between two electric fields, and then the RHCP can be achieved. The capacitively coupled feeding technique is used to compensate the high inductance caused by the very thin feeding line. The overall design and simulated results will be completed after the next subsection.

Fig. 1. Geometry of the antenna

B. Modified Ground

With the advantage of the high permittivity in piezoelectric material, the antenna size can be reduced theoretically. However, the location of the ground may affect the effective dielectric constant. Consider the first circumstance in Fig. 2(a), the ground is on the bottom of the FR4, and the piezoelectric material is placed on the board. Then the effective dielectric constant is almost equal to that of FR4 so that the merit of the piezoelectric material is useless here. In the second case in Fig. 2(b), the ground is located between the piezoelectric material and the FR4, and then the effective dielectric constant is almost up to 22. So, the dimension of the antenna can be designed in compactness under the circumstance. Based on the second case, the antenna is shown in Fig. 1. We elevate the ground to the location between the antenna substrate and the system board. The dimension of the ground would cause the resonant frequency shift. Fig.3. shows the simulated resonant frequency varying with the dimension of the ground. It is seen that resonant frequency shifts up to the higher band as the ground become larger. Nevertheless, as the ground enlarges to certain size, the shift becomes insignificant. For the above observation, we can see that for certain size of the system ground, it is better to co-design the antenna with the system ground so that the frequency shift can be controlled.

Fig. 2. The location of the ground (a) bottom of the PCB (b) between PCB and Piezoelectric material

Fig. 3. The return loss varying with the size of the ground

992

Consider the radiation efficiency in antenna design. We design the shape of the slot to be the same as the radiating patch, and the size of the slot is slightly larger than the patch. The aim of the slot is used to improve the radiation efficiency; however, the enhancement is not as expected. As we place the antenna on the system board, the antenna ground is shorted to the system ground. As shown in Fig. 4, the radiation efficiency of the antenna is improved to 33% with the ground dimension 45×90 mm². It can be deduced that the system ground provides the paths for current flow, and then enhances the radiation efficiency. On the other hand, as shown in Fig. 5, the system ground has less impact on the impedance matching of the antenna. The simulated result is consistent with the discussion in the previous paragraph.

Fig. 4. The comparison of the radiation efficiency

Fig. 5. The return loss varying with the size of the ground

C. GPS Module Integration

Currently, the antenna is often designed with the front-end circuit and modules. These modules may have impact on the impedance matching of the antenna. In this subsection, we will simulate the antenna with the GPS module, and provide the solution to the problem.

Fig. 6. The configuration of the simulation environment Fig. 6 shows the configuration of the simulation

environment. The antenna is located at the corner of the system board, and the GPS module is covered with shielding box which is shorted to the ground. Then the module is connected to the antenna with the microstrip line. The simulated return loss in Fig. 8 illustrates that the resonant frequency is shifted to the higher frequency, and the GPS band is not matched.

Conventionally, the matching could be better by tuning the structure of the antenna. However, if the antenna were implemented or packaged, the radiating structure cannot be altered. Alternatively, the matching network technique would compensate for the mismatch [9]. The structure of the impedance-matching network is shown in Fig. 7, the tuning stub length L=2 mm and the width W=1.5 mm.

Fig. 7. The feeding network (a) original (b) matching

Fig. 8 shows the comparison of the simulated return loss

between the original antenna and the antenna with matching network. It is seen that the antenna with matching network has wider bandwidth than original antenna, and the GPS band is also covered. Fig. 9 shows that the AR is lower than 3dB and the radiation efficiency is 33% over the GPS band. Fig.

993

10 shows the radiation pattern of the antenna, and the RHCP is obtained.

Fig. 8. Comparison of the return loss between original and matched feeding network

Fig. 9. The AR and radiation efficiency of the matched antenna

Fig. 10. The radiation pattern of the antenna

IV. CONCLUSION

The fractal GPS antenna on piezoelectric material is designed. The antenna size is reduced by the high dielectric constant associated with piezoelectric material and fractal structure. With the special arrangement of the ground, the high effective dielectric constant can be obtained so that the antenna size can be effectively reduced. The Koch fractal

structure is employed in antenna design. With the corner-truncated structure, the RHCP requirement in GPS band can be obtained. By introducing the modified ground, the radiation efficiency is enhanced to 33%. The impact of the GPS module is also discussed, and the matching circuit is added in the case. The simulated result shows that the antenna with matching circuit has broader bandwidth than original antenna. The design guideline of such an antenna will be very useful in application such as IPD, and other communication systems.

REFERENCES

[1] S. W. Su, K. L. Wong, C. L. Tang, and S. H. Yeh, “Wideband monopole antenna integrated within the front-end module package,”IEEE Trans. Antennas Propag., vol. 54, pp. 1888-1891, Jun. 2006.

[2] M. R. Hsu and K. L. Wong, “WWAN ceramic chip antenna for mobile phone application,”Microwave Opt. Technol. Lett., vol. 51, pp. 103-110, Jan. 2009.

[3] S. H. Wi, J. S. Kim, N. K. Kang, J. C. Kim, H. G. Yang, Y. S. Kim, and J. G. Yook, “Package-level integrated LTCC antenna for RF package application,”IEEE Trans. Adv. Packag., vol. 30, pp. 132-141, Feb. 2007.

[4] Y. P. Zhang, M. Sun, and W. Lin, “Novel antenna-in-package design in LTCC for single-chip RF receiver,” IEEE Trans. Adv. Packag., vol. 56, pp. 2079-2088, July. 2008.

[5] D. H. Werner and S. Ganguly, “An overview of fractal antenna engineering research,”IEEE Antennas Propag. Mag., vol. 45, pp. 38-57, Feb. 2003.

[6] J. Romeu and J. Soler, “Generalized Sierpinski fractal multiband antenna,”IEEE Trans. Antennas Propag., vol. 49, pp. 1237-1239, Aug. 2001.

[7] C. P. Baliarda, J. Romeu, and A. Cardama, “The Koch monopole: a small fractal antenna,”IEEE Trans. Antennas Propag., vol. 48, pp. 1773-1781, Nov. 2000.

[8] Colin Campbell, Surface Acoustic Wave Device for Mobile and Wireless Communications. Academic Press, San Diego, 1998.

[9] H. F. Pues and A. Van de Capelle, “An impedance-matching technique for increasing the bandwidth of microstrip antennas,” IEEE Trans. Antennas Propag., vol. 37, pp.1345-1354, Nov. 1989

0

90

180

270

-5

-5

-10

-10-15

-15-20

RHCPLHCP

xz plane

(+x)

(+z) 0

90

180

270

-5

-5

-10

-10-15

-15-20

yz plane

(+y)

(+z)

994