Fractal Composite Right/Left-Handed Transmission Line and its Application in Zeroth-Order Resonant

AntennaGeng Lin, Wang guangming, Zhang Chenxin, Zeng huiyong

microwave lab, 24 mailbox, Sanyuan Shaanxi Province 713800. P. R. China; Corresponding author: genglin8602@163.com

Abstract-This article proposes a fractal composite right/left-handed (CRLH) transmission line (TL). Its unit-cell is constructed by inserting a gap and 2-iteration Wunderlich-fractal short curves into a CPW-type structure. A two-unit-cell zeroth-order resonant (ZOR) antenna based on the CRLH TL is designed. The measured results show that the antenna provides smaller electronic size (0.16 0� �0.13 0� ) and wider bandwidth (1.78%) than reported compact ZOR antennas.

Index Terms-CPW; Wunderlich-fractal curve; composite right/left-handed transmission line; zeroth-order resonant antenna

I. INTRODUCTION

Metamaterials are understood as artificially engineered materials that exhibit unusual electromagnetic properties. They have become a field of intense research activities over the past decade in the area of microwave engineering [1, 2]. Particularly, the discovery of composite right/left-handed (CRLH) transmission line (TL) has facilitated the design and fabrication of low-profile and non-resonant type metamaterial components and also improved their performance, rending them suitable for practical applications. The CRLH TL has many unique properties such as supporting a fundamental backward wave and zero propagation constant at a discrete frequency. By using the backward wave property, CRLH TLs can be applied to dominant mode leaky-wave antennas radiating in the backward and forward directions [3]. Due to the zero propagation constant property, the resonator has an infinite wavelength and its resonant frequency is independent of the size. Therefore, zeroth-order resonant (ZOR) antennas are more compact than conventional half-wavelength antennas [4-6]. However, the narrow bandwidth limits their applications to modern wireless communication systems.

In this paper, a fractal CRLH TL is presented. Then, a two-unit-cell ZOR antenna based on the CRLH TL is designed. The measured results show that, although the electronic size of the antenna is only 0.16 0� � 0.13 0� , its bandwidth is increased up to 1.78%. Consequently, the antenna can find its application in modern wireless communication system because of the miniaturized size and extended bandwidth.

II. FRACTAL CRLH TL AND ZOR ANTENNA DESIGN

As shown in Figure 1(a), a fractal CRLH TL unit-cell is

constructed by inserting a gap and 2-iteration Wunderlich-fractal short curves into a CPW-type structure. It is fabricated on a F4BM-2 substrate with dielectric constant of 2.2 and thickness of 1 mm. The equivalent circuit model of the unit-cell is shown in Figure 1(b). The series capacitance and shunt inductance are provided by the gap and 2-iteration Wunderlich-fractal short curves, respectively. By cascading

unit-cells designed above, a fractal CRLH TL is realized. N

(a)

( )RL

( )LC

( )RC ( )LL

(b)

Figure 1 (a) The configuration of the fractal CRLH TL unit-cell with dimensions in mm(b) Its equivalent circuit model

With dimension as shown in Figure 1(a), the extracted values are: LC � 0.63pF, R 3.56nH, RL � C � 0.42pF,

LL � 11.17nH. The dispersion diagram is presented in Figure 2.

Then, a two-unit-cell ZOR antenna based on the CRLH TL is shown in Figure 3. A 50� CPW feeding line, a tuning stub and proximity coupling are used to construct the feeding network.

978-1-4673-2185-3/12/$31.00 ©2012 IEEE

Figure 2 The dispersion diagram of the proposed CRLH TL

17.7

21.5

3 5.7

5

3.2

2.7

0.4

7.55

0.35.7

0.2

0.2

x

y

Figure 3 The configuration of the two-unit-cell ZOR antenna with dimensions in mm

III. RESULT AND COMPARISON

The simulated return loss and the measured return loss are presented in Figure 4, where the measured resonant frequency of the zeroth-order mode and bandwidth are 2.25 GHz and 1.78%, respectively. So the electric size of the antenna is approximately 0.16 0� � 0.13 0� . Figure 5 shows the simulated and measured radiation patterns on the X-Z (E-plane) and Y-Z (H-plane) planes at 2.25 GHz that are monopole-like. The measured peak gain of the antenna is 1.21 dBi. As shown in Figure 6, the electric-field distribution of the antenna in the zeroth-order mode is in-phase.

The performances of the antenna are compared with those of reported compact ZOR antennas [4-6] in Table I. Although the antenna provides further size reduction than the reported compact ZOR antennas, it possesses significantly extended bandwidth.

Figure 4 Return loss of the antenna

(a)

(b)

Figure 5 Radiation patterns for the antenna at 2.25GHz (a) x-z plane (E-plane), (b) y-z plane (H-plane)

Figure 6 Electric-field distribution of the antenna in the zeroth-order mode

Table I Comparison of antenna performancesDesignedantenna [4] [5] [6]

Frequency

(GHZ) 2.25 4.88 3.38 0.917

Size

( 0� ) 0.16� 0.13 0.25� 0.25 0.17� 0.17 0.18� 0.15

Bandwidth

(%) 1.78 ~0.1 <1 1

IV. CONCLUSION

In this paper, a fractal CRLH TL is presented. By using the 2-iteration Wunderlich-fractal short curves and the CPW technology, the antenna based on the CRLH TL provides miniaturized size and extended bandwidth. The antenna with monopole-like radiation pattern can find its application in modern wireless communication system. Moreover, the CPW technology provides high freedom to change the shunt parameters, so miniaturized ZOR antennas with extended bandwidth operated at other frequencies can be easily designed by adjusting the structure of proposed CRLH TL.

ACKNOWLEDGMENT

The authors would like to thank the supports from National Natural Science Foundation of China under Grant 60971118. Thankfulness from the bottom of their hearts is also shown to the reviewers for their valuable comments.

REFERENCES

[1] V. G. Veselago, “The electrodynamics of substances with simultaneously negative values of � and � ,” Soviet Physics Uspekhi, vol. 10, pp. 509-514, 1968.

[2] R. A. Shelby, D. R. Smith, and S. Schultz, “Experimental verification of a negative index of refraction,” Science, vol. 292, pp. 77-79, 2001.

[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. 52, pp. 2678-2690, 2004.

[4] A. Sanada, K. Murakami, I. Awai, H. Kubo, C. Caloz, and T. Itoh, “A planar zeroth-order resonator antenna using a left-handed transmission line,” 34th European Microwave Conference, pp. 1341-1344, 2004.

[5] A. Lai, K. M. K. H. Leong, and T. Itoh, “Infinite wavelength resonant antennas with monopole radiation pattern based on periodic structures,” IEEE Trans. Antennas Propag. , vol. 55, pp. 868-876, 2007.

[6] J. X. Niu, “Dual-band dual-mode patch antenna based on resonant-type metamaterial transmission line,” Electronics Letters, vol. 46, 2010.