Metamaterial Enhanced High Gain Antenna for WiMAX Application

Hung-Hsuan Lin, Chun-Yih Wu, Shih-Huang Yeh

Industrial Technology Research Institute Information & Communications Research Laboratories

M400, Bldg. 14, 195 Sec. 4, Chung Hsing Rd., Chutung, Hsinchu 310, Taiwan

Abstract- A high gain low-profile antenna design using

metamaterial technology operating at WiMAX 2.5-GHz band (2.50-2.69 GHz) is constructed and measured. A metamaterial superstrate, composed of stacked S-shaped split-ring resonators, can modify the radiation pattern and exhibit gain enhancement functionality. In addition, the proposed structure can be made in economic 4-layer via-less FR-4 printed circuit boards. Therefore applying this technology to commercial applications becomes more practical. With this thin metamaterial superstrate (1.4 mm thick and 10.5 mm above the antenna ground), the patch-based antenna can achieve a maximum gain of 12.2 dBi. The impedance bandwidth of the proposed antenna is about 390 MHz (2.50-2.89-GHz, defined by 2:1 VSWR standard).

I. INTRODUCTION

Recently, the demand of planar high directivity antennas, which can be applied to high speed wireless LAN, satellite reception and various point-to-point radio links, are rapidly rising. There are many designs that can achieve high antenna gain, such as antenna array, Yagi antenna, disk antenna and etc. However these high gain designs usually lead to bulk size and high cost.

In the past few years, new methods for improving the antenna gain by metamaterial are proposed [1]-[5] and theoretically discussed [6]. In those works, various metallic structures are used as metamaterials to achieve certain unusual characteristics which are suitable for high gain designs. However, most of those former works are suitable for high frequency applications, such as X-band or mm-wave. We propose a laminar metamaterial structure which is based on S-shaped split-ring resonator proposed by Chen et al. [7] for low-GHz antenna design. This structure is via-less and can be realized by normal PCB fabrication process. We use this metamaterial plate as a superstrate to enhance the gain of a patch antenna for 2.5-GHz WiMax applications. The measured result shows that our proposed design can boost the antenna gain about 1.8 dB.

II. ANTENNA DESIGN

Fig. 1 shows the schematic configuration of the proposed antenna. We use an air-substrate patch antenna as the radiation element and cover this patch by a metamaterial plate. Traditionally an enhancing superstrate is a high dielectric constant material and is placed half-wavelength above the ground plane (h = 0.5 λ0) to meet the resonant condition [8]. For our application, half-wavelength is equal to 58 mm which is unacceptable for low-profile requirement.

We use a metamaterial plate to provide negative reflection phase, which can reduce the height h to 10.5 mm (0.09λ0).

Fig. 2 shows the detail of the unit cell which is composing the metamaterial plate. The coordinates are consistent with that in Fig. 1 and Z-axis is the broadside direction. Four reversed S-shaped split-ring resonators are separated by three 0.47 mm-thick dielectric layers (FR-4, εr = 4.4). Each cell has a planar dimension of 15 mm by 7.5 mm. Unit cells are placed in the form of a 2-D 10-by-11 periodic matrix as the metamaterial superstrate.

The radiation patch in our proposed antenna is 4.5 mm above the ground plane (d = 4.5 mm). A SMA connector (not shown in the figure) with a feed pin is used as the interface to RF system. The length of patch is trimmed to accommodate the loading from the superstrate. The ground plane is sized 70 mm by 136 mm.

Figure 1. Configuration of the proposed antenna

Figure 2. Proposed meta unit cell

meta superstrate

radiation patch

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feed pin

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III. MEASUREMENT RESULT

A prototype of our proposed high gain antenna was successfully constructed, as shown in Fig. 3, and its performance was measured. Fig. 4 shows the measured reflection coefficient, S11, of the constructed prototype. The measured impedance bandwidth is about 1.45% or 390 MHz (2.50-2.89 GHz, defined by 2:1 VSWR), which can meet the 2.5-GHz WiMAX system requirement.

Figure 3. Prototype picture of the proposed antenna (Meta superstrate is detached from the supporting posts)

Figure 4. Measured S11 of the constructed prototype The antenna gains across the 2.5-GHz band of the

constructed prototype as well as a reference patch antenna are shown in Fig. 5. The reference patch antenna has the same height from the ground plane and the same ground plane size as the constructed prototype. Nevertheless the patch lengths and feed points are different due to the loading effect of the superstrate. The reference antenna radiates a 10.4 dBi peak gain at 2.65 GHz. The antenna gain drops rapidly when the measured frequency moves away from 2.65 GHz. Our proposed design exhibits a maximum gain of 12.2 dBi at 2.65 GHz. It shows a 1.8 dB improvement in the peak gain and exhibits even higher gain improvement at higher frequency.

The radiation patterns of the proposed antenna are shown in Fig. 6 and Fig. 7. We can obverse the main lobe, as excepted, is in the direction of the positive Z-axis (θ = 0

and φ = 0). It also exhibits side lobes in E-plane, which does not exist in the radiation pattern of the reference patch antenna. This phenomenon is consistent with the analysis in ref. [9].

Figure 5. Measured antenna gain of the proposed antenna compared to a conventional patch

Figure 6. Measured E-plane radiation pattern of the proposed antenna Figure 7. Measured H-plane radiation pattern of the proposed antenna

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IV. CONCLUSION

A prototype of low-profile (total thickness ~ 0.1λ0) metamaterial enhanced high gain antenna suitable for WiMAX 2.5-GHz band (2500-2690 MHz) operation has been constructed and studied. Compared with the reference patch design, the prototype improves by 1.8 dBi and boosts the maximum gain to 12.2 dBi. The proposed metamaterial superstrate was fabricated by low cost via-less PCB, which makes it very promising for current wireless communication applications.

ACKNOWLEDGMENT

The Authors would like to acknowledge Mr. Wen-Chieh Yang for his help on assembly and measurement of the prototype and Prof. Ken-Huang Lin in National Sun Yat-Sen University for his comments and assistant in theoretical analysis.

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