Metamaterial Lens with Both Electric and Magnetic Resonance Structure for Antenna Gain Enhancement

Fan-Yi Meng*1, Yue-Long Lv2, Qun Wu3, and Li Sun4

1Harbin Institute of Technology, No. 92, Xidazhi Street, Harbin, Heilongjiang Province, China [email protected]

2Harbin Institute of Technology, No. 92, Xidazhi Street, Harbin, Heilongjiang Province, China [email protected]

3Harbin Institute of Technology, No. 92, Xidazhi Street, Harbin, Heilongjiang Province, China [email protected]

4Harbin Institute of Technology, No. 92, Xidazhi Street, Harbin, Heilongjiang Province, China [email protected]

Abstract

Metamaterial lens with both electric and magnetic resonance structure is proposed in this paper to enhance the directivity and gain of antennas. The electric resonance structure in the metamaterial unit cell can achieve near-zero permittivity and the magnetic resonance structure can achieve near-zero permeability in the same frequency band to make the characteristic impedance of the lensmatch that of air. The resulting near-zero refraction index can make the electromagnetic wave emitted by the antenna converge and the directivity of the antenna will be enhanced. Thanks to good impedance match between lens and air, the antenna efficiency will not drop and the antenna gain will be improved largely. The lens is simulated in CST and tested with a patch antenna and an H-plane horn antenna. Large gain enhancement is achieved in a wideband when tested with the horn antenna. In addition, the lens is insensitive with the distance between lens and antenna.

1. Introduction

Metamaterial is a kind of artificial electromagnetic (EM) material with period structure. It has gained extensive attentionfor its unique EM properties [1]. One can obtain metamaterial with proper constitutive parameters by adjusting the structure parameters. Particularly, metamaterial is a good approach to achieve antenna lens to improve the directivity and gain for near-zero refraction index at corresponding frequency[2]. Varieties of designs utilizing metamaterials with near-zero index to achieve antenna lens have been reported. Some lenses consist of electric resonance structure with near-zero permittivity[3] and some lenses consist of magnetic resonance with near-zero permeability[4]. However, these lenses cannot match the free space very well and the total efficiency of antenna and lens is low although the directivity enhancement is significant. In addition, the available frequency band is narrow. In 2011, Turpin et al. proposed a metamaterial lens with both electric and magnetic resonance structures, but the impedance match between lens and free space is still hard to controland the unit cell structure is complex to fabricate [5]. In this paper, a metamaterial lens (ML) with both electric and magnetic resonance structure is proposed for antenna directivity and gain enhancement. Not only is the refraction index near-zero beneficial for EM wave convergence, but also the characteristic impedance can match that of air very well. As a result, the proposed ML is universal for different kinds of antenna and not sensitive for the distance.

2. Metamaterial Structure Design and Analysis

The proposed metamaterial unit cell is illustrated in Fig. 1. It is composed of two parts: the modified split ring resonators (MSRRs),which is akind of strong magnetic resonance structure [6] and the metal patch, which is electric resonance structure similar to metal wire array [7]. The MSRR is a square loop with two slots at the opposite sides. In one metamaterial unit cell, two MSSRs are etched on the both sides of the dielectric substrate with rotation of 90°. The structure of the metamaterial unit cell is much easier to fabricate than the design in [5]. All the geometry parameters are

978-1-4673-5225-3/14/$31.00 ©2014 IEEE as below: l1 = 8 mm, l2 = 5.4 mm, l3 = h = 6.6 mm, t = 8.2 mm, t1 = 2.9 mm, t2 = 0.8 mm, w = 0.8 mm and s = 0.4 mm. The relative permittivity of the substrate is 2.2. The S-Parameters of the metamaterial unit cell are obtained by simulation in CST MWS and are depicted in Fig. 2. There is a wide pass band available centered at 9.9 GHz.The effective constitutive parameters of the unit cell can be extracted from the S-Parameters using the calculation procedure in [8] and depicted in Fig. 3. Both of the two constitutive parameters are of the same value of 0.26 (small enough for wave collimation [2]) at 9.9 GHz, and impedance matching are also achieved.

Fig. 1. Structure and parameters of the proposed metamaterial unit cell.

Fig. 2. S-Parameters of the metamaterial unit cell. Fig. 3. Refraction index of the metamaterial unit cell.

3. Metamaterial Lens Simulation and Experiment

The metamaterial lens (ML) is formed by arranging the metamaterial unit cell in one plane. The ML is simulated witha patch antenna operating at 9.9 GHz and an H-plane horn antenna with center frequency of 9.9 GHz in CST MWS respectively as shown in Fig. 4. Both of the simulated return losses and radiation patterns of the two antennas and the result discussion are given as below.

Fig. 4. The ML tested with (a) a patch antenna and (b) an H-plane horn antenna and (c) is the phototype of ML and the H-plane horn antenna.

3.1 Lens with Patch Antenna

For the patch antenna, the size of the ML is 7×9 unit cells to cover the antenna aperture. After placing the patch antenna behind the ML, the return losses of the patch antenna with and without the ML are given in Fig. 5. The return loss almost remain the same after loading the ML thanks to the good impedance matching property of the ML. E-plane radiation patterns are simulated and shown in Fig. 6. The main lobe width of E-plane radiation pattern is reduced remarkably from 123.5° to 31.2°, and the gain enhancement is 6.6 dB, which display the powerful directivity and gain enhancement of the ZIML for the patch antenna.

Fig. 5. Simulated return loss of the patch antenna with Fig. 6. Simulated E-plane radiation pattern of the patch and without the ML. antenna with and without the ML.

3.2 Lens with H-Plane Horn Antenna

For the H-plane horn antenna, the size of the ML is 19×13 unit cells. Similar to the case ofpatch antenna, the H- plane horn antenna with ML is both simulated in CST and fabricated as shown in Fig. 4(c). The measured return loss of the horn antenna is depicted in Fig. 7. The return loss of the H-plane horn antenna is also slightly affected by the ML.The measured E-plane radiation patterns of the H-plane horn antenna with and without ML are illustrated and contrasted in Fig. 8. The main lobe of the E-plane radiation pattern of the antenna at 9.9 GHz is obviously sharpened. The main lobe width is reduced from 91.4° to 14.8°. The directivity of the H-plane horn antenna is greatly enhanced. The antenna gain of the H-plane horn antenna is also enhanced by 4.43 dB, which is a significant improvement. The gain enhancement of the ML with different distance between the lens and antenna is also depicted in Fig. 9. The ML proposed in this paper is capable of enhancing antenna gain in a wideband from 9.5 GHz to 10.6 GHz. In additionally, the gain enhancement barely varies with the distance thanks to good impedance match between lens and free space, which is an advantage over lens based on Fabry-Perot resonance.

Fig. 7. The measured return loss of the H-plane horn Fig. 8. The E-plane radiation pattern of the H-plane antenna with and without the ML. horn antenna with and without the ML.

Fig. 9. The gain enhancement of the H-plane horn antenna loading ML with different distance.

4. Conclusion

In this paper, an ML with both electric and magnetic resonance structureis proposed for antenna directivity and gain enhancement. Good impedance match between ML and free space can be achieved by tuning the constitutive parameters of the metamaterial unit cell. And the near-zero index makes the ML converge the EM wave emitted from antenna. Significant directivity and gain enhancement is achieved for both patch antenna and H-plane horn antenna. The available frequency band is wide and the ML is insensitive to distance between ML and antenna.

5. References

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4. Q. A. Cheng, W. X. Jiang, and T. J. Cui, "Radiation of planar electromagnetic waves by a line source in anisotropic metamaterials," Journal Of Physics D-Applied Physics, vol. 43, no. 33, 2010, p. 335446(6).

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6. Q. Tang, F.-Y. Meng, Q. Wu, and J.-C. Lee, "A balanced composite backward and forward compact waveguide based on resonant metamaterials," Journal of Applied Physics, vol. 109, no. 7, 2011, p. 07A319(3).

7. J. B. Pendry, A. J. Holden, W. J. Stewart, and I. Youngs, "Extremely low frequency plasmons in metallic mesostructures," vol. 76, no. 25, 1996, pp. 4773-4776.

8. F.-Y. Meng, Q. Wu, D. Erni, and L.-W. Li, "Controllable Metamaterial-Loaded Waveguides Supporting Backward and Forward Waves," IEEE Transactions on Antennas and Propagation, vol. 59, no. 9, 2011, pp. 3400-3411.