CHENG Xiu-yang,TENG Fei

(College of Electrical Engineering and Automation,Shandong University of Science and Technology,Qingdao 266590,China)

In recent years,the microstrip antenna array has been widely used in many fields because of its advantages such as low profile,high gain and good directivity,which easily realizes the advantages of beam scanning[1]. A aperture coupled microstrip antenna is very suitable for an antenna array element,because it can be fabricated easily,realize impedance matching and avoid punching on the substrate. Moreover,the feed network has small influence on radiation pattern. However,for an antenna of array,there is mutual coupling between antenna elements because of the influence of space wave and surface wave. The mutual coupling can change the performance parameters of antenna array,such as the offset of main lobe,impedance,gain and radiation pattern[2].

In order to reduce the mutual coupling between antenna elements,the technologies used for suppressing the coupling recently have achieved great success mainly including: photonic bandgap (PBG) technology and defected ground structure (DGS) technology. For the forme because the antennas need many periodic PBG structures,it is difficult to design a compact microstrip antenna array[3]; for the latter,it just needs to slot in the ground. Because the antenna has a simple structure,it is easy to be fabricated[4]. However,there exists the defect that the gain of antenna may fall because of backward radiation of electromagnetic energy.

In Ref.[5],PBG technology is used. By penetrating cylindrical metal via hole through the dielectric layer,the square patch element on the medium plate is connected to the ground plate to form surface wave frequency band gap near the antenna resonant frequency. Therefore,the coupling among the antenna elements is reduced,and accordingly the performance of antenna radiation is improved. However,we need punch periodic holes in the substrate,but it is complex to realize this structure.

In Ref.[6],with DGS technology,by cutting twoU-shaped slots on the ground floor,the antenna's radiation frequency squarely lies in the band-stop DGS spectrum. Thus,the surface waves are suppressed and the coupling between the antenna elements is reduced by 9 dB with a gain drop of the antenna of 0.4 dB.

In this paper,we reduce the mutual coupling among antenna elements by cutting four small semi-circle annular apertures on the metal plate. The structure of antenna is simpler than PBG structure in Ref.[5],and the four semi-circle annular apertures cut off the coupling current field between the elements to suppress the surface wave. With the side and back lobe level suppressed effectively,the gain of the antenna is improved about 0.6 dB,and the coupling coefficient decreases significantly.

1 Antenna structure

1.1 Antenna element structure

The aperture coupling antenna element structure is shown in Fig.1. We choose FR4 whose dielectric constant is 4.4 as the dielectric layer,the thickness of the dielectric layer is 1 mm and the thickness of the air layer is 2 mm. Cutting “H” type slot on the metal board,the energy can be coupled into the radiation patch[7]. The purpose of cutting a 45° rectangular aperture is to form a circular polarized wave[8].

Fig.1 Antenna element structure

1.2 Antenna array structure

The aperture coupling antenna array structure is shown in Fig.2. The structure of antenna array is composed of four elements,and the distancedbetween the elements is 36 mm. The antenna elements adopt reverse feeding principle. There is 90° phase difference among adjacent antenna elements. The four feeding ports of the antenna have symmetry structure; Coupling coefficients of the neighbouring antenna elements are approximate,S12≈S23≈S34≈S14,and the coupling coefficients of the catercorner antenna elements is also approximate,S13≈S24. Therefore,we just need to study the coupling coefficientsS12 andS13[9].

Based on the above design,we use electromagnetic simulation software to build up the antenna array model and simulate it. The simulation is shown in Fig.3.

Fig.2 Antenna array structure before decoupling

Fig.3 Antenna array coupling coefficient before decoupling operation

It can be seen from Fig.3 that the coupling coefficient of adjacent elementsS12 is -31 dB and the coupling coefficient of the catercorner antenna elementsS13 is -28.3 dB. Therefore,there is influence of mutual coupling among the antenna elements,and it has bad impact on performance of the antenna array.

The structure of microstrip antenna array with semi-circle annular aperture symmetrically on the metal plate is shown in Fig.4.

Fig.4 Antenna array structure after decoupling

The four semi-circle annular apertures symmetrically on the metal plate have small influence on circular polarization characteristics of the antenna array[10]. The width of semi-circle annular aperture is 1 mm,and the inside radius is 5.1 mm.

In Fig.5,we can see the contrast of coupling coefficients before and after decoupling.

Fig.5 Contrast of coupling coefficients before and after decoupling

As it is shown in Fig.5,after decoupling,the coupling coefficient of adjacent elementsS12 is -51.6 dB and drops nearly 20 dB than before decoupling,so decoupling has great effect on decreasing coupling coefficient of adjacent elements. Similarly,the coupling coefficient of the catercorner antenna elementsS13 becomes -36 dB and drops nearly 10 dB after decoupling than before decoupling,with the coupling coefficient reduced obviously.

Fig.6 Contrast of radiation patterns before and after decoupling

The radiation patterns of the antenna array are shown in Fig.6. After cutting semi-circle annular aperture,the side and back lobe level are suppressed effectively,and the gain of antenna is improved about 0.6 dB. After slotting four small semi-circle annular apertures symmetrically on the metal plate,the distribution of effective dielectric constant of the dielectric layer is changed. And the microstrip has the characteristics of band-gap and slow-wave when the radiation frequency is included in the frequency of band-stop. The surface wave is suppressed,so the mutual coupling decreases significantly and the gain of antenna is improved. Furthermove,the radiation performance of antenna array is improved.

2 Conclusion

In this paper,we propose a mutual coupling suppression method for microstrip antenna array by slotting on the metal plate. With four semi-circle annular apertures cut symmetrically on the metal plate,the antenna array coupling coefficients decrease significantly. The side and back lobe level are suppressed effectively,and the gain of antenna is improved. As a result,this coupling suppression method is efficient and practical.

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