新型雙頻高阻抗反射面應用於天線增益改善

Abstract

本論文提出一新型雙頻人造磁導反射面的想法，設計工作頻率分別座落於LTE-3.5GHz和Wifi-5.2GHz兩頻段，並結合單極天線且天線與反射面之間以空氣層區隔以利阻抗匹配，利用此法來驗證其特性。人造磁導反射面的實現在此是以蕈狀結構為出發點，利用在上層金屬表面做出半對稱的結構，藉此提供兩組不同的共振路徑，令本身結構可以產生兩個不同操作頻率，且因兩組共振腔彼此獨立，所以可分別對高低頻進行獨立調整頻率的動作。 人造磁導作為反射面，其理論可從高等電磁學作解釋，傳統的金屬反射面反射係數為-1，所以當天線和金屬反射面之間距離小於四分之一波長時，彼此為破壞性干涉，所以效率低，增益也低，甚至不易達成阻抗匹配。有別於一般金屬面，人造磁導最主要的功能即為此反射面可令映像電流為同向反射，此舉可以提升整體天線的增益值，效率，和阻抗頻寬以及減少後向輻射。在本論文，我們可以透過人造磁導來減少天線和反射面之間的距離，同時，透過在蕈狀結構上板挖槽，利用增加電流路徑的原理，來使單元結構縮小，朝向輕薄短小的方向前進，並結合上述雙頻的設計原理，來達成最終的雙頻縮小化高阻抗面。在此，我們將會利用高頻電磁模擬軟體-HFSS來進行模擬，並藉由實驗來驗證其特性。

The thesis proposed an idea of designing a novel dual-band high impedance surface (HIS).The working frequency here are located at LTE-3.5GHz band and Wifi-5.2GHz band. In this thesis, CPW-monopole antenna and HIS are separated by air gap for impedance matching and validate the effect of HIS by overlapped simulation and experiment result. There are several ways to realize HIS and mushroom structure will be adopted here because of simplicity. By making a semi-symmetric structure on the upper metal sheet, it can obtain two different oscillating frequency. Further more, both frequency can be tuned independently because the LC parallel resonant cavities are separated. The principle of HIS can explained by advanced electromagnetic theory. It says the reflection coefficient of conventional metal sheet is -1,so the reflected wave here must be destructive interference for incident wave which will cause low efficiency , low gain and mismatched if air gap is smaller than wavelength/4. On the other side , HIS as reflector can make the different effect because the reflection coefficient at working frequency is 1 ,which means the phase of reflected wave and incident wave are the same at the surface. For these reason, HIS will be used which arranged periodically as the reflector of antenna. At last, the result shows that this kind of antenna can reduce the thickness of air gap by introducing the AMC reflector. Furthermore, the function of slot on the patch can increase the path of current to make the unit cell minimized and then the thesis will combine the idea for dual band design we have discussed. Finally, the thesis here can create small size and dual band HIS reflector.