基於交叉偶極負載裂環之雙圓極化反射式陣列天線

Abstract

建立在普通偶極反射天線單元的基礎上，本論文提出應用於雙圓極化反射式陣列天線的交叉偶極負載裂環之天線單元。每單元由一組正交且負載裂環之偶極組成，分別於支撐介質RT/Duroid 5880之印刷電路上下兩面，且此介質下方置有金屬板。其特色為結構簡單，對於兩正交方向之極化皆能提供360度以上的反射相位，且兩極化方向各自獨立。 本論文接著提出正向入射及TE斜向入射下，交叉偶極負載裂環天線單元的等效電路模型。只適用於正向入射的一般模型包含兩組共振的電容電感電阻，兩段相接的傳輸線，以及終端的短路電路。天線單元物理結構不同對於電容電感電阻的變化亦於文中討論。而適用TE斜向入射的廣義模型，只需要於正向入射的一般模型中，額外增加一段傳輸線及一顆電阻即可完成。此將模型廣義化的方法亦適用於偶極單元組成的頻率選擇表面及反射陣列天線單元，且將廣義模型會於代入正向入射條件時，廣義模型退化回適用於正向入射的一般模型。不同斜向入射角度對於電容電感電阻的變化也於文中討論。 最後，我們使用交叉偶極負載裂環單元設計，製作並量測一工作於10 GHz，包含1076天線單元的反射式陣列天線。受惠於單元簡單之特性及廣義模型對於相位之初步估計，全波模擬在無最佳化的情況下便能夠得到與前人特性相當的結果。模擬結果顯示反射式陣列天線只需要旋轉其中的饋入天線，便能得到右手圓極化31.5 dBi和左手圓極化31.1 dBi的高增益。若將饋入天線改為一雙線性極化之饋入天線，雙圓極化反射式陣列天線即可直接實現。此實作之反射式陣列天線在10 GHz量測之右手圓極化增益為29 dBi，對應天線孔徑效率23.5%，另外半功率波束寬度 ，旁波瓣低於主波瓣16.3 dB。造成輻射場型偏差以及增益降低的可能原因亦於文中討論。

From the plain strip dipole unit cell, the development of the crossed split-ring-loaded dipoles unit cell for a dual circularly polarized reflectarray antenna is presented in this thesis. Consisting of only dipoles and split rings printed on two faces of the supporting RT/Duroid 5880 substrate layer placed above the metal ground plane, the unit cell features simple structures, capability of providing an over 360° reflection phase range for both x- and y-polarization, and independence between the two polarizations. Then, the corresponding equivalent circuit model is presented, for both the normal incidence cases and the generalized TE oblique incidence cases. The conventional model valid only for normal incidence cases consists of two series RLC resonators, two connected transmission line sections, and a short circuit termination. Influences of the geometric parameters of the unit cell on corresponding RLC values are also discussed. For the TE oblique incidence cases, only an additional transmission line section and an additional resistor is added to form the generalized circuit model, compared to the conventional model under normal incidence. The proposed simple generalization method is also applicable for the dipole array FSS, and the dipole array unit cell, and the generalized models degenerate to the conventional model under normal incidence. The influence of the incident angle on corresponding RLC values are also discussed. Last, a 10-GHz prototype reflectarray antenna with 1076 crossed split-ring-loaded dipoles unit cell is designed, fabricated, and tested. Benefit from the simplicity of the unit cell structure and the preliminary phase estimation using the generalized circuit model, the full-wave simulation gives promising results comparable to prior works without any optimization during the full array design analysis. The simulation shows that peak gains of 31.5 dBi and 31.1 dBi for the right-handed and the left-handed circular polarization, respectively, could be obtained simply by rotating the feeding horn antenna in the reflectarray. By replacing the feeding antenna with a dual linearly polarized one, a dual circularly polarized reflectarray could readily to realized. The fabricated prototype of the reflectarray antenna exhibits a measured RHCP gain of 29 dBi with an aperture efficiency 23.5%, half-power beamwidth of 3.8°, and sidelobe level below -16.3 dB, and a measured LHCP gain of 28.8 dBi with an aperture efficiency 22.4%, half-power beamwidth of 3.5°, and sidelobe level below -15.9 dB, both at 10 GHz. The possible causes of the deviation between the simulated and measured results are also discussed.