本論文以陸基FMCW(Frequency Modulation Continuous Wave)雷達量測目標散射係數為出發點，設計一應用於X波段散射儀之電控相列碟型天線系統，特色為可調八角度之量測散射係數，其結構包含了饋入天線陣列、波束形成器與拋物反射面三部分。此天線系統原理是利用天線陣列間信號相位差，而產生多個不同入射角度波束來測量背向散射係數，同時可避免傳統以手動調整入射角度所造成之誤差，藉此提高量測的精準度與操作便利性。在饋入天線陣列設計選用，具有結構簡單、饋入方便，可以使用微帶電路來實現等優點準八木宇田天線 (Quasi Yagi-Uda antenna) 為單元天線，可大幅減少了單元天線的體積與重量，且其具有約40-50%頻帶寬、高增益、高指向性之單端輻射 (end -fire) 場型，可滿足在散射係數量測時，所需頻寬涵蓋範圍。在波束形成器部分，使用巴特勒矩陣 (Butler matrix)。巴特勒矩陣具有精確輸出相位、準位，且電路結構簡單易於設計之特性，可使用微帶線實現，因此易於與系統整合。最後再將饋入天線陣列所形成的波束，入射至拋物反射面，利用拋物面天線所具有之聚焦特性提高輸出增益、並縮小半功率波瓣寬度 (3-dB beamwidth )提升背向散射係數量測精度。 To facilitate a X-band ground-based radar scattering measurements, in this thesis, we design a scattering phased array reflector antenna system, with eight adjustable incidence angles to measure backscattering coefficient. The antenna system composes of feed antenna array, beam-former, and a parabolic reflector. By properly changing the phase difference among array elements, a number of different beam incident angles can be formed. The electronically steering incident angle is superior to the mechanical ones in setting the angle accuracy and easy operation. As for the feed antenna array element, a Quasi Yagi–Uda antenna is chosen for its simple structure, easy feeding, and implementable on microstrip circuit. In doing so, the size and weight of the antenna element can be greatly reduced, about 40-50% frequency bandwidth can be achieved. Its high gain, high directivity of the single-ended (end-fire) radiation pattern is well suited to scattering coefficient measurement with wide bandwidth coverage. Then it comes to the beam-forming. It is realized that the Butler matrix network bears high precision of phase and leveling, simple structure, and easily implemented by type of microstrip lines. Finally, a parabolic reflector is used to focus the antenna beam to produce high output gain, while improve the half-power beam width (3-dB beamwidth).
