本研究提出一種創新之基於再入式腔體設計之高靈敏度微波感測器，應用於地震P波之垂直震動量測。感測架構以腔體內部再入間隙作為電場集中感測區域，架構操作於圓柱腔體之TM₀₁₀模態，而利用再入式柱轉換形成再入式模態，藉由該模態具備電場沿軸心方向強烈集中於再入式間隙的分佈特性，藉由上層金屬薄膜在垂直震動下產生形變，再入式間隙電容進而引起共振頻率偏移作為感測機制。為有效控制耦合程度與系統可集成性，本研究採用平面式環形耦合槽作為激發架構，透過調整槽角與饋入位置實現磁場耦合，有效激發再入式模態。
針對不同再入柱幾何結構進行模擬分析與比較，提出一改量再入式柱體設計，結合錐形在再入式間隙區之強電場分布與反錐形之感測面積優勢，在維持一定電場強度下有效提升感測面積來提升偵測薄膜震動之靈敏度，靈敏度達到26.5%，為五種柱體裡最高者。腔體採用光固化3D列印與濺鍍技術初步驗證製程可行性，並最終以CNC加工實現高Q值金屬共振腔。薄膜選擇方面，設計可替換式薄膜，量測比較五種薄膜材料，最終選用同時具備乳膠之柔性回復能力與金屬薄膜之結構支撐力的複合乳膠銅箔結構，結合可調式質量塊設計，利用質量塊之慣性放大薄膜形變幅度，提升整體感測靈敏度與可量測震度範圍，成功實現一至四級地震之垂直震動量測，具地震預警應用潛力。

This study proposes an innovative microwave sensor based on a re-entrant cavity for the vertical vibration measurement of seismic P-waves. The sensing mechanism utilizes the strong electric field concentration within the re-entrant gap region of the cavity, operating in a mode derived from the {TM}_{010} mode of a cylindrical cavity. By introducing a central re-entrant post, the field distribution is transformed into a re-entrant mode, in which the electric field is highly concentrated along the axial direction within the gap. Vertical vibrations cause deformation in the upper metal diaphragm, modulating the effective capacitance of the re-entrant gap and resulting in a measurable resonance frequency shift. A planar annular coupling slot is adopted as the excitation structure. By adjusting the slot angle and feeding position, magnetic coupling is established, enabling efficient excitation of the re-entrant mode. Through simulation and comparison of five re-entrant post geometries, a modified post design is proposed, combining the strong electric field distribution of tapered post with the increased sensing area of inverted tapered post. This design achieves a balance between field intensity and sensing area, resulting in the highest sensitivity among five post structures, reaching 26.5%.
For fabrication, the cavity was initially realized using photopolymer-based 3D printing and metal sputtering, verifying the feasibility of the process. Final implementation employed CNC machining to obtain a high-Q cavity. In terms of diaphragm material selection, an interchangeable diaphragm design was implemented. After evaluating five different diaphragm’s materials, a composite structure of latex and copper foil was selected, offering both elastic recovery and structural rigidity. A tunable mass load was also introduced to amplify diaphragm deformation via inertia, effectively enhancing sensitivity and extending the measurable vibration range. The sensor successfully demonstrated vertical vibration detection for seismic events ranging from magnitude 1 to 4, showing strong potential for earthquake early warning applications.

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