| 摘要: | 微波輸出窗是微波管的關鍵組件之一，它的好壞直接影響著微波管的性能，甚至整管研製的成敗。因此，設計出低反射，能夠傳輸高功率的微波管輸出窗是一項重要工作。微波管所使用之輸出窗的形式是多樣的，目前用得比較普遍的是盒型窗，型態主要可分薄片陶瓷與二分之ㄧ導波長陶瓷兩種。當使用之微波波長進入毫米波的範圍時，薄片陶瓷盒型窗的陶瓷厚度變得更薄，設計與製造均很困難；而二分之ㄧ導波長陶瓷盒型窗有著容易產生電磁共振幽靈模式的缺點。 微波輸出窗還有波導半波窗的型態，只是較少關於它的研究，主要是因為製作困難，而且頻寬小於盒型窗。不過隨著科技的發展，製作不再是不可行，於是波導半波窗的研究便有其可行性。本研究將針對Ku與Ka頻段波導半波窗進行電性匹配優化，希望能獲得超過百分之二十之頻寬範圍，並消除電磁共振幽靈模式之現象。同時，藉此研究對波導半波窗的電磁場特性，能有更深入的了解，對以後的設計及製作能有效地節省時間與成本。 首先，利用等效電路的方法來研究波導半波窗之理論基礎，並藉由模型簡化的步驟，減少部分參數，在選定陶瓷介質與波導的頻段後，僅剩陶瓷厚度需要進行分析。 其次，我們利用基於有限元素法的電磁模擬軟體HFSS12進行數值模擬，先將Ku頻段盒型窗之模擬結果與文獻比對，確認模擬方法的正確性，之後再分別針對Ku與Ka頻段波導半波窗進行模擬分析，電性匹配經過優化後均可得到超過百分之二十之頻寬（RL>20dB之頻率範圍）。在陶瓷焊接邊角圓角化的分析，Ku與Ka頻段波導半波窗圓角半徑分別達到2mm與0.75mm才開始有些微的差異。另外，針對簡化模型理論分析之結果與模擬結果比對，兩者差異均不超過千分之二，準確度相當高。 最後在波導半波窗內電磁場分布的研究，因為它未產生電磁共振幽靈模式，使得陶瓷面的電場遠低於材料本身的介電強度，可以避免被微波擊穿的危險。 分析模擬結果，波導半波窗的頻寬與半波長陶瓷盒型窗相當甚至優於它，而且不會產生電磁共振幽靈模式，可以應用在毫米波領域的微波管，甚至波長更短的微波管。 Microwave output window is one of the key components of the microwave tubes. It has a direct impact on the performance of microwave tubes, or the success or failure for the development of the whole microwave tube. Therefore, design of an output window with low-reflection; high-power transmission is an important task in the development of microwave tubes. There are diverse forms of output window used in the microwave tubes. Currently, the most commonly used type is the pillbox-type window. The pillbox window is further divided into two main types of ceramics: thin slice and half-wavelength ceramic windows. When the use of microwave length is in the range of millimeter, the thickness of thin slice ceramic pillbox window becomes thinner. This makes the design and manufacture of the output window very difficult. In addition, the half-wavelength ceramic pillbox window also has the disadvantage of ghost mode resonance. The half-wavelength ceramic waveguide window is another form of microwave output window and its related research is less found in the literature. The main reasons for this are that it is difficult to manufacture, and that its bandwidth is less than that of the pillbox window. With the development of technology, manufacture of the half-wavelength ceramic waveguide window is feasible and the study of half-wavelength ceramic waveguide window becomes possible. The main purpose of this study is to optimize impedance matching for Ku- and Ka-band half-wavelength ceramic waveguide window; that is to obtain a 20% increase in bandwidth and to eliminate the phenomenon of ghost mode resonance. In addition, our study will have a better understanding of the electromagnetic properties of the half-wavelength ceramic waveguide window to provide a time saving and cost effective way for future design and production. We first applied the equivalent circuit model as theoretical basis to study the half-wavelength ceramic waveguide window and further simplified the model to reduce some parameters. At the end, we only need to analyze the ceramic thickness with the chosen ceramic material and waveguide operating bandwidth (Ku- or Ka-band). Based on the finite element simulation (HFSS12, ANSYS, Inc., Canonsburg, PA, USA), we simulated the coefficients of reflection and transmission for the Ku-band pillbox window and compared our simulated results with the literature to verify the correctness of simulation methods. Then, simulation of the Ku and Ka-band half-wavelength ceramic waveguide window was implemented. After optimization of impedance matching, we could obtain a 20% increase in bandwidth corresponding to a return loss that is greater than 20 dB in the pass band. Regarding the fillet analysis in the ceramic welded edge of Ku- and Ka-band half-wavelength ceramic waveguide window, it began slightly different when the fillet radius was 2 mm and 0.75 mm, respectively. In addition, we compared our simulated results with the results of theoretical analysis for the simplified model; the difference did not exceed 0.2%, which is quite accurate. Finally, the study of the electromagnetic field distribution in the half-wavelength ceramic waveguide window was conducted. We found that it did not produce ghost mode resonance because the electric field of the ceramic surface is far below the dielectric strength of the material itself. This could avoid the risk of breakdown caused by the microwave. In summary, our simulated results showed the bandwidth of the half-wavelength ceramic waveguide window is equal or even superior to that of the half-wavelength ceramic pillbox window. The half-wavelength ceramic waveguide window did not induce the ghost mode resonance and could be applied to the microwave tubes with millimeter or even shorter wavelength. |
