dc.description.abstract | 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.
| en_US |