71–76 GHz Sapphire Pillbox Windows

The transmission window, a critical component for the traveling wave tube (TWT), influences the transmission of high-frequency signals and also determines the bandwidth, output power, airtightness and stability of the TWT. Designing broadband, low-loss and low-reflection transmission windows is crucial for TWTs. In this paper, a sapphire pillbox window with an operating bandwidth of 71–76 GHz is designed and processed. It is found that at 71 GHz, the test results differ significantly from the simulation results. By analyzing and verifying, it is found that the phenomenon is caused by too thick a metallization layer. After optimizing the metallization process, the test obtained <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$S_{11} &lt; -24 \text{dB}$</tex> and S21 > -0.5 dB in 71–76 GHz. However, after soldering the sapphire pillbox window to the slow wave assembly, an air leakage problem occurred. Subsequently, the metallization process is optimized again. <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$S_{11} &lt; -18.5 \text{dB}$</tex> and S21 > -0.36 dB of the sapphire pillbox window are obtained, and it is experimentally verified that no air leakage occurs and it can be used for E- band TWTs.