Theoretical and Experimental Study of the Modified Pill-Box Window for the 220-GHz Folded Waveguide BWO

The modified pill-box window is designed and manufactured as a power-coupler part of the 220-GHz folded waveguide backward wave oscillator in the Institute of Applied Electronics. Owing to the modified structure, both the fringe area and the end area of the thin dielectric window piece can be metalized, thus the reliability of vacuum seal and strength can be greatly improved compared with its traditional counterpart. An analytical method is developed from the previous equivalent circuit theory of the traditional pill-box window. According to the analytical expressions, the inner parameter of the modified pill-box window is roughly designed. The 3-D computer code is used to verify and further optimize the analytical design. The calculation shows good consistency between the theoretical and the numerical methods. The resonant modes in this oversized window system are also numerically calculated to demonstrate that it is difficult to remove all the modes from the operating frequency range. Fortunately, the error analysis shows that these possible spurious modes are hardly stimulated in the window system with TE10 mode injected, when the misalignment of each transmission section can be limited to the practical mechanical tolerance. For this requirement, an innovative mechanical structure is then proposed to guarantee the precision of the parameters of window system which can nicely restrain the variations caused by the brazing process. By carefully controlling the errors in machining, assembly, and brazing process, the brazed and nonbrazed samples are finally manufactured and tested. The experimental result indirectly demonstrates that the microwave circuit design and the mechanical design is reasonable and reliable, where the reflection ratio is < 0.1 between the frequency range of 215-225 GHz.