Investigation on Out-of-Tube Reflection Oscillations and Suppression Technology for Wideband Sheet Beam TWTs

In this study, an analysis of the out-of-tube reflection oscillation of wideband sheet beam traveling wave tubes (SB-TWTs) is presented and an absorptive low-pass filter (ALPF) for oscillation suppression is proposed. The oscillation condition of the TWT is employed to investigate the effect of external devices on the stability of the system. The analysis reveals that high external reflection can trigger the out-of-tube reflection oscillation. A filter-loaded structure is proposed to suppress the oscillations, which employs multistep auxiliary dissipative waveguides and weak coupling technique to achieve low reflection in the operational frequency range and high insertion loss in the stopband. Two proof-of-concept prototypes are fabricated and tested. An appropriate error model is formulated to identify the source of the error, which helps improve the processing methods. The improved ALPF is tested and exhibits excellent agreement with the simulation results. The reflection coefficient from 26.5 to 40 GHz is less than −20 dB, while the average insertion loss is less than 0.8 dB in the passband (from 26 to 32 GHz) and surpasses 10 dB in the stopband (from 35 to 38 GHz). These results demonstrate the efficacy of the proposed filter-loaded technique, which suppresses out-of-tube reflection oscillations in wideband SB-TWTs.