Demonstration of an Even Symmetric Overmoded Coupler for Power Measurement in Sheet Beam Traveling-Wave Tubes

This study proposes a solution to the lack of effective measurement methods in hot tests for high-power sheet beam traveling-wave tubes (SB-TWTs) with ultra-wideband transmission. Specifically, an even symmetric overmoded coupler is suggested as a means of coupling quantitative signals from output power in an ultra-wideband range to enable the rapid measurement of output power. An overmoded waveguide is used to increase power handling capability. To extend the operating frequency band, a stepped secondary waveguide is employed, and coupling between different primary and secondary waveguides is utilized to maintain high mode purity. Additionally, the even symmetric structure is employed to suppress higher order mode. Based on 3-D simulation results, the proposed coupler exhibits 58% relative bandwidth and -40 +/- 0.2-dB coupling response. The entire even symmetric overmoded coupler, including the main coupling structure, a match waveguide, and a square-circular waveguide, is designed and manufactured. Appropriate measurement methods have been explored. The efficacy of the symmetric structure in suppressing higher order mode oscillations is verified, with the cold test exhibiting a relative bandwidth of 35% and a coupling response of -40 +/- 0.5 dB.