Design and Measurement of a Broadband Compact TE11 Mode Input Coupler for an X-Band Gyrotron Traveling Wave Tube

Design and measurement of a broadband compact circular TE ₁₁ mode input coupler for gyrotron traveling wave tubes (gyro-TWTs) are presented in this article. Different from the T-junction, V-type, and multiple-hole input couplers, the proposed coupler has a merit of compact dimensions formed by the H-plane of a rectangular cavity. The simulation results show that the port reflection S ₁₁ is less than -10 dB within 7.9-9.93 GHz. Also, the split transmissions S ₂₁ and S ₃₁ are over -3.4 dB in this band. It indicates that the relative bandwidth is 22.8%, and the transmission efficiency is higher than 90%. To further broaden the bandwidth, we propose an improved input coupler with ridges achieving an excellent bandwidth of 28.6%. The designed input coupler has been measured with open boundaries and matched output ports. The simulation and measurement results are in good agreement. The compact TE ₁₁ mode input coupler had been applied in our X-band gyro-TWT design.