Design and Experimental Measurement of Input and Output Couplers for a 6-18-GHz High-Power Helix Traveling Wave Tube Amplifier

In this article, two types of compact and effective input and output couplers for a 6-18 GHz 5-kW-level helix traveling wave tube (TWT) are designed, fabricated, and cold- tested. The input coupler section includes an impedance transformer, a conical coaxial window, and a transition to a subminiature version A (SMA) connector for a radio frequency (RF) input. For achieving a high-power output, a new type of double-ridged waveguide window is used in the output coupling section. The output coupling structure consists of a standard double-ridged waveguide (WRD650), an impedance transformer, a square sapphire window, and a coaxial to double-ridged waveguide transition. The commercially available 3-D electromagnetic simulation software package CST is used to further optimize these structures, including couplers terminated with an actual complex helix slow-wave structure (SWS). According to the simulation results, the input and output couplers are fabricated and tested. The experimental results of the optimized coupled structures are presented, and both couplers show excellent transmission performance for a 6-18-GHz high-power helix TWT.