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Design and Measurement of a Nonlinear-Curve Directional Coupler for Sheet Beam Traveling Wave Tube

A new type of input/output coupler for sheet beam traveling wave tube is designed, analyzed, and measured in this paper. In the design, a special sloped coupling aperture distributed between two paralleled rectangular waveguides in H-plane is employed for compact dimensions, ease of fabrication, attainment of high power capacity, and more convenience for periodic cusped magnet focusing system. A nonlinear-curve profile with Chebyshev-curve distribution is applied to extend the operating frequency bandwidth while suppressing reflections to avoid generating oscillation in the slow-wave structure. Based on the analytical investigation and numerical simulation, an optimized sloping angle of the coupling aperture is obtained to improved performance. Simulations imply that the coupler achieves a broadband bandwidth (S11 <; -20 dB) of 23 and 10 GHz in W- and Q-band, respectively, with coupling coefficient above -0.2 dB. Cold test results satisfactorily agree with simulations.