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A new topology to excite the HE04 mode in wide bandwidth for confocal gyro-TWAs is presented. The mode exciter consists of a TE04 mode exciter and a TE04–HE04 transition waveguide section formed by two cascaded one-stage tapers. In addition, the closed-to-open confocal waveguide boundaries are studied. Numerical results show that the port reflection of the whole structure is well below −10 dB over 90–110 GHz; the transmission of HE04 mode is better than −1 dB over 94–107 GHz. Even more important, a maximum transmission variation of less than ±0.15 dB over the 1-dB bandwidth and a mode purity of better than 97.2% over the 3-dB bandwidth are achieved. The mode exciter is compact and easy to be fabricated, which can be applied to cold-test experiments of confocal gyro-amplifiers circuits. [ABSTRACT FROM AUTHOR]

The propagation characteristics of a confocal waveguide are related to the radial spheroidal function, which is extremely complicated. A quadratic polynomial fitting (QPF) expression derived from numerical results by interpolation was successfully applied to the development of confocal gyrotrons. However, the QPF expression is valid only for a moderate spheroidal parameter. As a result, it cannot work well for confocal gyrotron travelling-wave tube (gyro-TWT) applications since the Fresnel parameter may be small for stable operations. For the first time, an efficient algorithm is developed to accurately calculate the spheroidal functions. Based on it, the MATLAB code with high accuracy and efficiency is developed to analyze the propagation characteristics of confocal waveguide. The calculated results and the QPF results are compared for different mirror apertures, and then demonstrated by high frequency structure simulator (HFSS). The calculated results show good agreement with the HFSS results for different mirror apertures while the QPF solution agrees with the HFSS results only for large apertures. The code is valid for arbitrary mirror apertures for both the confocal gyrotron and gyro-TWT applications. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM PUBLISHER]