In this paper, converters of TE

$^{\Box }_{{10}}$

–TE

$^{\circ }_{{21}}$

mode and TE

$^{\Box }_{{10}}$

–TE

$^{\circ }_{{01}}$

mode with similar exciting topologic structures are designed and verified for a cold test of gyro-TWT. Both the proposed converters are composed of three sections, including TE

$^{\Box }_{{10}}$

to TE

$^{\Box }_{{20}}$

mode converter with ridged twist waveguide, TE

$^{\Box }_{{20}}$

to TE ⁺ ₂₂ mode converter with variable cross-shaped waveguide, and the last section TE ⁺ ₂₂ to TE

$^{\circ }_{{21}}$

or TE

$^{\circ }_{{01}}$

mode converter (

$^{\Box }$

represents the rectangular waveguide, ⁺ represents the cross-shaped waveguide, and ° represents the circular waveguide). The mode converters can be designed step-by-step and exhibit excellent wideband performance and high conversion efficiency. Simulation results reveal that the proposed TE

$^{\circ }_{{21}}$

mode and TE

$^{\circ }_{{01}}$

mode converters have less than 0.04 and 0.03 dB loss over 13.5- and 12-GHz bandwidths, respectively, while the return loss is below −25 dB. Prototype TE

$^{\circ }_{{21}}$

and TE

$^{\circ }_{{01}}$

mode converters were fabricated and back-to-back millimeter-wave measurements were performed to verify the design.

Tinghui Peng

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