The rectangular gyrotron traveling wave tube (gyro-TWT) with a large aspect ratio (α) has the potential to achieve megawatt-class output power. As an essential component of gyro-TWT, a novel overmoded Ka-band mode converter with an α of 6.19 is designed, analyzed, and cold tested in this paper. Based on the magnetic dipole moment theory, the rectangular overmoded TE01 mode is excited via the rectangular fundamental TE10 mode. The cutoff waveguide is applied to prevent electromagnetic wave transport to the magnetron injection gun (MIG) region and also guarantee higher power electron beam transportation. Simulations predict an operation bandwidth higher than 4 GHz and greater than 99.8% mode purity between 33–37 GHz. To verify this design, the mode converter is manufactured and cold tested. The back-to-back measurement results exhibit a good agreement with the simulation. With similar topologies, the rectangular overmoded TE01 mode can be excited in a different α.
