Study of a Wideband TE° 21 /TE° 01 Mode Converter for Subterahertz Gyrotron Traveling-Wave Tubes

A novel design methodology of a TE[Formula Omitted]/TE[Formula Omitted] mode converter for gyrotron traveling-wave tubes (gyro-TWTs) is presented in this article. This type of mode converter is characterized by its broad bandwidth. The proposed mode converter consists of two-mode conversion sections (TE[Formula Omitted]–TE[Formula Omitted] and TE[Formula Omitted]–TE[Formula Omitted]/TE[Formula Omitted]) and an overmoded coupling section. The feeding TE[Formula Omitted] mode is first converted into the TE[Formula Omitted] mode, then coupled by using an overmoded coupling structure, and finally converted into the desired TE[Formula Omitted]/TE[Formula Omitted] mode. For verification, two [Formula Omitted]-band (220–325 GHz) back-to-back joined TE[Formula Omitted]/TE[Formula Omitted] mode converter prototypes were microfabricated and measured. The cold test showed that a measured 3-/2-dB transmission bandwidth of 58.9 GHz (235.2–294.1 GHz)/56.3 GHz (235.2–291.5 GHz) for the TE[Formula Omitted]/TE[Formula Omitted] mode converter was obtained.