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

A novel design methodology of a TE
$^{\mathrm {\circ }}_{21}$
/TE
$^{\mathrm {\circ }}_{01}$
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
$^{\mathrm {\pmb {\circ }}}_{10}$
–TE
$^{\mathrm {\pmb {\circ }}}_{20}$
and TE
$^{\mathrm {\pmb {\circ }}}_{20}$
–TE
$^{\mathrm {\circ }}_{21}$
/TE
$^{\mathrm {\circ }}_{01}$
) and an overmoded coupling section. The feeding TE
$^{\mathrm {\pmb {\circ }}}_{10}$
mode is first converted into the TE
$^{\mathrm {\pmb {\circ }}}_{20}$
mode, then coupled by using an overmoded coupling structure, and finally converted into the desired TE
$^{\mathrm {\circ }}_{21}$
/TE
$^{\mathrm {\circ }}_{01}$
mode. For verification, two
$H$
-band (220–325 GHz) back-to-back joined TE
$^{\mathrm {\circ }}_{21}$
/TE
$^{\mathrm {\circ }}_{01}$
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
$^{\mathrm {\circ }}_{21}$
/TE
$^{\mathrm {\circ }}_{01}$
mode converter was obtained.