An Easy-to-Fabricate Circular TE₂₁/TE₀₁ Mode Generator

The design, fabrication, and cold test of a mode generator to excite the circular TE
₀₁
or TE
₂₁
mode for
${H}$
-band (220–325 GHz) gyrotron traveling-wave-tubes (TWTs) are presented in this article. The mode generator is composed of two parts: a TE
$^{\Box }_{10}$
–TE
$^{\Box }_{20}$
mode converter based on a two-way H-T junction power divider and a TE
$^{\Box }_{20}$
–TE
$^{\circ }_{01}$
/TE
$^{\circ }_{{21{ }}}$
mode converter with a tapered waveguide (
$^{{\Box { }}}$
and ° represent the rectangular and circular waveguide, respectively). By adjusting the ratio of width and height of the tapered waveguide, the TE
$^{\circ }_{01}$
or TE
$^{\circ }_{\vphantom {D_{l}}21}$
mode could be excited. The primary advantage of this technique is that it has a relatively compact and easy-to-fabricate configuration, which is especially useful in the terahertz-band. To verify the design, two identical mode generators joined back-to-back were manufactured and measured. The measured
${S}$
-parameters were in good agreement with the simulated ones, which predicted that the TE
$^{\circ }_{\vphantom {D_{l}}21}$
/TE
$^{\circ }_{01}$
mode generator obtained a 1-dB transmission bandwidth of ~38.1 and ~48.0 GHz, respectively. Meanwhile the reflection coefficient
$S_{11}$
was below −10 dB over that bandwidth.