An Easy-to-Fabricate Circular TE/TE Mode Generator

The design, fabrication, and cold test of a mode generator to excite the circular TE01 or TE21 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.