A Terahertz-Band Rectangular TE $_{\text{10}}$ to Circular TE $_{\text{01}}$ Mode Converter

The design, fabrication, and experimental measurement of a broadband and compact TE $^{\Box}_{\text{10}}$ –TE $^{\circ}_{\text{01}}$ (symbols $\Box$ $^{^{^{}}}$ and $\circ $ represent the rectangular and circular waveguides, respectively) mode converter is presented in this article. The proposed mode converter consists of three components, including a TE $^{\Box}_{\text{10}}$ –TE $^{\Box}_{\text{20}}$ mode converter based on a two-way H-T junction power divider, a TE $^{\Box}_{\text{20}}$ –TE $^{+}_{\text{22}}$ mode converter with variable cross-shaped waveguide, and a TE $^{+}_{\text{22}}$ –TE $^{\circ}_{\text{01}}$ mode converter. Two distinct design schemes have been employed for the first part of the TE $^{\Box}_{\text{10}}$ –TE $^{\circ}_{\text{01}}$ mode converter, which are based on two-way $^{^{^{}}}$ H-T junction power dividers with a path difference of odd times half-waveguide wavelength (Scheme 1) and a 180 $^{\circ}$ phase shifter (Scheme 2), respectively. This mode converter is highlighted by the significant advantage of high conversion efficiency over a wide bandwidth with a relatively easy-to-fabricate structure, which is evidenced by the measurement of two identical back-to-back joined mode converters based on Scheme 1. Measurement results matched with simulation ones within reasonable accuracy, which predicted that a 1-dB transmission bandwidth of $\sim$ 56.7 GHz was obtained. Meanwhile, the reflection coefficient S $_{\text{11}}$ was below $-$ 13.2 dB over that bandwidth.