This paper presents a new approach for designing broadband, compact transitions between rectangular and circular waveguides using supershape geometries. The proposed structure incorporates a supershaped intermediate section that is less than one-quarter wavelength thick, enabling a highly compact profile. In addition to its wideband performance and suppression of spurious modes, the transition offers several practical advantages, including straightforward fabrication, low insertion loss, and an overall length shorter than 0.15 λ at the center frequency. To validate the concept, a back‑to‑back rectangular‑to‑circular mode converter was manufactured and tested. For a rectangular-to-circular waveguide transition, measurements of the back-to-back prototype, followed by extraction of the single-transition response, indicate that an individual transition exhibits a return loss of approximately 15 dB and an insertion loss of about 0.25 dB across the 12–18 GHz operating band.
Zarifi et al. (Wed,) studied this question.