Prof. Jiro Hirokawa
Institute of Science Tokyo, Japan

Title: Waveguide-Bend Mode Conversion for Omnidirectional Antennas

Abstract:
This keynote reviews mode conversion in waveguide bends from a common perspective, with particular emphasis on the principles used to control modal behavior in circular and rectangular waveguides. The discussion focuses on how a dominant mode can be coupled to a selected higher-order mode and how the cross-sectional geometry, bend angle, and radius of curvature determine the resulting coupling. Practical design issues are also considered, including the achievement of efficient conversion around the design frequency, suppression of unwanted higher-order modes, and realization of a useful operating bandwidth.

The first case concerns vertical polarization. A circular-waveguide bend is used to convert the TE11 mode into the axially symmetric TM01 mode. Two identical bends with opposite bending directions are combined to form a symmetric converter that permits bottom feeding while retaining broadband conversion characteristics. Tapered waveguide sections are incorporated to provide the required dimensional transitions and to control modal reflection and transmission during the conversion process.

The second case addresses horizontal polarization. Here, a rectangular-waveguide bend is designed to convert the dominant TE10 mode into the higher-order TE20 mode. The converted field is then transformed into the circular-waveguide TE01 mode by appropriately varying the waveguide cross section. The influence of waveguide dimensional ratios and the length of inserted straight sections on mode separation and operating bandwidth is examined.

The final case considers the simultaneous treatment of two orthogonal polarizations. To accommodate the two different conversion processes within a common structure, a super-elliptical waveguide cross section is introduced as an intermediate geometry between circular and rectangular waveguides. By adjusting its shape, the cutoff-frequency separation and bend-induced coupling of the polarization-dependent modes can be controlled. Conditions for realizing both mode conversions in a single bent-waveguide structure are discussed. A multistage tapered waveguide is further employed to make the cross-sectional transformation gradual, thereby maintaining high conversion efficiency while suppressing unwanted modal excitation for both polarizations.

Taken together, these three cases provide a unified view of mode conversion based on waveguide bends and cross-sectional shaping, and clarify a systematic approach to multimode control in high-frequency waveguide structures.

Biography:
Jiro Hirokawa received the B.S., M.S., and D.E. degrees in electrical and electronic engineering from Tokyo Institute of Technology (Tokyo Tech), Tokyo, Japan in 1988, 1990, and 1994, respectively. He was a Research Associate from 1990 to 1996, an Associate Professor from 1996 to 2015, and a Professor from 2015 to 2024 at Tokyo Tech. He is currently a Professor at Institute of Science Tokyo. He was with the antenna group of Chalmers University of Technology, Gothenburg, Sweden, as a Postdoctoral Fellow from 1994 to 1995.

His current research interests include analyses, designs, and fabrication techniques of slotted waveguide array antennas, millimeter-wave, and Terahertz antennas, and beam-switching circuits. He has authored or co-authored more than 200 peer-reviewed journal papers and more than 600 international conference presentations.

He served as an Associate Editor for IEICE Transactions on Communications during terms of 1999-2003 and 2004-2007. He also served as an Associate editor during 2013-2016 and as a Track Editor during 2016-2022 for IEEE Transactions on Antennas and Propagation. He serves as a Track Editor from 2023 for IEEE Antennas and Wireless Propagation Letters. He was the chair of the technical program committee for ISAP 2016. He was also the chair of IEICE technical committee on Antennas and Propagation from 2017 to 2019. He is the general chair of the IEEE International Symposium on Antennas and Propagation and JNC-USNC-URSI Radio Science Meeting in 2027.

He received IEEE AP-S Tokyo Chapter Young Engineer Award in 1991, Young Engineer Award from IEICE in 1996, Tokyo Tech Award for Challenging Research in 2003, Best Paper Award in 2007 and a Best Letter Award in 2009 from IEICE Communications Society, and IEICE Best Paper Award in 2016 and 2018. He also received Young Scientists' Prize and Award for Science and Technology from the Minister of Education, Cultures, Sports, Science and Technology in Japan in 2005 and 2026, respectively. He is a Fellow of IEEE and IEICE.