A Study of Planar-to-Waveguide Transition for Wideband Millimeter-Wave Signal Coupling Co-advisor: Professor Jae-Hyung Jang

Abstract

Millimeter-wave communication systems demand high-speed data transmission, minimal signal loss, and broadband signal coupling to support the performance requirements of next-generation applications. However, the integration of planar circuits and waveguide structures remains a significant challenge due to their differing guiding mechanisms, characteristic impedances, and field distributions. Efficient energy transfer between microstrip lines and rectangular waveguides is particularly difficult and often constrained by radiation leakage, fabrication complexity, and alignment sensitivity. This dissertation addresses these challenges by proposing and analyzing two novel planar-to-waveguide transition structures, focusing on broadband performance, ease of manufacturing, and high integration capability with planar and multilayer circuit platforms. The first transition introduces a partially covered back-short design in the microstrip-to-waveguide transition (MWT) to achieve wideband operation and low insertion loss without relying on a shorted waveguide cavity or additional matching structures. Implemented using a printed circuit board with three copper layers and two dielectric layers, the transition features a via fence and a partially covered microstrip line to suppress radiation leakage. It leverages multiple resonances to achieve broadband signal coupling while maintaining structural simplicity and cost-effectiveness. Back-to-back measurements validate its performance, demonstrating a return loss greater than 10 dB over the 41–63 GHz band, corresponding to a 42% operating bandwidth. The second transition focuses on enhancing manufacturability and misalignment tolerance in waveguide-to-microstrip interfaces. A wedge-waveguide to microstrip transition (WMT) is proposed, employing three key innovations. First, it utilizes broadside aperture coupling, which contrasts with conventional end-wall aperture methods, offering greater coupling area and improved misalignment tolerance. Second, it avoids the high losses and complexity associated with probe-insert broadside couplings by enabling robust inline coupling through a wedge-waveguide structure. Third, it eliminates the need for complex tapers or ridges, reducing both volume and fabrication difficulty. The design supports seamless MMIC integration and requires no waveguide splitting. It achieves a fractional bandwidth of 44% and a back-to-back insertion loss of 8.5 dB, including 5.5 dB from a 1-meter-long waveguide path, while tolerating ±250 µm vertical and ±300 µm horizontal misalignments with negligible performance degradation. Together, these transition architectures offer scalable, low-loss, and fabrication-friendly solutions that enable efficient planar-to-waveguide coupling for advanced millimeter-wave systems.