Background In the field of high-power microwave transmission, mode converters are essential components for transforming one waveguide mode into another. However, existing high-power mode converters often face a critical trade-off between broadband operation and compact physical size. Achieving both wideband performance and miniaturization simultaneously remains a significant engineering challenge, especially for applications requiring high efficiency and high power handling.

Purpose To overcome this difficulty, this paper proposes a novel broadband high-power TEM-to-TE₁₁ mode converter. The design integrates a broadband waveguide phase shifter with a radial-line waveguide power divider, aiming to deliver both excellent frequency response and a compact longitudinal dimension suitable for modern high-power feeder systems.

Methods The proposed converter consists of three functional stages. First, a TEM-mode radial-line waveguide power divider splits the input TEM microwave signal from a coaxial waveguide into four identical in-phase rectangular-waveguide TE₁₀ mode signals. Second, a phase-shifting section employs a capacitive-iris-loaded broadband phase shifter to introduce a precise 180-degree phase reversal between symmetrically paired paths. Third, a TE₁₁ radial-line combiner merges the four phase-adjusted microwave signals at the output port, synthesizing a linearly polarized coaxial TE₁₁ mode.

Results Full-wave simulation results demonstrate that the converter achieves a conversion efficiency exceeding 98% across the frequency band from 13.2 GHz to 15.4 GHz, corresponding to a relative bandwidth greater than 15%. Moreover, the power-handling capability surpasses 200 MW, while the overall longitudinal length is only 1.9 free-space wavelengths at the center frequency. These metrics indicate outstanding performance in terms of both bandwidth and compactness.

Conclusions The proposed TEM-TE₁₁ mode converter successfully addresses the longstanding conflict between broadband operation and compact size in high-power applications. With its high efficiency, broad bandwidth, substantial power capacity, and minimal axial length, the converter is well suited for integration into current and future high-power transmission feeder systems.