Background With the rapid development of high-power microwave (HPM) technology and the engineering application of relativistic backward wave oscillators, online power measurement has become a key requirement for device state monitoring.

Purpose Traditional mode-selective couplers based on magic-T synthetic networks suffer from large volume, heavy weight in HPM environments, which restrict integrated application of online measurement systems.

Methods This study aims to develop a miniaturized, lightweight and high-electromagnetic-compatibility mode-selective coupler for X-band HPM systems, to realize selective online measurement of the TM₀₁ mode and suppress the interference of the TE₁₁ mode, with goals of structural compactness and mode isolation improvement.The proposed coupler adopts a circular waveguide main transmission structure with coupling apertures and a stripline synthetic network. The conventional magic-T network is replaced by a combined topology of stripline hybrids and Schiffman phase shifters. The main waveguide and synthetic network are separately designed and optimized, and the mode coupling characteristics are adjusted by aperture parameters and stripline line dimensions.

Results The coupling level of the TM₀₁ mode is more than 20 dB higher than that of the TE₁₁ mode. Numerical simulations and experimental tests verify that the online measured waveforms are highly consistent with the radiated field waveforms. The overall size and weight are significantly reduced, and electromagnetic compatibility in HPM environments is effectively improved, which validates the feasibility of the optimization approach.The proposed stripline synthetic network-based parameter design method effectively realizes the miniaturization and high mode selectivity of the circular waveguide mode-selective coupler.

Conclusions The device has simple and compact structure, good measurement consistency and strong environmental adaptability, which can be widely used in online measurement systems of X-band relativistic devices and extended to other frequency bands, providing a reliable technical solution for power monitoring in high-power microwave.