Theoretical and Experimental Study on Mode Power Ratio Diagnosis of Dual-Mode Relativistic Backward Wave Oscillator

Hybrid mode converters (MCs) are critical components in high-power microwave (HPM) systems, particularly for devices operating at dual-mode or multimode regimes. Their conversion efficiency, however, may be significantly affected by different power ratios and relative phase differences of the constituent modes. Therefore, diagnosing the ratios of different modes is essential for designing hybrid MCs. This article presents an effective and straightforward method to diagnose the power ratios using a coaxial coupling probe through theoretical analysis, simulation, and experiments. By theoretical analysis and simulation, we demonstrate that the power coupled to the probe is proportional to the square of the amplitude of radial electric field at the diagnostic point. A periodic dependence between the coupled power and the relative phase difference of the modes is obtained. Experimentally, both small-signal and HPM experiments were conducted. In the small-signal experiment, the power proportions of TM01and TM02modes output from three different hybrid MCs were determined with no more than 5% error. In the HPM experiment, the method was applied to an X-band dual-mode relativistic backward wave oscillator (RBWO), indicating that the TM01mode constitutes approximately 25% of the total output power. With proper calibration of the probe coupling coefficients, this method can be extended to real-time mode ratio diagnostics.