Abstract Frequency tunability has become a subject of concern in the field of high-power microwave (HPM) source research. However, little information about the corresponding mode converter is available. A tunable circularly-polarized turnstile-junction mode converter (TCTMC) for high-power microwave applications is presented in this paper. The input coaxial TEM mode is transformed into TE10 mode with different phase delays in four rectangular waveguides and then converted into a circularly-polarized TE11 circular waveguide mode. Besides, the rods are added to reduce or even eliminate the reflection. The innovations in this study are as follows. The tunning mechanism is added to the mode converter, which can change the effective length of rectangular waveguide and the distance between the rods installed upstream and the closest edge of the rectangular waveguide, thus improving the conversion efficiency and bandwidth. The conversion efficiency of TCTMC can reach above 98% over the frequency range of 1.42 GHz-2.29 GHz, and the frequency tunning bandwidth is about 47%. Significantly, TCTMC can obtain continuous high conversion efficiency of different frequency points with the change of tuning mechanism.
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