A novel L-band coaxial Transit-time Oscillator (TTO) with mechanical frequency tunability is put forward and analyzed. A smooth inner conductor is adopted, and its radius is tenth of the one of the outer conductor. Compared with the traditional coaxial TTO with gaps in the inner conductor, the vertical distance between the outer and inner conductor is longer, as a result, higher power capacity is achieved. With the diode voltage and current as 600kV and 13kA, respectively, an L-band output microwave with frequency of 1.48 GHz and power of 3.2 GW is achieved. The efficiency can be 41%. Furthermore, by modulating the radius of the outer conductor, the working frequency ranges from 1.38 GHz to 1.52 GHz while the output power is higher than 1.0 GW. The frequency tuning range of the coaxial TTO is 9.5% of the central frequency. Besides, the electric field distribution is analyzed and shows that the longitudinal and transverse working modes are TM01 and π mode, respectively.
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