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The initial experimental results of an L-band coaxial relativistic backward wave oscillator with mechanical frequency tunability are presented. The key effects of the inner-conductor contributing to the mechanical frequency tunability are investigated theoretically and experimentally. In the experiments, the L-band microwave with frequency of 1.58 GHz is radiated when the inner-conductor radius is 1.5 cm. Meanwhile, the S-band microwave with frequency of 2.31 GHz is generated after removing the inner-conductor. In addition, the frequency tuning within 4% is realized by mechanically altering the radius of the inner-conductor at a half power level.

The initial experimental results of an L-band relativistic backward wave oscillator with a coaxial slow-wave structure are presented. The asymmetric-mode-competition mechanism in the device is investigated theoretically and experimentally. It is shown that the diode voltage, guiding-magnetic field, and concentricity play a key role in the suppression and excitation of the asymmetric-mode (coaxial quasi-TE11 mode). In the experiments, the asymmetric-mode with a frequency of 2.05 GHz is suppressed and excited, which is in good agreement with the theoretical results.