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[References: 11 tit.]

The paper reports on numerical and experimental research in a coaxial L-band backward wave oscillator (CBWO) with an electron beam energy of ~ 300 keV. The oscillating mode with a 150 MW power and ~ 30 % oscillator efficiency for the operating transverse electromagnetic (TEM)-wave at 1,2 GHz was calculated in axially symmetric geometry by the PIC-code KARAT. The radiation wavelength was three times larger than the mean diameter of the CBWO slow wave structure. In experiments, a coaxial TEM-to-circular waveguide TE 11-mode converter was connected to the oscillator output, making a radiation pattern with a maximum on the system axis possible. In preliminary experiments, stable single-mode microwave pulses with a peak power of ~ 110 MW at an oscillator efficiency of 11 % were obtained at the output of the microwave source; the operating frequency was 1,23 GHz. The design of the CBWO electrodynamic system ensured selective excitation of the symmetric operating mode.