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Characterization results of high-power microwave radiation, from an axial vircator driven by pulsed electron beam accelerator AMBICA-600 are reported in this paper. We present a study on variation in pulsed microwave power output and dominant frequency by discretely varying anode–cathode (A–C) gap. While keeping the cathode diameter fixed at 40 mm, for the A–C gap distance in the range 5–9 mm, dominant frequencies have been measured to be lying in the range 4.7–9.8 GHz. The trend of a subsequent increase in the dominant frequency at lower A–C gap distances (and vice versa) revealed that center frequency is mainly governed by the longitudinal size of the potential well. The highest microwave power of \sim14~MW for \sim75\-ns pulse duration was obtained at A–C gap of 7 mm having the dominant frequency in C-band at \sim6.9~GHz. The beam-to-microwave power conversion efficiency of \sim1.2\% has been demonstrated in our experiments. On the basis of power distribution pattern obtained by the gas breakdown technique, the dominant mode of emission is believed to be transverse magnetic mode. Relative analysis of frequency spectrums obtained for various A–C gap distances evidenced experimental recognition of optimum A–C spacing as a generation of narrowband distinct frequency peak of large magnitude with minimal mode hopping. [ABSTRACT FROM AUTHOR]