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This paper presents an overview of a coaxial free-electron maser (FEM) based on 2D and 1D distributed feedback, driven by a large diameter (70 mm) annular electron beam. Most recent experimental results and numerical simulations are presented and discussed. Particle in cell (PiC) code MAGIC is used to demonstrate the possibility to control the operating mode and frequency of the output radiation. From numerical simulation, it is possible to tune the FEM output in the frequency range associated with the band gap of the input mirror. FEM generated ~60 MW, 150 ns duration millimeter wave pulses, containing ~12 J of energy in the pulse, when operated in 35.9-38.9 GHz frequency region.