Context. It has been a long-standing puzzle on how to produce natural radio bursts of various cosmic objects, ranging from remote active galactic nuclei and pulsars to the nearest solar radio bursts and terrestrial auroral kilometer radiations. Aims. An electron-cyclotron maser (ECM) driven by fast electron beams trapped in magnetic fields has been suggested as a dominant mechanism of producing natural high-power radio radiation. However, there have been two serious difficulties: the

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... on condition of requiring the electron gyrofrequency over the plasma frequency and the inversion condition of the perpendicular velocity distribution of the fast electrons, which has held back the popularization of ECM in the astrophysical community. Methods. By including effects of self-generated Alfvén waves (AW) excited by the beam current, this paper proposes a novel, selfconsistent ECM model. Results. The results show that the self-generated AW can effectively make a density-depleted duct, in which the magnetization condition is easily satisfied, and result in the inversion condition of perpendicular velocity distribution of the beam electrons. Conclusions. This self-consistent ECM model can effectively overcome the two difficulties, make ECM very easily occur, and, hence, has greatly interesting implications and general significance in radio astrophysics because of its self-consistency, simplicity, and efficiency.