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Without requiring an externally applied magnetic field, the triple-beam TM040, 6.7-GHz monotron devised in this paper includes a simple structure where the elementary functions of electron bunching and energy transfer to the RF field are performed in a single circular cylindrical cavity, which makes such a microwave generator a compact lightweight device. The cavity geometry (radius 8.42 cm, length 1.0 cm) and injection beam energy (10 keV) are determined on the basis of a one-dimensional (1-D) Lagrangian treatment neglecting space-charge forces. A 2 1/2-D particle-in-cell simulation is carried out for 0.4-cm-thick beams at currents in proportion to the beams' cross section. This gives 560-kW output power at an overall tube efficiency of 14.8% in close agreement with 1-D theory.