We describe a simple dynamical model for an underdamped Josephson junction array coupled to a resonant cavity. From numerical solutions of the model in one dimension, we find that (i) current-voltage characteristics of the array have self-induced resonant steps (SIRS), (ii) at fixed disorder and coupling strength, the array locks into a coherent, periodic state above a critical number of active Josephson junctions, and (iii) when N_a active junctions are synchronized on an SIRS, the energy

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... ed into the resonant cavity is quadratic with N_a. All three features are in agreement with a recent experiment [Barbara et al, Phys. Rev. Lett. 82, 1963 (1999)].