We have studied the effect of an applied rf signal on the radiation emitted from a large-area Josephson junction by means of a model based on the sine-Gordon equation. The rms value of the voltage of the emitted signal has been calculated and a hysteresis loop found. An analysis shows that the hysteresis is due to the nonlinearity in the system, i.e., the dynamics of the lower branch can be described by a solution to the linearized system while the upper branch is described by a breather mode. These solutions are frequency locked to the driving signal. Various characteristics of the loop such as threshold value and level of the branch are predicted analytically.