Theoretical model for the radiation linewidth in a multi-fluxon state of a long Josephson junction is presented.
Starting from the perturbed sine-Gordon model with the temperature dependent noise term, we use a collective
coordinate approach in order to calculate the finite radiation linewidth due to the internal degrees of freedom
in the moving fluxon chain. At low fluxon density, the radiation linewidth is expected to be substantially
larger than that of a lumped Josephson oscillator. With increasing the fluxon density, a crossover to a much
smaller linewidth approaching the lumped oscillator limit is predicted.