  The current-voltage and spectral characteristics of a flux flow oscillator
(FFO), based on a long Josephson junction, are studied. The investigations are
performed in the range of small bias currents and magnetic fields, where the
FFO radiates a quasi-chaotic signal with extremely large radiation linewidth,
and the displaced linear slope (DLS) is observed at the current-voltage
characteristic. By direct numerical simulation of the sine-Gordon equation it
is demonstrated that for large lengths of the Josephson junction or in the case
of finite matching of the FFO with external waveguide system, the DLS with
extremely large linewidth is transformed into Fiske steps with very narrow
linewidth. While there is the common belief that the chaotic regime of the FFO
is due to excitation of the internal oscillation modes in the "soft" fluxon
chain, it is demonstrated that this regime is inspired by multiple reflections
of the traveling waves from junction ends.
