  Nonreciprocal microwave transmission through a long Josephson junction in the
flux-flow regime is studied analytically and numerically within the framework
of the perturbed sine-Gordon model. We demonstrate that the maximum attenuation
of the transmitted power occurs when the direction of the flux flow is opposite
to the direction of the microwave propagation. This attenuation is
nonreciprocal with respect to the flux-flow direction and can be enhanced by
increasing the system length and proper impedance matching of the junction ends
to external transmission line.
