Modal propagation algorithm for RCS prediction of inlet ducts

A modal propagation algorithm (MPA) for the computation of backscattering from arbitrarily shaped ducts in the intermediate frequency ranges is presented. The fields are propagated from the aperture toward the termination one small step at a time while ignoring reflections. Mode conversion from one section to the next is taken into account by a model conversion matrix which converts the modes in one section into the modes in the next section. For 3-D arbitrary geometries, the propagating modes in each section are found through solving a 2-D eigenvalue problem with the aid of the finite difference method. The same modal propagation steps are repeated to propagate the fields from the termination out to the aperture opening. The radar cross section (RCS) of the duct can then be found through the standard radiation integral. A comparison between the results generated by the present modal propagation algorithm and a reference hybrid ray-mode solution for a 2-D S-shaped duct is shown. Very good agreement is observed.<>