Scattering by over-moded, varying cross-section waveguide

An over-moded trapezoid-to-octagon waveguide transition was analyzed using the coupled mode analysis. The first five TE modes were used, and the first two TE-mode eigenpotentials at an intermediate location in the transition are shown. If the transition is matched, the mode reflections are the interior contributors to the RCS (radar cross section) and they determine the minimum RCS for the transition. In order to determine whether the intermodal coupling is significant, the mean RCS and mean RCS+or-one standard deviation were calculated for the cases of no intermodal coupling, for which only the diagonal elements of S(1,1:i,p) contribute, and all elements of S(1,1:i,p) contributing. The mean RCS for the case of intermode coupling is approximately 5 dB greater than for the case of no intermode coupling, which represents a 33% increase in radar detection range for the matched transition. Thus, intermodal coupling can be a significant contributor to the transition RCS.<>