High-frequency asymptotic methods for analyzing the EM scattering by open-ended waveguide cavities

An analysis of the electromagnetic (EM) scattering by electrically large open-ended cavities is of importance to radar cross section (RCS) and EM penetration problems. Four high-frequency methods are described for analyzing this problem. The first method is a hybrid combination of waveguide modal analysis and high-frequency asymptotics. This method is mostly restricted to cavities which can be built up by joining sections of uniform, separable waveguides whose modal fields are known. Elements of the scattering matrices associated with the interior discontinuities of the cavity are found accurately and efficiently using high-frequency methods. The second method is the geometrical optics (GO) ray shooting method. In this method the incident plane wave field which enters the cavity is represented as a dense grid of parallel ray-tubes which are tracked within the cavity using the laws of GO. This method is very versatile because it can handle relatively arbitrary geometries. It predicts the RCS envelope well but generally not the details of the RCS pattern. Finally, it requires one to track a large number of ray-tubes for each new incidence angle. The third method is the Gaussian beam (GB) shooting method where an array of shifted and rotated GB's in the open end are used to represent the fields coupled into the cavity. The relatively well focussed GB's are tracked inside the cavity using an axial approximation. This method gives good accuracy because the incident fields diffracted by the rim at the open end which enter the cavity are included; this is in contrast to the GO ray shooting method which neglects the latter. It is also relatively versatile and very efficient because the array of GB's needs to be tracked only once, independent of incidence angle. However, the GB method is limited to relatively shallow cavities due to beam divergence effects. The fourth method is the generalized ray expansion (GRE) method where an array of shifted and rotated diverging GO ray-tubes in the open end are used to represent the fields coupled into the cavity. The ray-tubes are tracked within the cavity using the laws of GO. The GRE is attractive because it retains all the useful properties of the GB approach mentioned above. Furthermore, it is not limited to shallow cavities because the ray-tubes can be made arbitrarily narrow, unlike the GB's; however, a larger number of ray-tubes are required.