Determination of the natural frequencies for conducting rectangular boxes

When the problem of the transient electromagnetic scattering from an object is researched by using the singularity expansion method (SEM), the natural frequencies (poles in complex s-plane) of the object are a very important parameter. It is very difficult to calculate the natural frequencies of three-dimensional scatterers. The natural frequencies of conducting rectangular boxes are determined originally and verified by other works. The degeneracy phenomenon of the poles due to the geometrical symmetry of the rectangular boxes and the distribution property of the pole in the complex s-plane are discussed