  This paper presents a novel approach for computing substructure
characteristic modes. This method leverages electromagnetic scattering matrices
and spherical wave expansion to directly decompose electromagnetic fields.
Unlike conventional methods that rely on the impedance matrix generated by the
method of moments (MoM), our technique simplifies the problem into a
small-scale ordinary eigenvalue problem, improving numerical dynamics and
computational efficiency. We have developed analytical substructure
characteristic mode solutions for a scenario involving two spheres, which can
serve as benchmarks for evaluating other numerical solvers. A key advantage of
our method is its independence from specific MoM frameworks, allowing for the
use of various numerical methods. This flexibility paves the way for
substructure characteristic mode decomposition to become a universal
frequency-domain technique.
