  A method to derive features of modal eigenvalue traces from known and
understood solutions is proposed. It utilizes the concept of subduction from
point group theory to obtain the symmetry properties of a target structure from
those of a structure with a higher order of symmetry. This is applied exemplary
to the analytically known characteristic modes (CMs) of the spherical shell.
The eigenvalue behavior of a cube in free space and a cuboid on a perfectly
electrically conducting plane are continuously derived from this. In this
process, formerly crossing eigenvalue traces are found to split up, forming a
split trace crossing avoidance (STCA). This finding is used to explain
indentations in eigenvalue traces observed for three-dimensional structures,
that are of increasing interest in recent literature. The utility of this
knowledge is exemplified through a demonstrator antenna design. The dimensions
of the antenna structure are chosen so the STCA is outside the target frequency
range, avoiding negative impacts on input matching and the frequency stability
of the far field patterns.

Signal Processing