I. Introduction
The theory of Characteristic Modes (CMs) [1] has led to an increased understanding of the fundamental properties of designing antennas in small multiple-input multiple-output (MIMO) terminals. CM analysis computes the radiation and scattering properties of any structure through solving a generalized eigenvalue problem which exploits the method of moments (MoM) impedance matrix. When the problem is solved, multiple eigenvalues ('s) are produced; eigenvalues over frequency provide information on the excitability as well as the obtainable bandwidth of a specific eigencurrent () [2]. Eigencurrents are useful to antenna design engineers as each eigencurrent provides information on how to adapt a structure to support multiple orthogonal antennas as well as how to excite each mode efficiently. CMs provide further insight when computed across a wide frequency band, allowing for the excitation of one or more CMs at multiple frequencies using a single feed element. Computing all modes over a wide frequency band becomes challenging as eigenvalues are not generally sorted between one frequency point and the next [3]. Furthermore, modes are not always maintained across frequency points, as modes can become unstable and cease to exist, whereas other modes can appear and become stable without having any relation to previous modes [4].