Characteristic-mode analysis (CMA) enables a systematic approach to antenna design and antenna placement. The approach is based on insight in the fundamental resonance characteristics of antenna geometries and of the structures on which they are mounted. This insight aids in choosing the locations of excitations on an antenna and of the antennas on a platform. Furthermore, knowledge of the coupling between excitations and modes enables a design engineer to synthesize the desired antenna pattern by exciting a linear combination of modal patterns. This is a deterministic approach, which is based on insight in physics. It contrasts with an approach in which an optimization routine is used to explore a large many-dimensional design space with few constraints. This paper offers a refresher on the theory of characteristic modes and presents practical examples that all use CMA in the design process and in the interpretation of results.

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An efficient method-of-moments (MoM) based domain decomposition technique, viz., the domain Green's function method (DGFM), is presented for analyzing large antenna arrays. The DGFM is a perturbation technique where mutual coupling between array elements is accounted for during the formulation of an active impedance matrix for each domain/array element. The active current distribution on the entire array geometry is obtained by solving the smaller matrix equations related to the elements, and not that of the problem as a whole. This leads to a significant saving in both runtime and memory usage. The method also takes into account the edge effects attributed to the finite size of the array, complex excitations with nonlinear phase shift and is not limited to periodic array configurations. The DGFM is an approximation and assumes a slowly varying current distribution between domains. A novel way to mitigate the aforementioned, by including secondary coupling effects, is also discussed. Furthermore, an efficient active impedance matrix fill strategy is presented where the active impedance matrix summation is truncated to include only a certain number of terms. Parallelization using both distributed and shared memory programming models have also been applied to the DGFM, to further optimize runtime and memory usage. Index Terms-Domain decomposition, finite antenna arrays, method-of-moments (MoM), non-periodic arrays. I. INTRODUCTION A NALYZING large, finite, irregular-spaced antenna arrays is of interest to various research groups. One such an application for which nonuniform spaced array configurations is specifically well-suited, are for high-sensitivity imaging in the field of radio astronomy-an example being the aperture Manuscript

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