Revisiting Sub-Structure Characteristic Mode Analysis

The energy-based sub-structure characteristic mode analysis (CMA) process is reviewed. From the process, the physical picture of sub-structure CMA and the physical feature of sub-structure characteristic modes (CMs) are revealed. The physical picture and feature clarify the fact that: the working of every sub-structure CM needs a driving field which acts on the active sub-structure of the objective electromagnetic (EM) system only, but does not act on the passive sub-structure of the objective EM system. This fact exposes the applicable range of sub-structure CMA: strictly speaking, the sub-structure CMA is not suitable for some EM systems, such as some aperture-fed antennas and the external-field-illuminated scatterers; the usually used microstrip-antenna-oriented sub-structure CMA is approximately correct, when the antenna is excited by probe and the probe is directly connected to the patch. Some numerical experiments are demonstrated for testing the above conclusions.