The Substructure Characteristic Mode Analysis for Low-Profile Cavity-Backed Slot Antennas

As a kind of useful antennas for wireless systems, low-profile cavity-backed slot antennas (LP-CBSAs) have many attractive advantages such as high gains, high front-to-back ratios, and easy planar integrations. Thus, efficient and accurate modal analysis for CBSAs is of great engineering significance. However, the conventional modal analysis method, i.e., the cavity-mode analysis, cannot be used to explore the external-resonance characteristics, and other existing modal analysis methods, such as the full-structure characteristic mode (CM) and classic aperture CM (CACM), are also inefficient for analyzing the CBSAs. In this communication, the concept of substructure is generalized to encompass not only the physical body but also an aperture acting as an interface for power flow. This generalization, along with proper electromagnetic (EM) modeling, leads to the development of a novel substructure CM formulation specific to the CBSAs. By employing this new formulation, the external-resonance characteristics can be accurately revealed. To validate effectiveness of the proposed formulation, a low-profile bandwidth-enhanced substrate-integrated-waveguide (SIW) CBSA and a low-profile circularly polarized CBSA are explored. Modal results obtained by cavity mode, full-structure CM, CACM, and substructure CM methods are compared, indicating that the proposed substructure CM formulation is more accurate and efficient for the modal analysis of CBSAs.