Characteristic Mode Analysis and Metasurface- Based Suppression of Higher Order Modes of a $2\times2$ Closely Spaced Phased Array

This paper studies the design tradeoffs and modal excitation of closely spaced finite-sized ground plane backed phased arrays in the framework of characteristic mode theory, and proposes a method to suppress the higher order modes using a metasurface. The characteristic mode analysis also reveals the dominant modes and the excited modes for wideband operation. From the generalized method of moments impedance matrix and N-port network impedance matrix, key parameters are derived for the analysis, such as modal currents, modal radiation patterns, modal significance, modal weighting, modal radiated power, and modal near fields. The analysis includes the broadside and beamsteering radiation of closely spaced bowtie array antennas. The simulated and measured results show that it is essential for the beamsteering to have multiple modes excited simultaneously. By suppressing the higher order modes using the metasurface, the aperture efficiency and the realized gain can be improved for beamsteering cases. This analysis can be extended to a larger array and used to identify array impedance bandwidth performance in scan cases.