Single-Layer Dual-Resonance Wideband Electrical Reconfigurable Intelligent Surface Element Design Using Characteristic Mode Analysis

The design of electrically reconfigurable intelligent surfaces (RISs) with zero-quantization loss is challenging, as each element requires a tunable phase shift exceeding 360<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^{\circ }$</tex-math></inline-formula> with a certain degree of linearity. Dual-resonance is an effective approach to extend the phase shift range. In this paper, a single-layer dual-resonance wideband RIS element is designed with characteristic mode analysis (CMA). By quantifying the resonance characteristics, analyzing modal currents and far-field radiation patterns, dual-resonance characteristics of the element can be constructed easily, and the linearity of the phase shift curve can be efficiently optimized. The proposed single-layer dual-resonance element only has two varactor diodes. Furthermore, it can achieve over 360<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^{\circ }$</tex-math></inline-formula> tunable phase shift from 6.4 to 7.5 GHz with good linearity. A PCB prototype is fabricated and tested using the waveguide method. The experimental results validate the usability of the proposed element.