Varactor-Based Reconfigurable Intelligent Surface for Continuous Phase Control at 28 GHz

This work presents a varactor diode-based reconfigurable intelligent surface (RIS) design for high-precision beam steering near 28 GHz. Two rectangular patches bridged by a varactor diode form the proposed unit cell and also reduce the biasing complexity. In an infinite periodic environment, without considering varactor loss, the unit cell exhibits a reflection phase coverage of 326° and maintains the reflection loss within -1.03 dB at 28 GHz. Furthermore, the unit cell's performance, including the parasitics of the varactor diode, is verified using a waveguide-based measurement technique, providing an accurate evaluation of its practical performance. A <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$10 \times 10$</tex> unit-cell RIS is constructed using the experimentally validated unit-cell results. Bistatic radar cross section (RCS) results show the precise beam steering capability of the proposed RIS for both normal and oblique incidences of plane waves while maintaining a sidelobe level better than -12.5 dB.