A 2-bit Tunable Unit Cell for Quad-Band Reconfigurable Intelligent Surface

This paper presents a reconfigurable intelligent surface (RIS) unit cell that can be tuned across the multiple frequency bands with low absorption. At the top surface of a low-loss substrate, a square patch with four U-shaped metallic patches is placed inside a square metal loop. Two varactor diodes are integrated to connect the square loop to U-shaped patches. The bottom surface is covered by a metal sheet. The unit cell has two vias inserted through the substrate from the top to the bottom to bias the diodes. Different phase shifts of the reflection coefficient have been generated by varying the biasing voltages. The proposed design operates at four frequency bands centered around 3.50 GHz, 5.10 GHz, 5.85 GHz, and 8.40 GHz. The simulation results determine the system's ability to span 360° of phase with 90° increments and analyze its polarization sensitivity and angular stability for different angles across all the operating bands.