A Low-Cost 1-Bit PIN-Diode Reconfigurable Intelligent Surface for the 5G NR n78 Band: Unit-Cell Design, Column-Biased Fabrication, and Anechoic-Chamber Characterization

Abstract

This paper reports a 10×10 1-bit reconfigurable intelligent surface (RIS) for the 5G NR n78 band, built for under USD 250 and used as a measurement-validated test vehicle: every as-built non-ideality is carried through the analysis, and every measured quantity is compared with simulation under a stated uncertainty. The H-slot patch unit cell switches with a USD 0.027 general-purpose PIN diode on an unbonded two-board Rogers RO4350B stack, with the bias network on the top copper and the interface air gap in the full-wave model; simulation gives ≈202° of reflection-phase contrast at 3.5 GHz (≈232° with the air gap), above 150° over 8.5% bandwidth. Column-wise control reduces the 100-element panel to ten bias lines. A co-rotating bistatic S21 measurement with time-domain gating, validated by three independent checks, shows the half/half coding forming a diffraction-limited lobe at +16° to +18° with 18.5° half-power beamwidth, 8.5–10 dB above the suppressed specular direction, and mirrored by coding reversal. The lobe-to-specular ratio bounds the effective two-state contrast to within ≈30° of 180°, consistent with simulation. The one systematic deviation, an ≈80 MHz downshift of the band center, is traced to the unbiased OFF state of the sink-only driver.