  Despite the growing interest in reconfigurable intelligent surfaces (RISs)
for millimeter-wave (mm-wave) bands, and the considerable theoretical work
reported by the communication community, there is a limited number of published
works demonstrating practical implementations and experimental results. To the
authors' knowledge, no published literature has reported experimental results
for RISs covering the n257 and n258 mm-wave bands. In this work, we propose a
novel wideband RIS design that covers the entire mm-wave 5G n257 and n258
bands. In simulations, the unit cell can maintain a phase difference of
180{\deg} +- 20{\deg} and a reflection magnitude greater than -2.8 dB within
22.7 to 30.5 GHz (29.3% bandwidth) using one-bit PIN switches. The proposed
unit cell design with four circular cutouts and long vias could realize
wideband performance by exciting two adjacent high-order resonances (2.5f and
3.5f). The periodic unit cells can maintain an angular stability of 30{\deg}.
Based on the proposed unit cell, a 20 by 20 RIS array is designed and
fabricated with a size of 7.1{\lambda} by 7.1{\lambda}. The measurement results
demonstrate that the proposed RIS could maintain a 3 dB peak gain variation
bandwidth among various array configurations within 22.5 to 29.5 GHz (26.9%)
and with a beam scanning capability of 50{\deg}, making this design a good
candidate for 5G mm-wave applications.
