Ultra-Miniaturized Conformal Polarization Insensitive and Incident Angle Stable FSS for n257 Band 5G EMI Shielding Applications

This article proposes a thin and flexible stopband frequency selective surface (FSS) for 5G n257 band electromagnetic interference (EMI) shielding. The unit cell of the FSS consists of a fractal square ring resonator and an interconnecting fractal cross resonator. The size of the unit cell is only 1.9×1.9 mm
²
, and the periodicity is 0.154λ
_o
where λ
_o
is the lower stopband frequency. The reformed geometry of the proposed FSS provides a high selective stopband filtering response that reflects the signal at 27.84 GHz with S
₂₁
< -10 dB stopband bandwidth (BW) percentage of 21.49%. The rotation symmetrical geometry of the designed FSS demonstrates unique stopband features for TM and TE polarization modes. The ultra-miniaturized unit cell is completed by fractal technology. It leads to high stopband stability with wide angular incident angle (AIA) up to 60° and 80° at TM and TE polarization modes, respectively. Moreover, the low periodicity of the FSS significantly reduces the peak transmission zero (TZ) frequency shifting at high AIA, which is only 0.21%. The peak TZ is tunable by adding an additional substrate layer with the FSS. Good agreement is accomplished between the simulation, equivalent circuit, and measurement results. Finally, as a conformal structure, the proposed FSS has the merits of EMI shielding on a curved surface at 5G n257 band applications.