Characteristic mode-assisted design of a broadband barbell-shaped reflective metasurface for linear cross-polarization conversion across the K-, Ka-, and V-Bands

Broadband linear cross-polarization conversion remains difficult to achieve with conventional single-layer reflective metasurfaces at millimeter-wave frequencies, primarily due to limited resonance coupling, narrow bandwidth, and limited polarization purity. In this work, a compact, barbell-shaped, anisotropic reflective metasurface is presented to overcome these constraints and achieve ultra-wideband cross-polarization conversion. The unit cell produces three principal resonances at 19.70 GHz, 31.80 GHz, and 44.30 GHz; their combined response yields continuous polarization conversion from 18.58 GHz to 46.90 GHz. Characteristic Mode Analysis (CMA) is used to clarify modal behavior and link successive geometric modifications to the observed broadband performance. Simulation results show that the structure maintains a polarization conversion ratio (PCR) above 97% across the operating band, corresponding to a fractional bandwidth of 86.5%. Within this range, the co-polarized reflection remains below −15 dB, while the cross-polarized component remains close to 0 dB, confirming efficient linear cross-polarization conversion and strong polarization selectivity. The proposed grounded metasurface employs an electrically compact 4 mm × 4 mm unit cell and maintains satisfactory polarization-conversion performance for oblique incidence angles < 20° under both TE and TM incidence. A 120 mm × 80 mm prototype was fabricated and characterized using a free-space measurement setup. The measured response reproduces the overall trends of the simulated reflection magnitude, PCR, and reflection phase over the experimentally accessible frequency range, with deviations attributed to fabrication and measurement uncertainties. The proposed metasurface therefore provides a compact, broadband platform for linear-polarization manipulation, with potential applications in polarization-rotating reflectors, radar systems, advanced antenna systems, and millimeter-wave wireless communication.