Miniaturized ultra-thin, ultra-wideband metasurface absorber with high angular stability for RF energy harvesting

This paper presents a cost-effective, ultra-wideband (UWB) high angularly stable ultra-thin metasurface microwave absorber (MMA) for radio-frequency energy harvesting, under both linearly and circularly polarized incident waves. It is able to display absorptivity surpassing 90% from 3.37 to 11.74 GHz, with exceptional relative bandwidth of 110.78% regardless of the wave polarization due to its 90° rotation symmetric design. To attain UWB absorption, four lumped resistors loaded meandered ring encompassing a square patch is printed on a 0.2 mm (0.002λ_L at 3.37 GHz) thin flexible FR4 substrate, having copper-backed air-spacer underneath it. The miniaturized absorber (0.157λ_L×0.157λ_L) offers absorptivity in excess of 85% up to an oblique incidence angle of 50° irrespective of transverse electric and transverse magnetic wave exposures. The working mechanism of the structure has been explained with the help of parametric optimization, electric field, and surface current distribution plots. An in-house equivalent circuit model incorporating transmission line topology has been devised and subsequently analyzed to emulate the full-wave simulation results. UWB absorption and conformal behavior have been validated through experimental measurements. Owing to these unique features, the proposed MMA is an intriguing choice toward applications including radio-frequency (RF) energy harvesting, electromagnetic interference shielding, radar cross-sectional reduction, defense stealth technology, and sensing. Lastly, the capability of the intended MMA as an RF energy harvester has been investigated, and it is found that suggested MMA demonstrates harvesting efficiency superior to 90% across an unprecedented 3.50-11.42 GHz frequency range.