In this paper, we propose an ultrathin and ultra-wideband metamaterial absorber based on periodically-arranged metallic square spirals. The design, characterization, and measurement of the proposal are presented as its equivalent circuit. The lumped elements of the equivalent circuit are extracted using a proposed algorithm based on the least-square method, which presents a straightforward and promising approach and shows a good matching with the electromagnetic simulation. The unit cell of the proposed structure has the square spiral mounted on an FR-4 substrate in front of a conductive plate. The simulated results show an absorptivity of more than 90% from 11.4 to 20.0 GHz, covering the Ku band for TM as well as TE polarization, and this broadband feature is confirmed by the experimental measurement. Furthermore, the proposed metamaterial absorber is λ/16:4 thick at the lowest frequency of absorption. The proposed metamaterial has proper response under oblique incidence from 0° to 50° and shows great performance with regard to the absorbers previously presented in the literature.

An ultra-thin and ultra-wideband anisotropic metasurface, acting as a linear polarization converter through reflection is designed and simulated in this research. The operation frequency range covers almost completely the Ku-band and the entire K- and Ka-band. The unit cell of the proposed metasurface is composed of a arrangement of concentric square metallic loops on a grounded dielectric substrate, in which a diagonal split is made, providing properties of an anisotropic material. Simulation results show that an efficient cross-polarization conversion, both for normal as well as for oblique incidence, is achieved with $-10$ dB fractional bandwidth of 98.7% from 13.8 GHz to 40.7 GHz and with an average polarization conversion ratio of 97.5%. The ultra-wideband of the proposed structure results from multiple plasmonic resonances occurring at three neighboring frequencies (15.2 GHz, 25.9 GHz, and 38 GHz) resulting in a significant expansion of the operating frequency range.

The objective of this paper is to explore the potential of integrating three distinct functionalities into a thin, single-layer metasurface. Specifically, the study introduces a metasurface design that combines absorption, polarization conversion, and transmission capabilities. The proposed structure consists of a double square loop disposed on a dielectric substrate, which is covered by a superstrate. In this study, the traditional ground plane was replaced with a periodic array, selectively reflecting frequencies of interest. Then, the absorption and polarization conversion characteristics were achieved by introducing the resonators in the front layer. By introducing asymmetry to the resonators and integrating PIN diodes for control, we demonstrated that the metasurface could efficiently absorb electromagnetic waves (with PIN diodes in the ON state), convert polarization (with PIN diodes in the OFF state), and enable signal transmission in a different frequency range. The numerical results indicated excellent performance in both absorption and polarization conversion. At a frequency of 3.05 GHz, the absorption rate reached 97%, while a polarization conversion rate of 98% was achieved at the resonance frequency of 4.37 GHz. Moreover, the proposed structure exhibited a thickness of λ/30.7 at the absorption peak.

In this paper we proposed a novel tri-band complementary frequency selective surface (CFSS) with a traditional geometry composed of double concentric rings. The choice of this geometry is because of its very good angular stability. The proposed resonant frequencies were 2.5 GHz, 3.5 GHz, and 4.5 GHz. The CFSS is low profile and presents angular stability and polarization independence. Simulations were performed using HFSS software and they are compared with the experimental results for validation purposes. A very good agreement is observed between them.