Square-loop Frequency Selective Surface Research Articles

60 GHz millimeter-wave indoor propagation path loss models for modified indoor environments

The 60 GHz band has been selected for short-range communication systems to meet consumers’ needs for high data rates. However, this frequency is attenuated by obstacles. This study addresses the limitations of the 60 GHz band by modifying indoor environments with square loop (SL) frequency selective surfaces (FSSs) wallpaper, thereby increasing its utilization. The SL FSS wallpaper response at a 61.5 GHz frequency has been analyzed using both MATLAB and CST Studio Suite software. ‘Wireless InSite’ is also used to demonstrate enhanced wave propagation in a building modified with SL FSSs wallpaper. The demonstration is applied to multiple input multiple output system to verify the effectiveness of FSSs on such systems’ capacity, as well as the effect of the human body on capacity. Simulation results presented here show that modifying a building using SL FSS wallpaper is an attractive scheme for significantly improving the indoor 60 GHz wireless communications band. This paper also presents and compares two large-scale indoor propagation path loss models, the close-in (CI) free space reference distance model and the floating intercept (FI) model. Data obtained from ‘Wireless InSite’ over distances ranging from 4 to 14.31 m is analyzed. Results show that the CI model provides good estimation and exhibits stable behavior over frequencies and distances, with a solid physical basis and less computational complexity when compared to the FI model.

Investigations on relevance of angular stability and polarization independence of frequency selective reflector in planar antenna gain enhancement

The relevance of angular stability (AS) and polarization independence (PI) is investigated by rotating the frequency selective reflectors (FSRs) kept behind a 10 GHz triangular slot antenna in orthogonal directions. Three distinct frequency selective surfaces (FSSs) based on closely packed rhombic loop (RL), square loop (SL) and merged dual rhombic loops (MDRL) are developed for the analysis under normal and oblique incidences. The gain enhancement of 4.87, 7.17 and 4.85 dB is achieved when the antenna is backed by 5 × 5 arrays of RL, MDRL and SL FSS, respectively. Later, with equal sized substrates, the gain enhancement of 7.25, 7.17 and 8.10 dB is achieved when the antenna is backed by 61 × 61 mm2 RL, MDRL and SL FSSs, respectively. It appears that stringent characteristics of FSSs i.e. AS and PI can be compromised for FSRs while employing antenna gain enhancement techniques.

Microwave Absorbing Properties of Carbonyl Iron Particle Composites with Frequency Selective Surface

With the increased use of diverse electronics in the high frequency spectrum, high-performance absorbing and shielding materials are needed to ensure electromagnetic compatibility. Microwave absorbance of polymer composites of carbonyl iron powders with frequency selective surface (FSS) is investigated, focusing on the frequency of 5.8 GHz. Composite sheets were prepared by hot pressing a mixture of carbonyl iron powders and silicone rubber. FSS with a square-loop geometry and surface resistance close to the optimum value was fabricated by a screen printing method using carbon black paste as conductive ink. The microwave absorbance for both normal and oblique incidence angles was determined using the commercial computational tool (ANSYS-HFSS). The particulate composites show a reflection loss of -21 dB at 5.8 GHz, with a small thickness of 2.5 mm. Further enhancement of microwave absorbance (less than -40 dB reflection loss) can be realized by attaching a resistive square-loop FSS on the grounded magnetic composite, which is attributed to the enhanced impedance matching driven by the additional resistive component of the attached FSS. The experimental results, obtained with a test sample of optimized composite structure with FSS, are in good agreement with the simulation results. In addition, it was found that the angular stability of the carbonyl iron absorber improved by attachment of FSS, particularly for TM polarization. Key words: carbonyl iron, microwave absorbers, frequency selective surfaces

Design of ultra wide‐bandwidth double‐layer electromagnetic wave absorbers with square‐loop frequency selective surfaces

AbstractA thin and ultra wide‐bandwidth microwave absorber is demonstrated theoretically and experimentally through simple layering 2 square loop‐frequency selective surfaces (SL‐FSSs) with different geometries on a grounded dielectric substrate. The circuit parameters of the SL‐FSS inductance (L) and capacitance (C) are retrieved using the equivalent circuit model and are utilized in the design of the wide bandwidth absorber. The L and C values increase with increases in the unit cell dimension, and thereby the absorption frequency band can be controlled by the SL‐FSS geometries. The optimal control for the surface resistance of the SL‐FSSs and the spacer (assumed by air) thickness of the double‐layer absorber provide a 10 dB absorption bandwidth of 6.3–38.3 GHz with a total thickness of 6.4 mm. The free‐space measurement with the test samples fabricated by the screen printing process provides a still wide bandwidth of 5.6–30.8 GHz with the same total thickness.

Bandwidth for the Equivalent Circuit Model in Square-Loop Frequency Selective Surfaces

This paper presents the results of a study, implemented for the equivalent circuit model (ECM), concerning the bandwidth in square-loop frequency-selective surfaces (FSSs). For the square-loop ECM, there is no formulation to quantify the effects of dielectric supporting layers, geometric features, and incident wave properties on the FSS bandwidth. Hence, it is not possible to control or evaluate the bandwidth of the FSS in the design from the knowledge of these parameters. In this paper, we addressed this issue using the ECM as an analysis method. In the first part of this study, we considered the square-loop FSS with thin dielectric supporting layers, where the bandwidth was expressed as a function of geometric parameters, incidence angle, and polarization of the incident wave as well as a factor that takes into account the effective permittivity of the dielectric support. Particle swarm optimization method was used to synthesize square-loop FSSs via the ECM with a specific resonant frequency and bandwidth requirements. In the second part, the behavior of bandwidth as a function of dielectric thickness was obtained by using a matrix cascading technique (MCT), implemented with ECM and a model for the dielectric effective permittivity. This approach takes into account the modifications on bandwidth due to multiple reflections on dielectric boundaries and allows calculating its value for any dielectric thickness and configuration. It is remarkable the low computational cost and acceptable accuracy obtained with the proposed approaches. All simulation results using ECM and MCT were compared with Ansys-HFSS commercial software simulations.

Miniaturization of Frequency Selective Surfaces Using 2.5-D Knitted Structures: Design and Synthesis

This paper explores the potential of using vias for the miniaturization of frequency selective surfaces (FSSs). A new concept of knitting the loop-type FSS elements in 2.5-D is proposed, where successive segments of the loop are placed alternately on the two surfaces of the substrate and then interconnected through vias. A 2.5-D square-loop FSS (2.5-D SL-FSS) based on the proposed method is designed with the inclusion of ten vias at each side and characterized by a full-wave simulator. The transmission curves indicate a significant size reduction with a figure-of-merit $\lambda _{0}/p = 16$ , where $\lambda _{0}$ is the free-space wavelength of resonant frequency and $p$ is the periodicity of unit element. In addition, the frequency response of this miniaturized FSS is also stable for various incident angles and polarizations. Furthermore, a general equivalent circuit model (ECM) is developed for 2.5-D SL-FSS by combining the prevailing electrical models of planar square loop and through-silicon vias. A wide set of parametric simulations for various element sizes, substrate thicknesses, and via counts are carried out with this ECM. Then, its performance is assessed on the basis of root-mean-square error (RMSE) criteria by comparing the results with appropriate electromagnetic simulations. The findings suggest that the ECM has sufficient accuracy for estimating the resonant frequency of 2.5-D SL-FSS with the RMSE values close to 3%. Moreover, the proposed concept of knitting is further validated by measuring two physical prototypes of the 2.5-D SL-FSS and the experimental results show a good consistency with full-wave simulations.

Broadband Microwave Absorption Properties of Ultrathin Composites Containing Edge-Split Square-Loop FSS Embedded in Magnetic Sheets

A convenient fabrication process about the ultrathin broadband composite microwave absorption material (MAM), which involves a frequency selective surface (FSS) sandwiched between two different magnetic sheets, is presented in this letter. The FSS consists of an array of elements with the geometry of square-loop having a split in each edge (denoted as edge-split square-loop FSS). The top and bottom magnetic sheets were prepared based on silicon rubber filled with sphere-shaped and flake-shaped carbonyl iron powders, respectively. It is revealed by measurement and simulation results that the composite MAM with the FSS that induces multiple resonances has a broad operating bandwidth compared to the traditional magnetic MAM without FSS. The low-frequency resonance can be manipulated by adjusting the size of the split $S$ , while the high-frequency resonance is generally invariable. The composite MAM with the thickness of 2.2 mm has an operating bandwidth with the reflectivity below −10 dB in the frequency range of 4–18 GHz, which is nearly 2.7 times the bandwidth of the traditional magnetic MAM.

Particle swarm optimization for the design of square loop frequency selective surfaces considering a model of dielectric effective permittivity

Purpose The purpose of this paper is to present the results of a particle swarm optimization (PSO) method applied in the design of a square-loop frequency selective surface (FSS) via the equivalent circuit model (ECM), considering the dielectric effective permittivity as a variable in the optimization problem. Design/methodology/approach In the optimization process considered, besides the FSS square loop geometric parameters, the thickness and relative permittivity of dielectric material used as support are included as variables in the search space, using for this a model of dielectric effective permittivity introduced by the authors in a previous work. Findings Square loops were designed and the obtained results were compared with designs reported in literature for applications in wireless local area network and long-term evolution 4G systems. The low computational cost is remarkable as well as the acceptable accuracy obtained with the proposed approach. The PSO method results were implemented with the ECM and compared with those obtained via Ansys – high frequency structure simulator commercial software simulations. Originality/value The lack of a model of dielectric effective permittivity for the ECM causes a restricted search space in the stochastic FSS design process limited to only geometric parameters, as it is reported in the available literature. The proposed approach simplifies and makes more flexible the design process, and allows guiding the FSS design to unit cell surface and/or dielectric thickness of small dimensions.

Design of wide-bandwidth electromagnetic wave absorbers using the inductance and capacitance of a square loop-frequency selective surface calculated from an equivalent circuit model

The design of wide-bandwidth microwave absorbers is conducted using a square loop-frequency selective surface (SL-FSS) on the surface of the grounded dielectric substrate. The parallel circuit combination of the input impedance of the grounded substrate and the complex impedance of the SL-FSS leads to impedance matching in a broad frequency range. The inductance (L) and capacitance (C) of the SL-FSS is calculated using the equivalent circuit model, which is dependent on the SL-FSS geometry. For the SL-FSS, the inductance and capacitance are calculated from the equations of reactance and susceptance at the resonance frequency (f0) of the equivalent L–C circuit. The circuit is capacitive below f0 and inductive above f0. For a grounded substrate with a quarter wavelength thickness, however, the input impedance is inductive at lower frequencies and capacitive at higher frequencies. Through combining these two impedances, impedance matching can be derived over a wide frequency range with the controlled FSS resistance matched to the free-space impedance. The optimized surface resistance of the FSS conductor is Rs=26Ω for the widest bandwidth (4.9–16.4GHz with respect to −10dB reflection loss), which is consistent with the simulation results obtained via computational tool.

Numerical Model of the Effective Permittivity for Square-Loop Frequency Selective Surfaces

This paper presents a model, obtained through numerical simulation and curve fitting, for the dielectric effective permittivity of frequency selective surfaces (FSSs). The numerical model is based on the permittivity and thickness of the dielectric layer supporting the FSS metallic elements, as well as on the period and on a factor dependent on the FSS geometry. The model is developed for square-loop FSS, using a given geometry reported in literature as a reference case. The results from electromagnetic simulations with the finite-element method are used for developing the numerical model.

Dual-band frequency selective surface with large band separation and stable performance

A new technique of designing a dual-band frequency selective surface with large band separation is presented. This technique is based on a delicately designed topology of L- and Ku-band microwave filters. The two band-pass responses are generated by a capacitively-loaded square-loop frequency selective surface and an aperture-coupled frequency selective surface, respectively. A Faraday cage is located between the two frequency selective surface structures to eliminate undesired couplings. Based on this technique, a dual-band frequency selective surface with large band separation is designed, which possesses large band separation, high selectivity, and stable performance under various incident angles and different polarizations.

The Design and Performance of a Novel Square-Loop Patch Frequency Selective Surface

A novel stop-band Frequency selective surface (FSS) is presented. By adding four narrow branches to conventional square-loop FSS, the proposed FSS can achieve a good frequency stability for different oblique angles and a high frequency selective ability. An equivalent circuit model is used to simply analyze its transmission property. The good agreement between simulated and measured results shows the validation of our design.

ANALYSIS AND DESIGN OF DUAL BAND HIGH DIRECTIVE EBG RESONATOR ANTENNA USING SQUARE LOOP FSS AS SUPERSTRATE LAYER

A superstrate layer is used to enhance the directivity of the small radiation sources such as Microstrip Patch Antennas. In this paper, we use square loop frequency selective surface (SLFSS) configuration to design the superstrate layer. To compact the structure, we propose a new single layer square loop FSS configuration that operates in two frequency bands such as multi-layer SL-FSS. Simulation results are shown to have good agreement with experimental results.