Loop Frequency Selective Surface Research Articles

A high Q factor sensor to measure the complex permittivity and effective permittivity based on a band stop frequency-selective surface

In this paper, a metamaterial sensor is proposed based on a band stop frequency-selective surface (FSS) with high Q factor of 421.9 at 8.032 GHz. The sensor consists of a square loop FSS and a F4B dielectric substrate. This is the first sensor based on a band stop FSS sensing the effective permittivity of 3D printed hollow structures. The equivalent circuit method is used to analyze the characteristics of the sensor, and the results agree well. Two materials under test and four different hole radii of 3D printed hollow structures are used to verify the sensing function. At the same time, the effective permittivity of the 3D printed hollow structure is analyzed, and the correctness of the effectiveness is verified by simulation inversion and Maxwell-Garnett formula. The sensor has the advantages of high Q factor, wide incidence angle and polarization angle insensitivity, which can provide some guidance in metamaterials sensing.

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.

Wave Impedance Equivalent Circuit Model for Square Loop Frequency Selective Surfaces

Wave Impedance Equivalent Circuit Model for Square Loop Frequency Selective Surfaces

3D-printed dual-band energy harvester for WSNs in green IoT applications

This paper describes the 3D-printed open Double Square Loop (DSL) Frequency Selective Surfaces (FSSs) based Radio Frequency energy harvester specifically designed for the purpose of capturing Wi-Fi ambient RF energy. The proposed dual-band energy harvesting circuit is a suitable applicant for low-power green IoT Wireless Sensor Networks. The inclusion of FSS in the receiver improved the antenna’s gain by 42% and 74% at 2.4 GHz and 5.2 GHz, respectively, and consequently, enhanced the conversion efficiency. Due to FSS-backed dual-band antenna, the proposed configuration gathers a significant amount of RF ambient energy from the surroundings. The collected RF energy is then converted into DC voltage using a three-stage Villard RF rectifier cum multiplier. The Fused Deposition Modeling (FDM) of 3D printing is employed to create the designed prototype on a biodegradable Polylactic Acid substrate. RF measurements are taken with horn antenna and signal generator to evaluate the proposed design. At 2.4 GHz, the constructed prototype has a conversion efficiency of 67.90% and a corresponding output voltage of 2.36 V, whereas, at 5.2 GHz, it achieves an efficiency of 34.01% with output voltage of 1.67 V, at 0 dBm input power. The proposed configuration has a good correlation between simulated and measured outcomes.

Power Handling of Slot Loop Frequency Selective Surface Based on Approximate Analytical Method

Power Handling of Slot Loop Frequency Selective Surface Based on Approximate Analytical Method

Double Sided Wideband Concentric Ring Spatial Filters for Mid-Band 5G Devices

This paper presents the methodology, development, and implementation of the double concentric ring band-pass frequency selective surface (FSS) based spatial filters. The filters are designed to improve the active responses of the 5G antennas by incorporating them as superstrate or as substrates. A comparable circuit model is used to anticipate the resonant frequency bands required for predicting the FSS band-pass behavior. The designed spatial filter is composed of a combination of periodic circular loop structures engraved on both sides of a thin dielectric substrate. The proposed design of a double circular loop FSS exhibits band-pass characteristics for n77, n78, and n79 bands of the mid-band 5G spectrum (ranging from 3 GHz - 6 GHz) and provides insensitivity to polarization angles of the incident plane wave. The computed, simulated, and measured findings are all depicting a similar pattern that validates our structure.

Bifunctional Full-Space Metasurface Combining Pancharatnam–Berry and Propagation Phases

A bifunctional full-space metasurface (FS-MS) capable of generating reflective multiple vortex beams and focusing transmissive electromagnetic waves within different frequency bands is presented in this letter. The generation of vortex beams is based on the Pancharatnam–Berry (PB) phase, which exhibits the property of full phase control by rotating the orientation angle of an I-shaped patch. Meanwhile, this PB unit is backed by a metallic square loop frequency selective surface (FSS), which behaves as stopband and passband responses around 18 GHz and 12 GHz, respectively. By encoding PB phase to generate a certain phase response, four reflective vortex beams carrying orbital angular momentum (OAM) are achieved at a higher frequency band under normal incidence. On the other hand, the incident waves operating at a lower frequency band can pass through such an FSS-backed PB unit and be focused by a three-layer composite unit consisting of a polarization-conversion unit sandwiched by two orthogonal polarizers. Therefore, upon illumination of normal incident plane waves, the proposed FS-MS works as a multiple OAM beams generator and a focusing lens simultaneously. The proposed design can motivate the realizations of FS-MS and hold potential applications in intelligent housing systems.

Wideband dual-polarized linear-circular and linear-cross angular stable THz reflective polarizer based on a modified square loop FSS.

A simple wideband reflective type linear-cross and linear-circular polarization converter for terahertz (THz) applications is proposed in this paper. The top frequency selective surface (FSS) consists of a thin gold coated figure of eight square loop with the connected strip line corners on a thin grounded polyamide dielectric substrate. For the y/x-polarized incidence, the design exhibits linear-circular conversion with an axial ratio (≤3dB) from 0.49-0.50, 0.60-0.83, 1.16-1.62, and 1.81-1.85 THz with 2.02%, 32.17%, 33.09%, and 2.19% fractional bandwidth (FBW), respectively. In addition, it also performs the linear-cross conversion with a minimum 90% polarization conversion ratio (PCR) from 0.53-0.56, 0.92-1.07, and 1.69-1.75 THz having 5.50%, 15.08%, and 3.49% FBW, respectively. Multiple plasmonic resonances are the reason behind different polarization conversions and are confirmed with surface current distribution profiles of the FSS and ground. The metasurface's performance is stable up to 45° for both transverse electric (TE) and transverse magnetic (TM) oblique incidences. The polarizer's unit cell architecture is compact with structural dimensions of 0.121×0.121×0.041λL3, where λL is the lowest operating frequency's free-space wavelength. The authors believe this design is compact, with angular stable multi-band multi-conversion ability that will significantly impact the THz applications in real time.

On the Design of Ultralow-Profile Broadband Absorber Based on Non-Foster Circuit

In this letter, a novel and effective method for non-Foster active absorber with ultralow profile and wide absorption band is proposed. The three-layer absorber consists of a square loop frequency selective surface array printed on the FR-4 dielectric substrate, a non-Foster matching layer, and a backed metal plane.Equivalent circuit model for the active absorber is proposed and analyzed, providing great insight into the effect of non-Foster matching layer on breaking the Rozanov limit effectively. A 1×10 absorber absorbing structure sample is fabricated.It is shown by measurement that the absorber has a fractional bandwidth of 36.0%, with a total thickness of only 0.008 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">λ_L</i> (wavelength at the lowest absorption frequency). The simulation and experimental results are in good agreement in the operating band, which verifies the accuracy of the design method.

A Compact High Gain Printed Antenna with Frequency Selective Surface for 5G Wideband Applications

In this article, a frequency selective surface (FSS) based compact wideband printed antenna radiator with improved gain and directivity is proposed for sub-6 GHz 5G wireless networking applications. Due to their inherent property of possessing spatial filtering characteristics, FSSs are attracting the interest of researchers. An approach for increasing the gain and the directivity by integrating a band pass FSS on a compact built patch antenna radiator is proposed here. The architecture equations for designing the band pass FSS using double square loop geometry are defined. The printed patch antenna radiator (PAR) and a double square loop frequency selective surface (DSLFSS) are designed and integrated. The simulation results are verified using the results from the measuring setup. The output response is giving a fractional bandwith of 19.14% with 5.5 dBi gain and 6.2 dBi of directivity and thus makes it the good choice for 5G applications.

Design and Analysis of Frequency Selective Surface for Ism and Lower Ultra-Wideband Antenna

A Compact, dual-layer Frequency Selective Surface (FSS) is presented for electromagnetic shielding applications. The square loop FSS is modified to provide band rejection characteristics for ISM band and lower Ultra-Wide Band (UWB) covering from 2.2 GHz to 7.1 GHz with the bandwidth of (S21 < -20dB) 4.9 GHz. The novel design exhibits symmetric property due to which it achieves polarization-insensitive stable band rejection of the desired frequency band for various angles of incidences up to 60°. The proposed FSS structure has compact dimensions of 0.170λ x 0.170λ x 0.0117λ. The design was fabricated on either side of the substrate made of FR-4 with a dielectric constant of 4.4 and loss tangent of 0.02. The measured results are well per the simulation results. The proposed FSS in this paper has various applications, including the aerospace industry, hospitals, random, and in the scope of electromagnetic filters, etc.

Fabrication of RF elements on thin plastic substrate using laser engraving technique

A novel fabrication process using a laser machining is proposed, allowing for the creation of arbitrary conductor geometries on an ultrathin dielectric substrate. During the engraving process, a thin aluminum layer is removed from a Mylar substrate, leaving a periodic conductive array. This process provides several advantages over existing techniques and may open additional applications due to the ability to economically create ultra-thin, conformal, optically transparent, light weight surfaces. The proposed technique is modelled and validated through experimental measurements of fabricated simple band-stop square and circle loop frequency selective surfaces (FSSs). Good agreement is observed between the simulated and measured results with resonances 9.9 GHz and 10 GHz for the square loop and 14.2 GHz and 14.4 GHz for the circle loop.

ECM Enabled Whale optimization assisted facile design of dual-band conformal FSS for WLAN shielding applications

A new modified Equivalent Circuit Model (M-ECM) enabled Whale Optimization (WO) assisted electromagnetic (EM) approach is presented for an efficient design of a double square loop frequency selective surface (DSL-FSS) based conformal shielding structure. A simple single-layer DSL-FSS is designed, fabricated, and measured to suppress both wireless local area network (WLAN) bands without affecting the rest of the considered spectrum using ECM enabled WO. The analytical and measured results have been found in quite good congruence with each other. The measured transmission coefficient at 2.4 and 5.0 GHz shows suppression of 35 and 38 dB, respectively. Further, the angular stability, polarization insensitivity, and physical interpretation of the DSL-FSS shielding structure are investigated. Finally, the conformal behavior of the structure has been investigated for different bending radius and good agreement between simulated and measured results shows the effectiveness of the aforementioned technique for planar and conformal WLAN shielding applications.

Reduced specific absorption rate compact flexible monopole antenna system for smart wearable wireless communications

In this paper, design and analysis of a flexible antenna backed with Frequency Selective Surface (FSS) is presented for wireless body area communications. Textile based compact monopole antenna (50.5 × 38 mm2) is backed with a modified square loop FSS textile based superstrate (120 × 120 mm2), to improve the wearable antenna’s performance in terms of SAR reduction and gain enhancement. Modified square shaped monopole radiator with a rectangular slit defected ground structure provides the radiation of the 2.45 GHz ISM band with a bandwidth (S11 ≤ −10 dB) of 2.075 to 2.625 GHz. The bending analysis of antenna is also investigated in operating frequency band. Achiral FSS provides gradual increment in inductance and stable performance at both TE and TM polarization for oblique incidence of signals up to 60°. The proposed antenna system with the FSS, yields the fractional bandwidth of 22.44% and peak gain of 7.76 dBi with the dimensions of 120 × 120 × 30 mm3. The inclusion of FSS, reduces the back radiation there by decrease in SAR level is achieved from 3.197 W/kg to 0.164 W/kg. The reduced SAR value and its stable frequency performance over flexed conditions, ensures that the proposed system is a suitable entrant for wearable applications.

On the Design of Wideband Absorber Based on Multilayer and Multiresonant FSS Array

In this letter, a novel and effective design method for multilayer and multiresonant absorbers is proposed. The two-layer absorber is composed of two resistor-loaded square loop frequency selective surface (FSS) arrays printed on the Rogers 4003 dielectric substrate and a backed metal plane. Equivalent circuits for two-layer absorbers with resistive sheets and resonant FSSs are proposed, analyzed, and further improved to provide great insight into the existence of the four resonances and ultrawide bandwidth. In the equivalent circuit model, the substrate is transformed to a parallel admittance and a positive susceptance is introduced in the whole operation band, which contributes to an extra resonance. After the measurement of a fabricated sample, a fractional bandwidth of 171.2% is obtained for at least 10 dB reflection reduction, with a total thickness of only 0.087λL (wavelength at the lowest absorption frequency). The good agreement between the calculated, simulated, and measured results validates the designed absorber.

Characterization of the koch fractal hexagonal loop frequency selective surface for X-band application

&lt;p&gt;This paper presented the bandstop Koch fractal hexagonal loop frequency selective surface (FSS) for the X-band application. The simulated transmission coefficient response (S_21) had been obtained by using CST software. The proposed Koch fractal hexagonal loop FSS structure is highly insensitive towards angular stability and also incident polarization up to 60 degree , with deviation of resonant frequency, f_r below than 1%. The parametric analysis on the effect of the periodicity, width, and height of the fractal FSS structure on the S_21 has been illustrated and discussed thoroughly.&lt;/p&gt;

Study on an Accurate and Efficient Design Method of Resonant FSSs Based on the Macro-Model of Units in the Basic Strip-Gap FSS

An accurate and efficient design method for resonant frequency selective surfaces (FSSs) based on the macro-model of units in the basic strip-gap FSS is proposed. Several variables are introduced in the formulas to improve the accuracy of computation. Without dielectric substrate, two effective widths are introduced in the calculation of equivalent L and C due to the current inhomogeneity caused by the gap. Novel equivalent circuit (EC) formulas are proposed after full-wave simulation and parameter fitting. Furthermore, with substrate, another two variables, the effective permittivity for inductance and capacitance, are introduced with detailed calculation equations. The results obtained by simulation and EC calculation show that not only the resonance frequencies but also the frequency responses over a wide frequency band are found to be in reasonable agreement, when the wavelength of the operating frequency is smaller than the period. For verification of complex structures and actual application, two absorbers based on cross dipole and square loop FSSs, respectively, are proposed. Good agreement is gained between the measurements of a fabricated sample working from 2.03 to 9.36 GHz, and the results obtained by EC calculation and simulation. The results verify that it is feasible to use this method in the study of more complex and novel structures which are based on resonant FSSs.

Novel, straightforward procedure to design square loop frequency selective surfaces based on equivalent circuit model

A new procedure to synthesis three unknown dimensions of a square loop frequency selective surface SL-FSS (periodicity, width and the gap between loops) is presented. SL-FSS can be treated as a series LC circuit. Some well-known analysis equations which relate the LC values to the dimensions are considered as bases. They are simplified algebraically assuming normal incidence. The reduced equations are used in order to reach the synthesis procedure. The action is divided into two parts: First, resonance frequency is considered as known and the periodicity is related to the other two unknowns, in terms of roots of a quartic polynomial. Second, it is assumed that the normalized L and C impedances are known. A numerical but efficient method is presented in this part to synthesis the unknowns. In each part, a discussion about the domain of known values in which the output dimensions are acceptable, is presented. Finally the procedure is employed to design an absorber for 8.4-15.2 GHz, consisting of resistive SL-FSS over grounded dielectric. An instruction is presented to combine the method with full-wave simulations to take into account the effect of resistivity. Practical results show reasonable agreement with the simulations.

A 3-D printed square loop frequency selective surface for harmonic radar applications

This paper presents a 3-D printed frequency selective surface (FSS) for harmonic radar applications. The proposed FSS design consists of a square loop printed on the cost-effective acrylonitrile butadiene styrene (ABS) plastic material to pass the signals at 5 GHz resonant frequency band. The proposed FSS is tested for various oblique angles up to 45° and even for different polarization modes. A prototype of the proposed design FSS is fabricated. The results demonstrate a good agreement of simulated results with experimental ones. The 3-D printing technique provides a simple and low-cost method to fabricate FSS and make it suitable for future FSS mass production.

Study of 5.8 GHz Band-Stop Frequency Selective Surface (FSS)

This paper presents the study of 5.8 GHz frequency selective surface (FSS) acts as a band stop to eliminate unwanted radiation signal at 5.8GHz. The FSS was designed using computer simulation technology (CST) Microwave Studio software. The paper shows the comparison of square loop, octagon loop and hexagon loop of Band stop FSS (BSFSS) performance at 5.8 GHz. Besides, the BSFSS design using four different type of dielectric substrate such as FR-4, TLY-5, Roger RT5870 and Roger RT5880 were compared. The results obviously show that the Rogers RY5880 has the attenuation -44.72 dB. The fabricated FSS were measured by using free space technique with two horn antennas connected to performance network analyzer (PNA). The measured and simulated results were compared. The results show that the square loop FSS structure have the better attenuation -26.76 dB (simulated) and -38.34 dB (measured) at 5.8 GHz.