Frequency Selective Surface Unit Cell Research Articles

Design of high roll‐off frequency selective surface based on single‐layer SIW cavity with fully canonical filtering responses

In this paper, a novel design of high roll-off frequency selective surface (FSS) is proposed based on a single-layer substrate integrated waveguide (SIW) cavity. Fully canonical filtering responses with three transmission poles (TPs) and three transmission zeros (TZs) are obtained by employing transversal signal-interference techniques (TSITs). The FSS unit cell can be regarded as a combined triple-mode cavity consisting of one cross-slot resonator and two SIW cavities. By using the filtering coupling matrix, the FSS can be well designed and analysed. Accordingly, to verify the design concept, a prototype is fabricated and experimented. Measured and simulated results are in good agreement, showing high performances in terms of high roll-off and low profile.

Design of frequency selective surface based on Minkowski fractal and interdigital capacitance

In this paper, the Minkowski fractal is used to design the geometry of a frequency selective surface (FSS) unit cell, which resembles the shapes found in nature. The proposed FSS unit cell has both angular and polarization stabilities of incident wave. For the miniaturization of FSS and decrease of resonance frequency, interdigital capacitances (IDCs) are devised in the FSS structure, which do not eliminate angular and polarization stabilities. The dimensions of the unit cell are 5 mm × 5 mm and miniaturized 0.095λ0×0.095λ0. The resonance frequency is specified as 5.75 GHz with 155 MHz bandwidth. An equivalent circuit is derived for the unit cell to evaluate its frequency responses. The effects of variations of the geometrical dimensions of the FSS unit cell on its performance are studied. A prototype model of proposed FSS is fabricated and measured. The performance of its full-wave computer simulation results and measured data are compared and are shown to be in good agreement.

Angularly stable frequency selective surface for the gain enhancement of isolated multiple input multiple output antenna

AbstractA metasurface is highly useful for improving the performance of patch antennas and reducing their size due to their inherent and unique electromagnetic properties. In this research work, we design a wideband bandstop reflector‐based frequency selective surface (FSS). It consists of a periodic array of circular shape metallic rings with four stubs. The FSS unit cell resonates over a frequency band from 7.2 to 9.2 GHz and has excellent stability under different polarizations and incident angles. A microstrip patch‐based isolated multiple input multiple output (MIMO) antenna is designed that operates over a frequency band of 7.9GHz to 8.7 GHz. The MIMO antenna and FSS are printed on the one face of a substrate. The radiation performance such as gain and directivity of the antenna is improved due to parasitic radiation of the surface and coherent‐ superposition of energy in the far‐field, between the antenna and FSS unit cells. Where the other parameters such are efficiency, radiation pattern and isolation are well preserved. The designed structure shows low complexity during fabrication. The simulated and experimental results confirm the predicted antenna performance and gain enhancement.

An ultra-wideband frequency selective surface with high stability for electromagnetic stealth

Aiming at the problems of low-frequency coverage, angle and polarization instability of ultra-wideband (UWB) frequency selective surface (FSS) in practical application, this paper introduces the additional circular parasitic patches and two-stage rectangular bent grids to enhance coupling, miniaturize geometry size, broaden bandwidth and suppress the drift of resonance frequencies based on the construction of a FSS unit cell by using a basic circular patch and a Jerusalem cross grid. The simulated and measured results show that the bandwidth of the FSS is 13.47 GHz, and its fractional bandwidth is 140.53%, which fully covers the UWB band (3.1–10.6 GHz). The UWB FSS has the flat transmission passband, and high transmission characteristics, and maintains stable performance at large incident angles and different polarization modes. The size of the UWB FSS unit cell is 0.25 λc × 0.25 λc × 0.12 λc. Therefore, the UWB FSS can be applied to electromagnetic stealth and the related fields of UWB communications.

A switchable frequency selective surface with polarization selectivity in microwave based on the rotatable magnet

In this letter, a switchable frequency selective surface (FSS) based on the rotatable magnet is first proposed for polarization selector. The unit cell of the switchable FSS is composed of a rotatable cuboid-magnet and a F4B substrate. The cuboid-magnet is inserted into the substrate which controlled by the orthogonal dual-U-shaped electromagnetic control system. The FSS has a characteristic of polarization selectivity at 8.66 GHz by rotating the cuboid-magnet. To confirm the simulation results, a prototype of the proposed FSS is fabricated and measured. The measured result shows that the switchable FSS can selective the passband in transverse-electric polarization or transverse-magnetic polarization.

Gain Enhancement of CPW Antenna for IoT Applications using FSS with Miniaturize Unit Cell

Wireless connectivity is a critical enabler for many IoT applications. Antennas are often required to be installed inside the device cover, which usually occurs in small sizes with optimal performance. On the other hand, a suitable antenna should also have high efficiency, gain and adequate bandwidth covering the desired frequency range. Here, we proposed new type of Frequency Selective Surface (FSS) with miniaturized resonator element to enhance the gain of an CPW antenna. Furthermore, the miniaturization of the Frequency Selective Surface unit cell is attained by coupling the two meandered wire resonators. The wire resonator is separated by thin and single substrate layer. The structure of the FSS is shown to have a FSS unit cell dimension that is miniaturized to 0.057λ. The CPW antenna size is only 28.8mm × 46.5mm operating at 2.45 GHz frequency. With the additional of the FSS, the antenna’s gain reaches up from 1.8 dBi to 2.6 dBi with omnidirectional radiation pattern.

Quad-band FSS for Electromagnetic Shielding

In this paper, a novel method to design Quad-band stop frequency selective surface (FSS) is proposed to shield medical/non-medical devices from electromagnetic radiation of wifi/Bluetooth, GPS,1G, 2G, 3G, 4G and 5G frequencies. The proposed device can also be used in security and surveillance devices to protect them from electromagnetic attack. The FSS unit cell consists of convoluted structures on FR4 substrate. The study was performed by designing individual frequency selective surfaces for 1G, 2G, Wi-Fi/Bluetooth, GPS, 4G and 5G, then combining it to form a single Quad-band FSS unit.

Tunable liquid crystal frequency selective surface with the compact unit cell, large tuning range, and the passband of flat-top and sharp roll-off

In this paper, a design method for tunable liquid crystal (LC)-based frequency selective surface (FSS) with the compact unit cell, large tuning range, and the passband of flat-top and sharp roll-off is proposed. A very compact LC-based bandpass FSS unit cell with the dimension of 0.077 λ × 0.077 λ is designed based on a pair of planar vortex-shaped metallic structure with two large parallel patches, between which the LC is sandwiched. Results show that the central frequency of the FSS is tuned from 11.56 to 12.58 GHz, and the tuning range reaches up to 8.5%. In order to obtain the transmission characteristics of flat-top and sharp roll-off, a double-layer structure is proposed and combined with the LC-based FSS design. Theoretical analyses are conducted to declare the working principle of the double-layer LC-based FSS. A prototype sample for the double-layer LC-based FSS is designed and fabricated. Results show that the double-layer LC-based FSS structure achieves frequency selective characteristics with a flat-top passband of about 450 MHz and sharp roll-off for arbitrary LC states with −15 dB rectangle coefficient of 2.44 for arbitrary LC states. Moreover, stable frequency responses of the double-layer LC-based FSS under various incident angles from 0° to 45° are obtained for arbitrary LC states.

Polarization‐insensitive frequency selective rasorber with higher transmission band using modified double cross slot based bandpass layer

In this study, a frequency selective rasorber (FSR) is proposed where a transmission band at a higher frequency is achieved besides the lower broadband absorption. The FSR is designed by combining the individual designs of broadband absorber and bandpass frequency selective surface (FSS). The broadband absorption is achieved by placing the resistive and ground layer at a distance of around λ/4 at center frequency. The unit cell of bandpass FSS is designed by etching a double cross slot. The performance parameters at the transmission band are improved by a modified double cross slot, wherein a quarter of the slot is replicated on each corner of the unit cell. The bandpass FSS replaces the ground plane of broadband absorber, in the design of proposed FSR. The proposed FSR is a polarization-independent structure exhibiting a broadband absorption from 3.1-8.7 GHz (94.91%) along with a transmission band at 11.4 GHz. The working principle of proposed FSR is illustrated by an equivalent circuit model. A range for higher transmission frequency is determined for the proposed FSR, by studying the effects of bandpass FSS parameters on the transmission performance. A prototype of the proposed FSR is fabricated, and the results are experimentally verified.

Toward Wideband Frequency Rejection: An Efficient Frequency Selective Scheme

This paper outlines a Frequency Selective Surface configuration designed for wideband filtering characteristics. The proposed design topology includes two-layer conductive elements printed on an FR4 substrate with a thickness of 1.6 mm. On the front side, a square loop and four branches are placed, and on the other side of the substrate, there is another square loop supported by two modified vertical arms. The use of square loops as one of the trendy elements in frequency selective designs provides the opportunity to tune the resonances and obtain the desired Performance; The proposed 10 mm × 10 mm frequency selective surface unit cell structure rejects the frequency band of 2-14 GHz covering WiMAX, WLAN, and X band in-service frequency ranges. It has three resonant frequencies and works well in various incident angles. There is a good match between the simulated results and the tested results. Stable frequency response, simple structure, and easy production are useful features of the developed scheme.

Design of a triple band notched polarization independent compact FSS at UWB frequency range

The design of a compact triple band notched Frequency Selective surface (FSS) at Ultra-Wide-Band (UWB) frequency range is presented. To make the proposed FSS polarization insensitive, on the upper part of the substrate a single metallic square loop (SL) and four identical rotational symmetric square split ring resonators (SSRR1) are placed, whereas at the bottom part another four identical rotational symmetric SSRR loops (SSRR2) are printed. Here each SSRR is rotated in 90°clockwise direction over a single layered FR4 substrate. Triple band notches are obtained at WiMAX and Satellite communication downlink C-band (3.3-4.2 GHz), WLAN (5.1-5.9 GHz) and Satellite communication X-band (7.2-8.4 GHz) for both TE and TM polarizations. Angular stability is achieved until 70° and attenuation of more than 25 dB is observed at resonance. The passband is obtained at 4.37 and 6.22 GHz. The proposed structure separates the closely spaced frequency bands by a ratio of 1.44 and 1.39 at the stop band and 1.42 at the pass band. The dimensions of the proposed FSS unit cell are 0.13λ0 × 0.13λ0 × 0.020 λ0, where λ0 represents the free space wavelength at the lowest resonance frequency. Good agreement between modeled and measured results is obtained.

Analysis and Design of Stopband FSS Unit Cell on Textile Substrates

A single layer Frequency Selective Surface (FSS) unit cell on a textile substrate with a stopband frequency characteristic is presented in this study. A rectangular unit cell with a slot is designed by using CST and fabricated on jeans and Tyvek substrate. In order to obtain consistency with the measurements, the waveguide method is used in the analysis of the unit cell in simulations. The resonance frequency achieved through simulation for jeans is 9.52 GHz with 56 dB insertion loss, and for Tyvek, it is 9.13 GHz with 55 dB insertion loss, respectively. Three different conductive materials: copper tape, copper thread, and conductive textile are used to fabricate the design, and their performance is compared with simulations. The experimental results confirm that all fabricated designs provide Shielding Effectiveness (SE) of more than 10 dB from 8 GHz to 10 GHz. The frequency behavior of the designs fabricated with copper tape resembles more to the simulated design as compared to other conductors. It provides SE more than 60 dB at 9.52 GHz for jeans and more than 100 dB at 9.13 GHz for Tyvek substrate, respectively.

A frequency selective surface with 3-D unit cells

This paper conceptualizes the development of a frequency selective surface (FSS) with 3-D unit cells. The FSS unit cells individually present a three-dimensional structure, whereas collectively these are more like planar FSS. The unit cell substrate has been printed using 3-D printing technique and metallized using copper paint. The 3-D printing offers a degree of freedom to fabricate substrate material in three dimensions. Various designs of unit cell have been studied by fabricating a dual-band FSS which highlights the significance of the dimensional change of the 3-D unit cell over FSS performance. The designed FSS has been fabricated and tested using VNA where test results are found as per desire. The main objective of the paper is to develop a comprehensive method for rapid prototyping of FSS in applications in the area of radar, satellite and navigation.

A novel miniaturized dual-layer frequency selective surface

A novel dual-layer frequency selective surface (FSS) with extended cross-dipole structure is reported in this paper. The elementary form of the FSS is designed at high frequency which is then made to operate at low frequency by increasing the arm length without increasing the footprint of the FSS. The extended arm runs on both the sides of the printed circuit board (PCB) where the connection is accomplished using plated through-hole (PTH) vias. The miniaturized FSS unit cell provides band-stop response at 1.6 GHz within a small footprint of 0.026λo × 0.026λo (5 mm × 5 mm) where λo is the free-space wavelength. The proposed unit cell has rotational symmetry and hence provides good polarization stability. The angular stability is investigated and stable shielding effectiveness performance is witnessed. The proposed FSS offers angular stability up to 60 degrees. The prototype FSS screen is fabricated and the measurement results are presented. The measured shielding effectiveness is greater than 26 dB for both TE and TM mode of operation. The proposed FSS is suitable for antenna gain enhancement and radome applications in L-band.

Independently Tunable Closely Spaced Triband Frequency Selective Surface Unit Cell Using the Third Resonant Mode of Split Ring Slots

In this paper, a tunable triband frequency selective surface unit cell based on varactor-loaded split ring slots is presented. By using the third resonant mode, a considerable mutual coupling reduction between adjacent ring slots is obtained ensuring both close band response as well as independent band tuning. Simulated results show the tunable properties of the proposed approach by shifting the three passbands resonant frequencies from 8.368, 10.276, and 12.010 GHz (for the unloaded split ring slots), to 7.732, 9.202, and 10.900 GHz, respectively, when each split ring slot is loaded with a capacitance of 140 fF. Furthermore, the metal shorts that split the ring slots as well as the metal rings located in between the slots are used to build the conducting paths to bias each varactor with minimum disruption in the RF response. A demonstrator based on an X-band diaphragm was designed, fabricated, and experimentally tested. Measured results using a varactor with capacitance range of 30-65 fF show resonant frequencies for the unbiased varactors at 8.53, 9.70 and 11.47 GHz with insertion loss of 1.1, 2.3 and 2.4 dB, respectively, while biasing the diodes at maximum reverse voltages (24 V) provides resonant frequencies at 8.74, 10.03 and 11.77 GHz with insertion loss of 0.8, 1.4 and 1.7 dB, respectively. Frequency shifts of 210, 330 and 300 MHz for the first, second and third resonant frequencies were obtained, respectively. The -10 dB reflection coefficient bandwidths were 495 MHz for the first band, 436 MHz for the second band and 418 MHz for the third band. Close band response is verified obtaining frequency band ratios of 1.18 and 1.14 for the third to the second resonance, and for the second to the first resonance, respectively.

Close Band Spacing Pentaband Frequency Selective Surfaces Based on Concentric Ring Slots

This paper presents a polarization insensitive frequency selective surface (FSS) based on concentric ring slots with five closely spaced passbands. The proposed FSS unit cell contains five ring slots coupled with a supplementary innermost ring slot. The close band response within the first five transmission bands is achieved by the influence of the modified geometry obtained with the inclusion of the sixth ring slot. This added ring slot causes the appearance of a stopband, due to the conductive ring between the fifth and the sixth ring apertures, and a passband produced inherently by the sixth ring aperture. The effect of the additional stopband and passband in the FSS response has been investigated in the paper. Full-wave simulations reveal that a modified geometry with proper sizes effectively enables the close band response of the pentaband FSS. To validate the proposed approach, a 729-element penta-bandpass FSS prototype operating at millimeter-wave frequencies has been designed, fabricated, and tested. Experimental results in a free-space measurement setup indicate resonances at $f_{1} =26.7$ , $f_{2} =29.8$ , $f_{3} =33.6$ , $f_{4} =38.1$ , and $f_{5} =44.2$ GHz, with resonant frequency ratios of $f_{2} / f_{1} =1.12$ , $f_{3} / f_{2} =1.13$ , $f_{4} / f_{3} =1.13$ , and $f_{5} / f_{4} =1.16$ . This FSS demonstrates an extremely low ratio of the highest and the lowest resonant frequencies ( $f_{5} / f_{1} =1.66$ ) for a penta-band operation. This polarization insensitive FSS demonstrates good performance at oblique angular incidence for elevation angles up to 45°.

Design and analysis of angular stable antipodal F‐type frequency selective surface with multi‐band characteristics

This article presents frequency selective surface (FSS) with multi-band characteristics that features tri band band-stop and dual-band passband filter. The proposed FSS unit cell comprises the F-type resonance elements disposed of antipodal (diametrically opposed to) one to another on both layer and a square loop surrounding all resonators on the top layer of the dielectric substrate. The structure is specifically designed to filter out some extensively used broadband communication signals, such as, WiMAX and WLAN while enabling C-band satellite communication in the earth station and provides three stopbands operating at 2.4, 5.2, and 5.9 GHz besides two passbands of 4.1 and 5.5 GHz. Although this structure has a multi-band structure, it exhibits high stable performance for oblique incidence ranging from 0° to 60° for both transverse electric (TE) and transverse magnetic (TM) polarizations. Considering the resonant frequency deviation (RFD) at the incidence waves with different angles of arrivals, it was found that RFD at 60° is less than 0.6% on average and very low when compared with existing studies about multi-band FSS. The detailed equivalent-circuit model (ECM) is employed to predict and analyze the transmission characteristic of the proposed structure, and a good agreement between the simulated and measured transmission coefficients is obtained.

Yüksek İzolasyonlu MIMO Anten Dizisi İçin FSS Duvar Tasarımı

In MIMO antennas, mutual coupling that adversely effects antenna performance is due to the surface currents. To date, many techniques have been proposed to suppress these surface currents such as Electromagnetic Band Gap (EBG) structures, metamaterial unit cell, band-stop filters and Frequency Selective Surfaces (FSS). FSSs, when properly designed, can act as a band-stop filter in a given frequency range and thus prevent the propagation of these surface waves in the microstrip patch arrays. In this study, a new FSS unit cell is designed to reduce the mutual coupling effect caused by surface currents between two identical 2x1 patch antenna arrays with a 3.49 GHz operating frequency. These designed FSS unit cells are placed between the patches with a 1x5 periodic sequence. According to the simulation results, mutual coupling effect decreased by 26.6 dB without any shift in the transmission band with only 0.48 dB change in S11. Finally, ECC curves are plotted separately for the antenna array with and without the FSS wall. It is observed that the ECC of the antenna array is dropped from 0.16 to 0.07 by applying the proposed isolation wall.

Stacked Slotline Structure-Based Unit Cell and Its Application for Synthesis of 3-D Bandpass Frequency-Selective Surfaces

This article presents a basic 3-D frequency-selective surface (FSS) unit cell and its application for synthesis of high-order bandpass filtering response. The proposed FSS unit cell is constructed by stacked slotlines loading with shunted capacitive and/or inductive strip lines, which can be easily fabricated with traditional printed circuit board techniques. First, by analyzing the propagation characteristics of the stacked slotline, it can be found that the stacked slotline is seen as a transmission-line transformer, which can convert spatial electromagnetic waves into guided waves. Then, by loading shunted capacitive and/or inductive strip lines, equivalent capacitors and/or inductors are formed with the slotline section, which can be used to realize desirable resonators for bandpass filtering response. Equivalent circuit models are established and circuit parameters are extracted. With the aids of the generalized filter synthesis theory, the proposed unit cell is first applied to synthesize bandpass FSS with 1/4-wavelength transmission-line inverters. An example of a second-order Chebyshev bandpass FSS with the center frequency of 10 GHz is synthesized and designed. The measured results demonstrate that the desired bandpass filtering response is stable for oblique incidence. Furthermore, to obtain high-order response with a reduced FSS thickness, the proposed unit cell is then applied to synthesize bandpass FSS based on a lumped-element bandpass filter. The detailed synthesis procedure and derivations are given. Another example of a third-order Chebyshev bandpass FSS with the center frequency of 10 GHz is also designed, fabricated, and measured. The measured results show that the desired stable performance is realized and the total FSS thickness is $0.21\lambda _{0}$ ( $\lambda _{0}$ is the free-space wavelength at the center frequency).

Miniaturized Frequency-Selective Surfaces Based on Monolithically Integrated Components

In this article, a novel approach to design low-profile miniaturized frequency-selective surfaces (FSSs) is proposed. The effective miniaturization of the FSS elements, based on square-ring-slot resonators, is accomplished by reactively loading the FSS unit cell with monolithically integrated components. The reactive components, such as capacitors and inductors embedded in the resonant unit cell, are fabricated using a multistep thin-film process based on benzocyclobutene (BCB) resin. The capacitors are built as planar metal-insulator-metal structures using an ultra-thin dielectric layer of BCB, while the multiturn inductors are fabricated as planar spiral structures with two metal layers and BCB as an interlevel dielectric for crossover. The advantages of the proposed approach are the capability to obtain large values of capacitances and inductances within small areas with low parasitics as well as the benefits in cost reduction and easy fabrication by using a planar microfabrication process. A component value variation analysis is performed to study the effect of fabrication tolerances in the reflection and transmission coefficients of the FSS when non-identical small cells are obtained. It has been experimentally validated that a 2.46 mm unit cell resonates at 4.175 GHz with transmission loss of 0.86 dB, achieving a miniaturization level of $\lambda $ /29.