Double-layer Frequency Selective Surface Research Articles

Tunable liquid crystal double-layer frequency selective surface with wide transmission band and small insertion loss

ABSTRACT In this paper, a tunable Frequency Selective Surface (FSS) based on liquid crystal (LC) with a wide transmission band, large tuning range, high tuning rate, and miniaturization is designed. First, a single-layer FSS is developed with two frequency selective layers and one LC tunable layer. The simulation and experiment results show that the single-layer FSS has a transmission bandwidth greater than 4.25 GHz, and the LC change can make the center frequency of the transmitted band shift to the lower frequency by 576 MHz, with a tuning rate of 4.2%. Subsequently, a double-layer FSS is designed by adding a coupling layer to the single-layer FSS, achieving a flat-top passband and enhancing the tuning rate. Simulation and experimental validation show that altering the LC state enables continuous tuning of the center frequency of the double-layer FSS from 14.95 GHz to 13.86 GHz, with a tuning rate of 7.2%. This structure exhibits a flat-top passband wider than 5.7 GHz, a relative bandwidth of 41%, and a rectangular coefficient below 1.877. Moreover, the double-layer FSS demonstrates favorable polarization stabilization and wide incidence angle stabilisation properties in various states.

Dual-polarized angle-selective surface based on double-layer frequency selective surface

In this Letter, we propose a dual-polarized angle-selective surface (ASS) based on a double-layer frequency selective surface (FSS) to achieve angular selectivity. By appropriately constructing the structure of the single-layer FSS, bandpass and band-stop are produced by the ASS under normal and oblique incidences, respectively, resulting in exceptional angular selectivity. Simulated results show that the proposed low-pass ASS achieves a bandpass (|S21| > −1 dB) from 0° to 10° and a band-stop (|S21| < −20 dB) from 31° to 85° with a profile thickness of 0.6 λ. A low-pass ASS prototype with an operating frequency of 10 GHz is fabricated to demonstrate the design concept. The measured results of the transmission coefficients are in good agreement with the simulated results. The proposed ASS can play an important role in many applications, as in minimizing the side lobes of the antenna.

A Broadband Double-Layer Transmitarray Antenna Based on Polarization Conversion Elements

A broadband double-layer transmitarray antenna (TA) is proposed in this letter. In order to obtain broadband performance in a double-layer structure, we use a “receiving-transmitting” (RX-TX) structure instead of a double-layer frequency selective surface. The elements of the new TA are composed with metal layers and dielectric substrate. Both the metal layer are connected by a metallized via and etched with orthogonal slots. The RX and TX layers work without interfering while converting the linearly polarized incident wave into orthogonal polarization. Phase compensation can be achieved by changing the direction and length of the microstrip meander ling. Then, a broadband TA is designed. Considering the simplicity of the design, we sequentially selected eight phase states at 45° intervals for phase modulation. The measured maximum antenna gain is 24.5 dBi. The center frequency is 10 GHz. The measured 1-dB and 3-dB gain bandwidth(BW) are 18.7% and 29.4%, respectively. The maximum aperture efficiency (AE) is 51%.

Fabrication of the double-layer millimeter wave frequency selective surface by femtosecond laser

With the rapid development of communication technology, frequency selective surface (FSS) operating at higher frequencies is required and it is necessary to design and fabricate new FSS. In this paper, a broadband double-layer FSS is proposed which is composed of one layer of foam, two metal layers, and four layers of substrate. The simulation result shows that a high transmission in the band of 60.03–89.33 GHz (covering the E band) with a relative bandwidth up to 29 % is achieved. In addition, the effects of the key structural parameters, oblique incidence angle, and TE/TM polarization on the transmission performance are further explored. The results indicate that the proposed FSS has good oblique incidence angular stability and polarization insensitivity up to 60°. Finally, the proposed FSS is fabricated by the femtosecond laser and measured by the waveguide method. Experimental results show that the machined sample has high precision and the machining error is within 1 µm, and good agreement between the simulated and measured results is observed. Therefore, the proposed FSS could be used as spatial filters or sub-reflectors of multiband antennas and the corresponding processing method could be extended to other millimeter wave functional devices, such as absorbers, radomes, etc.

Broadband Magnetic Absorber Based on Double-Layer Frequency-Selective Surface

A thin and broadband frequency-selective surface (FSS)-based magnetic absorber is presented for low-microwave frequency applications. The design consists of double layers of FSS with the combination of two different magnetic substrates, which provides a much broader bandwidth compared to existing single-layered designs. The magnetic materials are in-house made by mixing different ratios of carbonyl-iron powder with silicone elastomer polydimethylsiloxane (PDMS). The simple single-square loop (SSL) structure is used for both FSSs that are printed on a standard FR4 material using lossy resistive sheet. This configuration allows the use of a closed-form equivalent-circuit model for a fast and efficient bandwidth optimization. A genetic algorithm (GA) is utilized to obtain the design parameters for broadband absorption with minimum overall thickness. The measured results confirm that the proposed absorber operates from 0.78 to 4.8 GHz with −10 dB reflectivity. The design achieves 144% fractional bandwidth with a 10.12 bandwidth-to-thickness ratio.

A High-Temperature and Frequency-Reconfigurable Multilayer Frequency Selective Surface Using Liquid Metal

A high-temperature and frequency-reconfigurable multilayer frequency selective surface (FSS) based on liquid metal is proposed in this work. After construction of a multilayer dielectric cavity with specific dielectric-shaped pillars and the injection of liquid metal (EGaIn) into each layer of the cavity, the liquid metal and dielectric pillars form a slot-type bandpass FSS, and frequency reconfiguration is realized by switching the injection layer. A prototype of the proposed FSS was designed, fabricated and measured. The measured results validated the design well. The good conductivity and fluidity of the liquid metal are utilized to realize the reconfigurable electromagnetic performance of the proposed FSS. This works in the C-band (5 GHz) and Ku band (13 GHz) and has a frequency switching time between 13.1 to 14.2 s. Moreover, due to the good thermal conductivity of liquid metal, the flowing liquid metal can take away heat, and the multilayer FSS can work as cooling plate. The heat transfer performance was investigated by numerical simulations. A 1000 ° plane heat source was set on the bottom surface of the double-layer FSS. The average temperature on the top surface was 62.2 °, when the liquid metal fluid in the FSS had a speed of 1 m/s. The proposed multilayer FSS has strong potential for applications in some special application fields involving stealth and hypersonic aircraft.

A double layer FSS filter for sub-THz applications

This work presents the simulated and measured performance of single- and double-layer frequency selective surface filters for operation at sub-THz frequencies (250 GHz center frequency). They were composed of concentric square loops with a split as a unit cell resonator on top of a low dielectric permittivity, low thickness material (RT5880). Both a single layer filter and a cascaded two layer filter with varied distances were investigated. The simulated bandwidth for the cascaded filter was 27 GHz and 16 GHz and 9 GHz bandwidth measured with a THz-TDS and microwave system.

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.

Equivalent Circuit Design Method for Wideband Nonmagnetic Absorbers at Low Microwave Frequencies

An accurate equivalent circuit (EC) design approach for wideband nonmagnetic absorbers operating at the low microwave frequency (1–10 GHz) is presented. Following the impedance matching approach, this communication introduces an EC model based on the simulated data and synthetic asymptotes for single- and double-layer frequency-selective surface (FSS)-based nonmagnetic absorbers. Two simple and commonly used resistive FSSs, i.e., square patch and single square loop, are considered in this communication. Compared to the full-wave simulations, the proposed EC model shows more than 95% accuracy. By employing the proposed model and genetic algorithm-based optimization, several designs of broadband absorbers are demonstrated. The presented single- and double-layer FSSs show 126% and 161% fractional bandwidth, respectively, with the total thickness close to the Rozanov limit. The results confirm that the proposed method is a simple and efficient way of designing thin wideband absorbers using single- or double-layer FSS configurations.

Resonant Coupling Effects in a Double-Layer THz Bandpass Filter

Cascading similar frequency selective surfaces (FSSs) improves the roll-off rate and frequency selectivity of THz bandpass filters. However, resonant coupling between FSS layers causes anomalous transmission peaks. In this work, we have employed an equivalent circuit model to analyze a double-layer FSS structure with periodic cross apertures at different interlayer spacing. We found that the anomalous peaks can be attributed to the resonant coupling between the upper and lower layer FSSs, and their periodic appearance with changing interlayer spacing is related to the half-wavelength repeatability of the circuit. We have fabricated a double-layer FSS sample using femtosecond laser micromachining and measured it using time-domain THz spectroscopy. The results using the equivalent circuit model agree with the Finite-Difference Time-Domain (FDTD) and experimental results.

Design of a Dual-Band Electromagnetic Absorber With Frequency Selective Surfaces

A dual-band electromagnetic absorber based on double-layer frequency selective surface (FSS) is proposed for broadband applications. The layer I is designed with an absorption performance at higher frequency and a transmission window at lower frequency. Simultaneously, the layer II is designed with an absorption performance at lower frequency and a strong-reflection window at higher frequency. Due to the low-pass property of the layer I, this layer is on the top of the absorption structure, allowing the incident waves to transmit to the layer II at lower frequency and be absorbed. Meanwhile, the incident wave is absorbed at higher frequency by the layer I since the layer II acts as the strong-reflection substrate in the bottom of the absorption structure. The reflectivity of absorber below -10 dB covers a bandwidth of 1.3-4.1 GHz and 7.2-10 GHz at normal incidence. Furthermore, the absorber can still maintain an absorption bandwidth below -10 dB at 1.4-4.1 GHz under the 30° oblique incidence. The good agreement between simulation and measurement validates the proposed absorber.

Broadband Double-Layered Huygens’ Metasurface Lens Antenna for 5G Millimeter-Wave Systems

A double-layered Huygens’ unit cell is proposed to design a broadband metasurface lens (meta-lens) for 5G millimeter-wave antennas. The Huygens’ unit cell consists of a pair of antisymmetric conducting semicircle arc elements on both surfaces of a thin dielectric substrate. The surface currents flowing at the opposite directions on both conducting elements form an electric current loop to induce the orthogonal magnetic current, and then the Huygens’ resonances are stimulated. The Huygens’ unit cell provides the transmission phase coverage of over 400° with transmission amplitude better than −2.3 dB. This breaks through the phase shift limitation of a conventional double-layer frequency-selective surface (FSS) element. It shows that induced magnetism makes such a Huygens-based meta-lens very compact with only one printed circuit board. As an example, a double-layered meta-lens on a 1.5 mm-thick dielectric substrate is designed and experimentally verified. The meta-lens antenna achieves the measured peak gain of 30.7 dBi at 26.2 GHz with an aperture efficiency of 42.25% over the 3 dB bandwidth of 15.7% from 24.1 to 28.2 GHz, fully covering the proposed 5G spectrum from 24.25 to 27.5 GHz. This proposed method greatly helps in the application of promoting planar lightweight low-cost broadband lens antennas in the coming 5G systems.

Double Layer Frequency Selective Surface for Ultra Wide Band Applications with Angular Stability and Polarization Independence

In this paper, a double layer stopband Frequency Selective Surface (FSS) for Ultra-Wide Band (UWB) applications is presented. The proposed FSS consists of two cascaded conventional FSS, one with double square loops and other with square loops. The − 10dB frequency band from 3.00 GHz to 11.64 GHz is achieved, with a fractional bandwidth of 149 %. Effects of angular stability and polarization independence over the frequency response are investigated and conclusions are presented. In order to show transmission performance, the FSS was simulated by ANSYS HFSS. The FSS shows angular stability and polarization independence in entire UWB band (3.10 – 10.60 GHz). A prototype was built and measurements were performed. A good agreement between simulated and measured results was observed.

Miniaturized-Element Bandpass FSS by Loading Capacitive Structures

A bandpass frequency-selective surface (FSS) element loaded with capacitive structures is proposed to miniaturize FSS unit cell size. In the FSS design, the capacitive structure, implemented by convoluted slot grids (CSGs), is introduced to increase the equivalent capacitance of the bandpass FSS, thus leading to a lower resonance frequency. The FSS unit cell size is then reduced accordingly. To validate this design concept, a double-layered bandpass FSS consisting of convoluted slot dipole and CSG structures is designed to provide a unit cell size as small as $0.035\lambda _{0} \times 0.035\lambda _{0}$ , where $\lambda _{0}$ refers to the resonance wavelength in the free space. Equivalent circuit of this FSS is established for explaining the resonance mechanism and modeling the frequency performance. Finally, a prototype of the proposed FSS is fabricated and measured. The measured results exhibit the stable filtering response under different polarizations and incidence angles (up to 60°), which also agree well with those obtained by full-wave simulations.

High-transmittance double-layer frequency-selective surface based on interlaced multiring metallic mesh.

In this work, we proposed an optically transparent double-layer frequency-selective surface (FSS) based on interlaced multiring metallic mesh. By changing the large metal area of a conventional double-layer FSS into triangular-orthogonal distributed basic rings and nested rotated subrings, we achieved an FSS with high optical transmittance and low normalized high-order diffraction intensity while maintaining a flat passband and steep transition band. The results showed that our fabricated FSS had a normalized visible transmittance of 90.31%, stable filtering passband of ∼33.9 GHz, 3dB bandwidth of 13.4GHz, and uniform diffraction distribution, which are favorable characteristics for optically transparent FSS applications.

Frequency Selective Surface With Two Quasi-Independent Notch Frequencies

A double-layer frequency selective surface (FSS) built on FR4 substrate exhibiting two notch frequencies in the 0–10-GHz range is introduced. Recently published examples demonstrate that further research needs to be undergone for obtaining separate tuning of the resulted notch frequencies. Thus, this issue is addressed in this paper: the notch frequencies can be quasi-independently set by appropriate values given to the geometrical parameters of the two metal patterns in the unit cells of the structure. The FSS behavior is stable with respect to the angle of incidence of impinging electromagnetic waves up to 45°, as proved by simulation and validated by experiments.

Ultrawide Bandwidth Electromagnetic Wave Absorbers Composed of Double-Layer Frequency Selective Surfaces with Different Patterns

A novel design for an ultra-wide bandwidth and thin microwave absorber is introduced utilizing two frequency selective surfaces (FSSs) with different patterns of resonating frequencies. The circuit parameters, inductance and capacitance, of the three types of FSS (square loop, cross, square patch) were determined using an equivalent circuit and strip wire conductor model. The square loop FSS indicates a low frequency resonance (10 GHz) due to its high inductance and capacitance. On the other hand, the square patch of small inductance reveals a high resonating frequency (36 GHz). By optimizing the combination of the two FSSs, an ultra-wide absorption bandwidth (6.3–40.0 GHz for −10 dB reflection loss) was designed with a small total thickness of 5.5 mm, which is close to the theoretical limit. The free space measurement result with a test sample prepared by the screen printing method was in good agreement with the simulation result and verified the validity of the proposed design method. For these periodic array structures, however, the grating lobes were observed above the high frequency limit, and it needs to be emphasized that the further control of the unit cell periodicity is important, particularly for large oblique incidence angles.

Double-Layer Frequency Selective Surface for Terahertz Bandpass Filter

A thin, low-cost, double-layer frequency-selective surface (FSS) bandpass filter has been designed, fabricated, and measured for operation in the terahertz band. The present FSS structure is composed of double-layer tin foil with a hexagonal lattice array of circular holes. A flat passband is observed with a 3-dB bandwidth of 0.20 THz from 0.81 to 1.01 THz, which is about 22% relative to the center frequency. This bandpass filter based on double-layer FSS structures achieved average insertion loss of 0.7-dB and 106.31-dB/THz band-edge steepness. We have found good agreement between theoretical calculations and experimental results.

Terahertz Bandpass Filter Based on Frequency Selective Surface

We designed a terahertz bandpass filter based on double-layer frequency selective surface. Details of the proposed terahertz wave filter design and of the related parametric analysis are presented and discussed. The proposed terahertz filter was fabricated by laser technology. The transmission spectrum was characterized by terahertz time-domain spectroscopy. The results show that the center frequency of the terahertz filter is about 1 THz with 3-dB bandwidth of 400 GHz.

A glass based dual band frequency selective surface for protecting systems against WLAN signals

A polarization independent, double layer frequency selective surface on glass substrate is proposed that can transmit 900MHz and 1.8GHz GSM bands but inhibit transmission of wireless local area network (WLAN) bands, 2.4–2.5GHz and 4.98–5.825GHz. A parametric analysis is conducted to tailor the resonance frequencies at WLAN bands and the effect of each geometrical parameter on band stop response is investigated. Band stop responses of each FSS layer and the double layer final design are presented both numerically and experimentally. The simulation results showed a good agreement with the experimental results and supported the polarization independent behavior. In particular, very wide rejection bands are obtained (−10dB bandwidths more than 1.75GHz) for the 5GHz band, both numerically and experimentally, for vertical and horizontal polarizations, under normal incidence. The design can be used in double layer window glasses for protecting systems against interference with WLAN signals in indoor environments and buildings.