Tunable 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.

Design of Dual-Band Frequency-Selective Surfaces With Independent Tunability

A design approach for dual-passband frequency-selective surface (FSS) with independent tunability is proposed and implemented. The principle of the proposed design approach is elaborated based on an equivalent circuit model, and it is demonstrated that the construction of FSS unit cell with two independently controlled transmission poles plays a decisive role. Referencing to the proposed approach, a varactor-based polarization-insensitive dual-passband FSS with independent tunability is demonstrated experimentally for the first time. In this prototype, a shunted substrate integrated waveguide cavity (SIWC) like structure is designed to realize electromagnetic isolation and serve as the dc ground for varactors to simplify the dc bias feed network. Simulated and measured results show that both passbands located in L-band and S-band can be tuned independently, and the transmission characteristic of the designed tunable FSS (TFSS) possesses good angle stability.

Origami tunable frequency selective fabric and its tuning mechanism

The tunable frequency selective surface can change its resonant frequency to better meet the complex and changeable electromagnetic (EM) environment. The traditional metal frequency selective surface (FSS) has high precision, but it is hard, making it not easy to bend and adapt to the surface. In this paper, an origami tunable frequency selective fabric (oTFSF) is proposed, which is thin, flexible, and bendable with precise resonant frequency adjustment function. The oTFSF is made by folding and ironing a single layer of FSF with a special three-dimensional spatial structure. By changing its state, the angle of the EM wave interacting with conductive units of the oTFSF can be altered to realize the precise adjustment of the resonant frequency. When the folding trace is parallel to the electric field, the three-dimensional structure of the oTFSF can be equivalent to the projected plane structure through simulation. Therefore, multiple oblique incidence situations can be converted into vertical incidence situations. The equivalent EM parameters of the oTFSF can be obtained by particle swarm optimization algorithm (PSO) to analyze the influence of unit size change on oTFSF. The results show that when the folding angle increases, the equivalent inductance (L) decreases, and the equivalent capacitance (C) increases. The oTFSF can be used to prepare intelligent EM functional materials with characteristics of broadband, lightweight, flexible, and adjustable frequency.

A tunable frequency selective surface integrated high isolation MIMO antenna for THz applications

A novel tunable triple stop band frequency selective surface (FSS) is designed and integrated with the multiple inputs multiple outputs (MIMO) antenna as an isolation structure to verify its validity. The proposed FSS renders excellent isolation characteristics for the reported MIMO system which is verified from the ECC and DG plot. In FSS, the graphene materials are attached at the end of three stubs to realize tuning over the respective bands. The center frequency of the attained triple bands shows an obvious shift from 2.2 to 2.4 THz, 3.15 to 3.5 THz, and 4.6 to 4.9 THz on increasing the chemical potential of graphene from 0 to 1.5 eV. The tuning range can be enhanced by further increasing the applied potential and hence its service might extend to multi or wideband MIMO systems. The proposed FSS unit cell achieves miniaturization to the level of 0.17 λ o. The validity of the proposed FSS is tested with a linear array MIMO antenna. The designed MIMO antenna consists of two linear array antennas separated by 40 μm with each linear array is made up of three individual antenna elements. The introduction of FSS between the linear arrays provides the minimum isolation of 30 dB. The presented highly isolated linear array MIMO antenna realizes maximum efficiency and gain of 83%, 12.2 dBi respectively. The FSS unit cell structure is modeled and their electromagnetic characteristics are simulated by the 3D electromagnetic simulator CST Studio Suite. The auto meshing technology and frequency domain solver are used to acquire S-parameter. This wearable antenna is a significant component in tracking and monitoring systems including the miniaturized multiband antennas exhibiting stable radiation characteristics during bending, stretching, and crumpling conditions to suit military applications. Nowadays, wearable antennas are being designed for military applications by perfectly matching the industry requirements such as thin, low profile, lightweight, low maintenance, robust, conformal, easy integration into clothing without affecting the movement of the soldier.

Tunable frequency selective surface using crossed shaped graphene metasurface geometry for far infrared frequency spectrum

Tunable frequency selective surface using crossed shaped graphene metasurface geometry for far infrared frequency spectrum

Stretchable tunable frequency selective fabrics and their electromagnetic response mechanism

Tunable frequency selective surface (TFSS), whose resonant frequency can change with the external complex and changeable electromagnetic (EM) environments. In this study, using the composite thermoplastic polyurethane (TPU) film/spandex elastic fabric as the substrate, stretchable tunable frequency selective fabric (sTFSF) with excellent tensile resilience is prepared. When the sTFSF is stretched, the strain is linearly proportional to the resonant frequency, so the resonant frequency can be precisely regulated according to the strain. Due to the change in the size and shape of conductive units as well as the size of units, the equivalent inductance (L) and capacitance (C) of sTFSF can change during stretching, which can be used to tune the resonant frequency. The values of L and C are obtained by particle swarm optimization (PSO) algorithm, and the different widths of resonant peaks and the tuning mechanism of resonant frequencies are theoretically explained. Furthermore, the sTFSF exhibits good mechanical performance and durability. After 1000 stretching cycles, the tunable performance is basically unchanged. Overall, the sTFSF is a thin, flexible, and precisely tunable fabric with facile preparation process, which makes it an effective functional composite material with great application prospect.

Dielectric Properties of Liquid Crystal Polymer Substrates in the Region from 90 to 140 GHz

In this article, we present a study of the equivalent dielectric constant of two nematic liquid crystals (LCs) with different thicknesses, in the region from 90 to 140 GHz. The equivalent dielectric constant of the LCs was measured using a frequency selective surface (FSS). The LC-based tunable FSS with 22 × 25 unit cells was printed on a quartz substrate with an area of 4 × 4 cm2 and a thickness of 480 µm; the LC layer with thicknesses ranging from 30 to 100 µm acted as a substrate. The FSS featured a maximum frequency-shifting range of 8.15 GHz and 30 µm-thick LC layers with mutually orthogonal rubbing directions were deposited on it. The results show that the initial equivalent dielectric constant of the LC layer increased with the LC layer thickness, while the saturation-equivalent dielectric constant remained almost constant. This work provides LC parameters that can be useful for the design of LC-based devices in the millimeter and terahertz ranges.

Neural Network Topology-Based Terahertz Absorber Using Fractal Frequency Selective Surface

This work offers a new kind of polarization-insensitive, angular stable, tunable fractal frequency selective surface (FSS) based terahertz (THz) absorber. An equivalent circuit model (ECM) of the absorber is developed and impedance is obtained analytically. Further, an ECM-backed artificial neural network (ANN) approach is proposed and used to optimize the absorber to the desired frequency and bandwidth. The minimum mean square error between targeted and observed outputs is only <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.86\times10$ </tex-math></inline-formula> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-6</sup> . The maximum absorption of 99% is achieved at a central frequency of 1.5 THz. The tunability is accomplished by varying the chemical potential ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu _{c}$ </tex-math></inline-formula> ) of the graphene layer introduced in the proposed absorbing structure. The resonance frequency shifts to 1.66 THz from 1.39 THz with a change in <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu _{c}$ </tex-math></inline-formula> from 0.3eV to 0.6eV.The dual-band absorption peaks are noticed with fractal FSS based absorber within the operating frequency range of 0.1-4.0 THz. The maximum absorption of 0.99 and 0.98 at the resonant frequency of 1.0 THz and 3.1 THz with fractal FSS, respectively is achieved. Furthermore, the designed absorber works well for both perpendicular and parallel polarizations up to 60 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sup> angle of incidence. Moreover, the effect of structural parameters is also observed on the equivalent inductance and capacitance to understand the physics behind the structure. All the results obtained from ANN are verified with particle swarm optimization (PSO) and Whale optimization (WO) techniques. Good congruence among ANN, PSO, and WO results shows the effectiveness of the aforementioned technique. The possible applications of the proposed tunable absorber are THz imaging, sensing, detection, and stealth.

Symmetry-breaking manipulation in the design of multifunctional tunable frequency selective surface

The design and characteristics of a PIN diode based tunable Frequency Selective Surface (FSS) is provided. Spatial filtering, polarization filtering and polarization control are all features the FSS exhibits in the considered 2-14 GHz frequency band. The proper symmetries required for the different features are explained and controlled by a holistic approach that includes the effects of the biasing network in the design. Symmetry breaking due to the presence of the biasing network used to bias the active elements are compensated by predistorsion of the initial geometry of the main structure that presents rotational symmetry. The proof of concept presented here allows extending the method to achieve proper design goals.

A dual‐function switchable and frequency tunable active frequency selective surface

A dual-function switchable and frequency tunable active frequency selective surface (AFSS) is proposed in this article. The proposed AFSS consists of a dielectric substrate and metal patches on the top and bottom surfaces of the substrate, while varactor diodes and PIN diodes are soldered on the top and bottom layers, respectively. A simple bias network is designed to facilitate the function adjustment of the AFSS. By controlling the OFF and ON states of the PIN diode, the AFSS can switch between filtering and wave absorbing functions. Furthermore, by adjusting the bias voltage of the top varactor diode, the AFSS can achieve continuous frequency tunability. Simulation and experimental results confirm the dual-function switchable and frequency tunable performance of the proposed AFSS.

A novel 4-DOF wide-range tunable frequency selective surface using an origami “eggbox” structure

AbstractShape-changing mechanical metamaterials have drawn the attention of researchers toward the development of continuous-range tunable frequency selective surfaces (FSSs). In this paper, a novel tunable FSS utilizing an origami-inspired “eggbox” structure is presented featuring four-degrees of freedom that can change the frequency response of two orthogonal linear polarizations. The centrosymmetric “eggbox” structure can be folded or rotated along two axes that lead to unprecedented reconfigurability compared to traditional Miura-Ori-based structures which have fewer degrees of control. The utilized cross-shaped dipole FSS element shows enhanced bandwidth, support for orthogonal linear polarization, and ease of fabrication. The prototype is fabricated using a low-cost fully additive inkjet printing process with silver nanoparticle conductive ink. The outcome of this study shows a 25% frequency tunable range over two polarization directions. The design can be an ideal spatial filtering candidate for advanced ultra-wideband terrestrial and space applications.

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.

Tunable Frequency Selective Surface With Angular Stability

In this letter, a paradigm for designing tunable frequency selective surfaces (FSSs) with angular stability is presented. The tunable FSS is implemented by nonresonant subwavelength unit cells loaded with varactor diodes, which can be applied in a dynamic and efficient compensation mechanism. The red shifts introduced by varactor diodes are used to compensate the deviation under oblique incident angles. The proposed tunable FSS is a two-layer structure composed of an array of wire grid and an array of split square ring loaded two varactor diodes. The periodicity of one split square ring is same with that of 2 × 2 wire grids. The equivalent circuit based synthesis method is analyzed to investigate the underlying principle of the proposed FSS. The experimental measurement is carried out in C-band frequency, which demonstrates the feasibility of this method. The measured results are in good agreement with the simulations, manifesting the angular stability at 4.25 GHz.

A Cascaded Tunable Wide Stop Band Frequency Selective Surface with High Roll-off Band Edge

A Cascaded Tunable Wide Stop Band Frequency Selective Surface with High Roll-off Band Edge

Tunable Frequency Selective Surface Based on a Sliding 3D-Printed Inserted Dielectric

A novel mechanically tunable frequency selective surface (FSS) based on a sliding 3D-printed inserted dielectric is presented in this paper. This paper first introduces the working mechanism of the tunable FSS, which indicates that the variable capacitance is the key to tunability. Based on this, we design an all-metal bandpass FSS with a certain thickness and a matching cross-shaped inserted dielectric. By changing the insertion depth of the dielectric, the resonance frequency can be adjusted. To optimize the performance, an improved structure based on a hollow support and a tripole slot is proposed. A tunable range from 3.24 to 5.52 GHz (53%) and good angular stability is obtained in the simulation. To verify this idea, we fabricate a cross slot prototype by 3D printing and mechanical engraving. Measured results show that this prototype has a continuous tunable range of 3.61 to 5.89 GHz (34.4%).

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.

A Low-Power Tunable Frequency Selective Surface for Multiplexed Remote Sensing

In this interdisciplinary work, a low-power varactor-tunable frequency selective surface is presented for use with a multiplexed remote sensor system. Combined with a low power sensor and control system, voltage biasing of the proposed frequency selective surface panel can be used to modulate the center frequency of a reflection peak, which in turn may be detected by remote interrogation through a radar system. The control board and frequency selective surface were found to draw only $200~\mu \text{A}$ of current during typical operation from two 3V CR2032 button batteries, enabling continuous operation for weeks at a time (even longer if operated in wake-up mode) when used in conjunction with a suitably low power sensor. Centered at 3 GHz, the backscatter peak can be modulated across a range of 200 MHz, and the combined system was shown to successfully differentiate between various concentrations of ammonia under test conditions.

A Dual-Band Tunable Frequency Selective Surface With Independent Wideband Tuning

In this letter, a dual-band tunable bandstop frequency selective surface (FSS), which is capable of exhibiting independent tuning for the top and bottom layers, has been presented. The proposed design comprises periodic patterns of cross diagonals printed on the opposite sides of a thin substrate, where the varactor diodes are mounted symmetrically. Both the layers use novel biasing circuitries to independently regulate their corresponding responses, thereby resulting in two tunable stopbands. Each of these two stopbands has wide bandwidths (having fractional bandwidths of 110.71%) as well as offers large tuning ranges of 128.21% (with respect to their center frequencies). Furthermore, the geometry is low profile, polarization insensitive, and angularly stable. A prototype has been fabricated, and the varactors have been modulated through parallel biasing. The experimental results show a reasonable agreement with the theoretical responses, thus confirming the proposed concept.

Thermally Tunable Frequency‐Selective Surface Based on VO2 Thin Film

An approach to a design of tunable frequency‐selective surfaces (FSSs) for radio frequency (RF) domain based on vanadium dioxide (VO2) thin films is proposed. As an example, a thermally tunable annular slot FSS with integrated VO2 switchable elements for C‐band of the RF domain is described. Switching is based on abrupt change of conductivity of the VO2 film, initiated by heating from room temperature to a temperature of the metal–insulator phase transition of 44 °C. A tuning mechanism of the FSS allows continuous change of frequency response from room temperature state, where the FSS acts as a band‐pass filter, to high‐temperature state, where the FSS works as a reflector. In the process of tuning, a reflection coefficient at a central frequency of the filter can be increased from −26.4 dB up to −2.2 dB. The proposed approach can be used for the design of tunable FSS for applications that allow thermal control of the frequency response.

Study on Tunable Magnetized Plasma Frequency Selective Surface Using JEC-FDTD Method

The tunability of traditional frequency selective surfaces (FSSs) is inconvenient using metal material patch as periodic element, utilizing genetic algorithms to model FSS layers obtaining desired filtering electromagnetic waves effect. However, plasma tube is used as an element of the frequency selection surfaces substituting traditional metal patch in this article. A 3-D finite-difference time-domain (FDTD) formulation for dispersive media called the JE convolution (JEC) method is derived using the convolution relationship between the current density $J$ and the electric field $E$ to simulate the interaction of incident wave and magnetized plasma FSS. Numerical experimental results show that the electron number density of magnetized plasma has a significant effect on the resonance frequency, which can be modulated to the desired filtering by varying the density in the magnetized plasma elements. Whereas the magnetized plasma FSSs behave exactly like a perfect electric conductor (PEC) increasing the electron number density to a certain degree. Thus, modulation based on the ionized electron number density of the magnetized plasma can be designed into a tunable plasma FSS. Both the collision model and the noncollision model of the magnetized plasma are proposed to research electromagnetic transmission characteristics of FSS. It shows that the collision frequency only affects the reflectivity while has no effect on the resonance frequency in the following numerical experiments. In addition, magnetization intensity of the plasma FSS is also taken into consideration. The users obtaining the desired resonance frequency and transmission characteristics of plasma FSS by simply tuning the plasma’s electron number density, collision frequency, and magnetization intensity. So the magnetized plasma FSS can not only improve the electromagnetic shielding effect but also facilitate the modulation and rapid response to achieve the desired filtering effect. What is more, the magnetized plasma FSS can be turned off to achieve transparency, which greatly expands the universality of magnetized plasma FSS.