Composite Frequency Selective Surface Research Articles

Electromagnetic wave characteristics of composite frequency selective surfaces with patch‐grid‐patch type structure fabricated by e‐beam deposition

AbstractIn this study, a new method of fabricating a composite frequency selective surface (FSS) using an e‐beam evaporator is proposed. The FSS is designed and fabricated as a part of the structure surface. A new concept of FSSs integrated into a composite skin structure provides a design that is electrically and structurally effective. This manufacturing process has the advantage of direct application of FSS function in a real structural surface with varied conformal skin. The multilayered FSSs of the patch‐grid‐patch type consist of a copper layer as a capacitive or inductive surface on a glass/epoxy pre‐impregnated (prepreg) substrate. The copper layer forming the FSS is manufactured using the e‐beam deposition on a glass/epoxy prepreg. The bandwidth of the designed FSS increases three times. The difference between the simulated and the measured results is due to the different evanescent wave interactions between the capacitive layers and inductive layer such as the local misalignment of the patch and grid, size of the specimen, and change in the thickness of the dielectric substrate after the manufacturing process. The bandwidth is successfully modified by correcting the design parameters such as the period, patch gap, grid width, and dielectric thickness. The band‐pass filtering and skirt characteristics appear in the experimental results, which agree well with the simulated results. This method is remarkably simple, and the manufactured FSS can function as both a structural skin and an FSS. Moreover, it has the merit of manufacturing simplicity and realization of structural shape.

Circular-to-linear polarization converter based on composite via-coupled patch frequency selective surfaces

In this paper, we present a design of circular-to-linear polarization converter operating in C-band based on a composite frequency selective surface which consists of two types of via-coupled patch modules. The building unit cell is built with four modules, and each one is a truncated patch pair printed on the double-layer substrate and vertically connected by a metal via-hole. A perforated common ground plane is sandwiched between the double layers to support transverse electric and magnetic mode coupling between the front and back patches. The front diagonally truncated patch array responds to the incident right-handed circularly polarized wave, and it is identically converted into general right- and left-handed elliptically or circularly polarized waves with same phase shifts, amplitudes and axial ratio, thus a resultant linear polarized wave is generated in forward transmission mode.

A Broadband Circular Polarizer Based on Cross-Shaped Composite Frequency Selective Surfaces

A cross-shaped composite with frequency selective surfaces is applied in the design of a broadband polarizer. The structure is multilayer, and the unit cell of each layer consists of a composite cross and a metal strip. The design takes advantage of both the similar amplitude and the stable phase difference of nearly 90° of the two orthogonal components of transmitted waves. The bandwidth of the polarizer reaches 74%, from 5.15 to 11.20 GHz, which is able to generate circular-polarization waves from incident linear waves. A sample polarizer is fabricated and tested. According to the measurement results, the polarizer operates from 5.75 to 11.20 GHz at normal incidence. In addition, its working frequency band can still cover the range of 5.80 to 11.00 GHz when the incident angle increases to 20°.

Miniaturised meandered square frequency selective surface on a thin flexible dielectric with selective transmission

A low profile, thin, flexible dual layer frequency selective surface (FSS) operating in L-band has been designed and fabricated using screen printing technology. The composite FSS structure was achieved by meandering the basic λo/22 square loop at its four corners filling the space on the top layer and half period offset pattern on the bottom, which produces a reject band at 1.89 GHz. The frequency response of the FSS structure for both transverse electric and transverse magnetic polarisations and various angles of incidence using measured and simulated results for single as well as double sided FSS show good agreement. The main purpose of this paper is to propose a design applicable for FSS windows with selective transmission and shielding at frequencies such as mobile communication signals (GSM) working around 1.89 GHz.

Acrylic substrate based low profile wearable button antenna with FSS layer for WLAN and Wi‐Fi applications

ABSTRACTIn this letter, a low profile circularly polarized wearable antenna with the appearance of a button on clothing for Wi‐Fi and WLAN applications is presented. The radiating elements of the antenna are made of copper sheet while a transparent acrylic fiber sheet material is used as substrate and fed by a coaxial line through the ground plane. The proposed antenna is simulated by Ansoft high frequency structure synthesis finite element method‐based (HFSS FEM)‐based EM simulator and has been optimized to operate within the Wi‐Fi and WLAN frequency bands (5250–5850 MHz) with center frequency of 5.25 GHz. A split ring‐shaped slot‐based frequency selective surface (FSS) is designed to enhance the gain of the proposed antenna. The peak gain of the antenna is found to increase from the conventional antenna gain of 2.1 to 5.1 dBi with the superstrate along with a bandwidth enhancement of 290 MHz. The composite FSS antenna is fabricated and the maximum measured gain of 5.1 dBi matches well with theoretical value. The antenna gets miniaturized around 34% by the slits with its circular polarization characteristic well maintained. © 2015 Wiley Periodicals, Inc. Microwave Opt Technol Lett 57:1033–1038, 2015

Combined‐element frequency selective surfaces with multiple transmission poles and zeros

A novel combined-element frequency selective surface (FSS) is proposed for multi-band applications. In this design, complementary square loops and meandering slots are combined to construct the composite FSS. The proposed FSS provides three transmission poles and two transmission zeros, transmitting the desired signals at 8.5, 11 and 16 GHz while reflecting the unnecessary signals at 9.6 and 13.1 GHz. This FSS has the advantages of flexible and simple structure, easy processing and low profile compared with traditional multi-pole FSSs. The design strategy of the proposed FSS and the associated equivalent circuit model are provided to analyse the frequency characteristics. A prototype is fabricated and measured, showing a superior and stable frequency response in case of oblique incidence for both the transverse-electric (TE) and transverse-magnetic (TM) polarisations. Good agreement between the simulated and measured results is observed. Finally, the higher-order FSS with good frequency selectivity is investigated.

Electromagnetic wave characteristics of second‐order composite frequency‐selective surfaces fabricated by e‐beam evaporator

A composite frequency-selective surface (FSS) fabricated by using an e-beam evaporator is proposed. This manufacturing process is advantageous in that it can directly achieve an FSS function for real structural surfaces with various skin conformations. A multilayered patch–grid–patch-type FSS consisting of copper layers as capacitive or inductive surfaces and a glass/epoxy prepreg as the substrate is produced. It operates at 9–10 GHz for dual polarisation. This technique can be used to manufacture a stealth technology for communicating in a specific frequency range.

In brief

PAGE 189 A manufacturing process for composite frequency selective surfaces (FSSs) is presented in work from Korea. The simple method, using an e-beam evaporator, copper layers and glass/epoxy prepregs can produce an FSS that can also function as a structural skin. The researchers believe the method can be used to manufacture a ‘stealth’ technology for communication in a specific frequency range. PAGE 153 Researchers in the UK present a complete four scale continuous wavelet transform (CWT) chip for real-time, multi-level, advanced analysis of ECG signals using only 1.3 nW of power. This is the lowest reported power consumption for a complete wavelet filter bank, and the first demonstration of the use of pico-amp scale currents in a 0.18 µm CMOS process to provide complicated signal processing functions. The FSS produced can also be used as a structural item PAGE 144 Work from Korea has studied the use of the human body as the transmission medium for wearable and implantable medical devices in an RF body area network (BAN). The feasibility of this method was studied using deformable electrodes in the ear canal for in vivo measurement of transmission loss for this mode of transmission between the ear and the wrist and chest. The CWT chip has the lowest power consumption to date and shows the first use of pico-amp currents in 0.18 µm CMOS signal processing PAGE 136 A method for frequency-controlled beam-scanning in the THz band based on a diffraction enhancement grating reflector is proposed in work from China. The concept is verified with a 0.2 THz device using a bilayer structure to introduce multiple reflections to suppress the specular beam. The work aims to provide an alternative to phased array scanning based on frequency sweeping for low cost THz video systems. The work studies feasibility of using of the body itself to transmit RF BAN signals between devices PAGE 146 Researchers in Korea have implemented a precedence walking assistance mechanism in an exoskeleton without using bio-signals based on human gait pattern analysis. They found that, using the proposed method, the operator could reduce muscle force required for step initiation. Using a bilayer structure to introduce multiple reflections to suppress the specular beam yielded diffractive efficiency over 89% Using the no-bio-signal precedence walking assistance method the operator does not need to use as much force to initiate each step

Electromagnetic transmission characteristics of composite frequency selective surfaces coated with conductive polymer–silver paste

Abstract In this work, a kind of conductive polymer–silver paste was used to fabricate composite frequency selective surfaces (FSSs) with four-legged slot elements. Screen printing and 3D engraving process were carried out to coat the conductive polymer–silver paste onto the fabric preform and sculpt composite panels into composite FSSs, respectively. The substrates of the composite FSS were carbon and quartz glass fabrics, respectively. The equivalent electrical conductivity of the composite material FSS was measured and the results showed that it increased significantly after the fabrics printed by the conductive silver paste. Free space method was adopted to test the electromagnetic transmission characteristics of the composite FSS. Experimental results showed that the conductive silver paste was effective to decrease the minimum transmission loss of the composite FSS and the minimum transmission loss of the quartz glass fabric composite FSS was lower than that of the carbon ones. A finite element model was put forward to calculate the electromagnetic transmission characteristics of the composite FSS. The calculated results were in agreement with the experimental data. Effects of the equivalent electrical conductivity and the thickness of the composite substrate on the minimum transmission loss of the carbon fabric composite FSS were investigated.

The Cascaded Effects in the Frequency Selective Surfaces Sculpted from Carbon Fabric Reinforced Epoxy Composite Panels Coated with Conductive Silver Paste

In this work, a kind of conductive polymer-silver paste was used to fabricate carbon fabric composite frequency selective surfaces (FSS) with four-legged slot elements. Screen printing and 3D engraving process were carried out to coat the conductive polymer-silver paste onto the carbon fabric preform and sculpt composite panels into composite FSSs, respectively. Free space and finite element methods were carried out to measure and calculate the electromagnetic transmission characteristics of the composite FSS, respectively. The calculated results are in good agreement with the experimental data. The cascaded effect in double-layered FSS were investigated by finite element method and design of experiment technique (DOE).

The use of cutting carbon fiber fabric/epoxy composites as band-pass frequency selective surfaces

Sandwich structures with frequency selective surfaces have been widely used to fabricate low-observable radomes. Traditional frequency selective surfaces were made of metals, such as copper and aluminum. There were problems when used the frequency selective surfaces with the sandwich structures, such as a bonding layer and thermal mismatch. Because of the thermal mismatch between the sandwich structures and the metals, mechanical properties of the sandwich structures usually decreased. To avoid these disadvantages, a new type of composite frequency selective surface was put forward in this study. A four-legged slot-array frequency selective surface and a square-aperture array frequency selective surface were fabricated by cutting carbon fiber/epoxy composites. Free-space method and finite-element method were carried out to evaluate the electromagnetic transmission characteristics of the frequency selective surface specimens, respectively. Results show that the composite frequency selective surface with four-legged slot array can realize the function of frequency selection, and its minimum transmission loss can be decreased by increasing the electrical conductivities of the composite material, adjusting the thickness, and increasing the aperture-to-cell ratio of it. For the composite frequency selective surface with square-aperture array, grating lobes come closely after its resonant frequency, although its minimum transmission loss is very close to zero.

A FSS WITH STABLE PERFORMANCE UNDER LARGE INCIDENT ANGLES

In this paper, we propose a novel frequency selective surface (FSS) with stable performance under large incident angles. The FSS is composed of hexagon metallic lines and hexagon patches. Using such a hexagon arrangement, the periodicity size could be miniaturized and thus the FSS unit cell is compact. The composite FSS has an excellent stability under large incident angles. In the passband 10.58{ 11.06GHz, the insertion loss is still less than i1dB for both TE and TM polarizations, even under incident angle up to 85 degree. Both the design procedure and experimental results of the novel FSS are presented and discussed.

The Use of Fabric Composite Frequency Selective Surface in Sandwich Structures for Low-Observable Radomes

In this work, the hybrid fabrics of carbon and quartz fibers as filters were adopted in PMI core sandwich structures for low-observable radomes. The advantages of fabric composite frequency selective surfaces (FCFSSs) are easily fabricated by braiding and shaped into curved surfaces. This paper mainly studied the electromagnetic (EM) transmission characteristics of the low-observable radome wall with composite FSS by 3-dimensional electromagnetic analysis software. First of all, the accuracy of the simulation model composed of up and down composite face sheets, a layer of composite FSS and PMI core was validated by comparing with experimental results. Then, the studies on electromagnetic transmission characteristics of the sandwich structure with composite FSS and the sensitivity analysis about the thickness and permittivity of face sheets and PMI core were carried out. The results show that the sandwich structure with single layer composite FSS can realize the function of frequency selection, limit the reflection at the resonance frequency and decrease the sensitivity of EM transmission loss about the thickness of sandwich structures.

Dispersive Periodic Boundary Conditions for Finite-Difference Time-Domain Method

A dispersive periodic boundary condition (DPBC) is developed for the finite-difference time-domain method to analyze periodic structures with dispersive media on the boundaries of a unit cell. The formulation is based on the auxiliary differential equation (ADE) method with a two-term Debye model and the constant horizontal wavenumber approach. The developed formulation is easy to implement and is efficient in both memory usage and computational time. The validity of this formulation is verified through several numerical examples such as infinite dispersive slab and sandwiched composite frequency selective surface (FSS) structure.

Characterization of Switchable Substrateless Frequency Selective Surfaces

In this paper, a new means for characterization of switchable substrateless frequency selective surfaces (FSS) was demonstrated with numerical and experimental approaches. The diodes model loaded active doubly periodic flat strip FSS acts as a dynamic screen, in which its surface reflectivity and transmittivity can be switched alternately and tuned through d.c. bias voltage control. The properties of FSS layer are characterized using a specially designed parallel plate waveguide (PPW) simulator that permits normal incidence excitation of the FSS under test and the responses are found between 3 to 5 GHz. It is shown that by means of d.c. bias control, the screen can be utilized in, (a) transmission mode as a dual band electromagnetic shutter, or, (b) reflection mode, as an amplitude shift keying (ASK) spatial modulator or dual band reflection canceller. In addition a hybrid de-embedding technique is employed to remove the influence the PPW simulator and reveal the true property of substrateless FSS. Typical experimental results are presented and validated against a new 3D electromagnetic (EM) modeling led de-embedding method for the composite FSS layer and PPW structure.

Low-observable radomes composed of composite sandwich constructions and frequency selective surfaces

In this work, low-observable stealth radomes composed of composite sandwich constructions and FSS (Frequency Selective Surfaces) were developed for selective wave transmission and reflection characteristics. The FSS was composed of a copper foil and a polyimide film. The sandwich constructions were composed of plain weave fabric E-glass/epoxy composites and PVC foams, which protect the radar antenna in the radome from external loads. The electromagnetic transmission characteristics of the low-observable radome wall were numerically calculated with 3-dimensional electromagnetic analysis software, and compared with the measured results by the free space measurement system. Also the mechanical properties of the sandwich constructions were investigated.

EM characteristics of the RAS composed of E-glass/epoxy composite and single dipole FSS element

From the methods to reduce radar cross section (RCS) such as shaping of the target, radar absorbing material (RAM), and radar absorbing structure (RAS), the RAS composed of frequency selective surface (FSS) screens and low-loss composite materials is used widely because the FSS screen transmits or reflects electromagnetic (EM) waves selectively and the composite material withstands external loads. In this study, the RAS composed of the E-glass/epoxy composite and single dipole FSS element was fabricated by printed circuit board (PCB) manufacturing process, and their EM transmission characteristics, such as a resonant frequency, a minimum transmission loss, and a transmission bandwidth, were measured in the X-band frequency range by the free space method with respect to the size of dipole element and its periodicity of array.