Study of dual-frequency absorbing materials based on frequency selective surfaces

This paper presents the results of our research in the field of frequency selective surface in recent years. Aiming at the traditional passive frequency selective surface (FSS) in this paper, we mainly do two aspects of research work, which is the research of the small frequency selective surface and the research of the ultra wide band frequency selective surface. The center frequency value of the miniaturized FSS is only 2.47GHz, and the relative size of the unit is only 0.0494λ×0.0494λ and. That is to say, the unit size is less than λ/20. Stop band for the double layer structure FSS we proposed is from 11.03 to 13.98 GHz, with a relative bandwidth 23.6%. Pass band for the three dimensional structure FSS we proposed is from 8.1 to 14.6GHz, with a relative bandwidth 57.3%. The miniaturized FSS structure and wide band FSS structure have high practical application value.

Frequency selective surface (FSS) is a spatial filter that has frequency selection characteristics for electromagnetic waves with different operating frequencies, polarization states and incident angles. At the same time, frequency selective surface has the advantages of simple structure and easy processing compared with the traditional filters. FSS have been investigated for many decades and extensively utilized in various applications. It can used in radar radomes to reduce the radar cross section (RCS) of an array antenna, as a reflector to transmit and reflect electromagnetic waves in different frequency bands. Normally, electromagnetic waves are incident at an oblique Angle onto the target surface. Therefore, the research of FSS absorbers with stable absorbing performance under oblique incidence has tremendous practical application value. This paper proposes a new design scheme for designing a FSS absorber with stable oblique incident response range from 0° to 70°. A single-square-loop array with lumped resistors and square patch printed on the dielectric substrate is proposed, and it exhibits a characteristic impedance matching for wide incident angles from 0° to 70°. The design scheme of the FSS layer under the condition of oblique incidence is introduced in detail, and the equivalent circuit is adopted to make the design clearer and more direct. The results show the absorption bands with oblique incident angles from 0° to 30°, 30° to 50°, and 50° to 70° reflection coefficients below –10 dB are 3.44 to 9.03 GHz, 4.49 to 9.06 GHz, and 4.87 to 8.96 GHz for TE polarization, respectively. The absorber proposed in this paper will contribute much to the development of ultra-wide-angle oblique incidence.

Thicknesses of window glasses, radomes, plastic cases of wireless systems are comparable with the wavelength in the millimeter-wave band. The reflections from the dielectric plate surfaces are canceled out of phase when the thickness is identical to a half of the wavelength. However, they are not canceled at any other frequencies except integer multiple of a half wavelength. Therefore, return loss is a serious problem in the millimeter-wave band. A frequency selective surface (FSS) loaded to the dielectric plate controls reflection amplitude and phase, thus, return loss is canceled for various dielectric thickness and frequency. A complex scattering matrix (S-matrix) of FSS is represented by only reflection amplitude |S 11 | from the condition of zero thickness of FSS expressed by shunt suceptance. The total reflection is calculated by converting S-matrices of FSS and a dielectric plate to a transfer matrices (T-matrix) and synthesizing them. We propose to find the required characteristics of FSS from the desired characteristic of whole structure. Broadband characteristic was designed by inserting FSS at the center of the dielectric plate.

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

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.

Frequency Selective Surface (FSS) is a specialized structure used in the field of electromagnetic waves and radio frequency (RF) engineering. It is designed to exhibit selective transmission or reflection properties based on the frequency of the incident electromagnetic waves. This article describes the design, construction, and analysis of a textile-based band-stop frequency selective surface for use in the highly EM-polluted GSM, Wi-Fi, LTE, and WiMAX bands. A full-wave EM solver called CST Microwave Studio was used to develop and simulate the unit cell of the proposed FSS at the relevant frequency. In this study, embroidered and screen-printed textile based FSSs were designed. According to the results of this study, it was demonstrated that both embroidery and screen printing FSSs exhibit resonance at a frequency of 3.5 GHz. The screen printing method yielded the best results in terms of resonance frequency sensitivity, while the embroidery method showed a resonance frequency shift. It was observed that the stitch directions and density are important parameters in the embroidery method. Gaps between the embroidery paths in the production of embroidered FSSs resulted in differences from simulations due to the disruption of the structural integrity of the unit cell. Consequently, textile-based FSSs offer advantages over traditional FSSs. This study highlights the potential of textile FSSs as an effective means of reducing electromagnetic pollution, and suggests that further improvements in the design and production processes of textile FSSs can be made.

This paper proposes a technique that uses the small-metal width of a frequency-selective surface (FSS) element to suppress the change in beamwidth due to the undesired reflection from other FSSs. FSSs reflect low-level electromagnetic waves that are expected to pass through essentially the FSSs outside of the desired reflection bands, and generate these low level reflections from other FSSs as undesired reflections. Electromagnetic field analysis based on a dual-band antenna structure shows the relationship between the metal width and the reflection coefficient of the FSS element, and the relationship between the metal width and the beamwidth in the desired band and the other band. This paper clarifies that the small-metal width of the FSS element can be used to reduce the change in beamwidth outside the desired band.

FSS superstrate antenna for satellite cynosure on IoT to combat COVID-19 pandemic

A new bandpass frequency selective surfaces (FSS) is designed and the composite system (patch antenna covered by an FSS) is analyzed using the finite difference time domain (FDTD) method. Far-field patterns of the patch antenna, with and without FSS radomes, are obtained and compared with each other. The feed is modeled by using a coaxial line extension, which enables us to compute the return loss of the patch and the composite system accurately. Numerical results show a frequency shift in the return loss response of the patch in the presence of the FSS cover.

This study proposes a rational optimum design procedure for broadband radar absorbers composed of a square-loop-shaped resistive frequency selective surface (FSS) that are placed on an E-glass fabric/epoxy laminate and grounded by Cu foil covering the X-band (8–12 GHz). A rational optimum design procedure for the broadband absorbing performance was presented based on the analytical and numerical illustration. The effects of the surface resistance of the FSS and dielectric substrate on the design process and performance of the absorber are examined. The resistive FSS was fabricated using screen-printing technology with carbon-type ink as a demonstration. The demonstration samples were developed considering the sectional profile of the circuit lines composing the printed FSS. It revealed that the experimental reflection loss encompassing the frequency range over the entire X-band was repeatable and similar to the simulation result.

Performance analysis of frequency selective reflectors based on rectangular loop pair for wideband applications

Metamaterials (MMs) are periodic and/or non-periodic subwavelength structures with a unique response to electromagnetic (EM) waves compared to conventional materials. MMs possess negative permittivity and/or permeability; the combination of both negative is not found in conventional materials. The shape, dimensions, and materials used in the MM determine its ultimate working frequency and response. Owing to their peculiar properties, MMs have found new-age applications such as cloaking, perfect absorber, hyperlenses, modulators, holography, etc. Graphene is a remarkable material with distinctive properties, including excellent thermal and electrical conductivity, very high mechanical strength, along with having a zero bandgap. Further, one of the most important properties of graphene, which has enabled it to be widely used as/along with MMs, is its tunable response to EM waves via a change in conductivity when its chemical potential is varied by applying a bias or doping. Frequency selective surfaces (FSSs), or flat MMs, and MMs utilizing graphene, find applications for defense and security systems in infrared, terahertz, and microwave frequency range. The broad areas of application include stealth, communications, and imaging. Stealth applications aim to prevent radar detection by absorbing or dissipating the incoming EM waves using graphene MMs or FSSs [which act as radar absorbing surfaces (RASs)]. Another important stealth application, although a futuristic one, is cloaking. Fast, secure, and high volume data transfer is critical for any security system, and graphene MMs can improve upon the existing technologies, particularly in the terahertz frequency range. Graphene MMs can be used to develop lenses, mirrors, hyperlenses, modulators, etc., for infrared (thermal) and terahertz imaging systems. In the current chapter, the basic properties of graphene, the working of FSSs and RASs, and applications of graphene-based MMs in defense and security systems are discussed.

This paper deals with a novel Frequency Selective Surface (FSS) Electromagnetic shield that operates in the low 5G vertical applications. This FSS is designed for the purpose of serving as an electromagnetic shield to eliminate EMI. The FSS designed operates at a frequency of 5GHz with a bandwidth of 1.45GHz providing a shielding of more than 40dB. The proposed FSS uses a polyester sheet as substrate and uses silver for the metallization of the FSS. Since the proposed FSS is symmetric, it behaves similarly when a Transverse Electric (TE) or a Transverse Magnetic (TM) wave is incident. The structure was designed and simulated using CST software. The results show that this FSS blocks electromagnetic radiation at lower frequencies as intended.

A simple microstrip patch antenna was first used to study and its resonant frequency and bandwidth were observed. Based on the observations a Frequency Selective Surface (FSS) is proposed to improve the bandwidth, antenna gain and return losses of a patch antenna. Fabricated results of the return losses, radiation pattern and antenna gain of this patch antenna are also presented. Through the Fabricated results it has been proven that the microstrip antenna implanted with an FSS structure containing fractal cross elements has been converted into an antenna with good performance along with sufficient bandwidth and higher gain.

Wearing electromagnetic protective equipment enables the absorption or reflection of electromagnetic waves that are incident on the human body, thereby reducing the risk of health issues associated with electromagnetic pollution. Although fabric‐based metamaterial absorbers can overcome the drawbacks of traditional electromagnetic shielding gear, such as excessive weight and low efficiency, achieving broadband absorption under low‐profile conditions remains challenging due to the inherently low dielectric constants of fabric materials. This study proposes a lightweight metamaterial absorber utilizing wool felt as the dielectric substrate and a polyethylene (PE) conductive film as the resonant layer for the frequency‐selective surface (FSS). Leveraging the excellent conductivity and high sheet resistance of the PE film, the nested square‐ring structure of the FSS exhibits strong resonant behavior. As a result, the absorber demonstrates absorption rates exceeding 90% across the 3.91–27.52 GHz frequency range, achieving a relative absorption bandwidth of 150.24% with an overall thickness of only 0.109 . Experimental measurements in a microwave anechoic chamber validate the absorber's angular and polarization insensitivity, in conjunction with excellent flexibility that maintains high absorption efficiency even under mechanical deformation. Furthermore, numerical simulations confirm that the absorber retains stable performance under humid environmental conditions.