Uniformity improvement of microwave heating with switchable frequency selective surface

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.

This paper proposes a novel switchable Frequency Selective Surface (FSS) design. The designed FSS unit size is only 0.12 λ 0 (where λ 0 represents the wavelength corresponding to the lowest resonant frequency in free space), and its angle stability can reach up to 65°. Meanwhile, it can form stable resonances at the three commonly used frequency bands of 2.4, 3.3, and 5.2 GHz in communication systems. During the switching process, the difference in transmission coefficients between different states exceeds 15 dB, and the insertion loss in the transmission state is stably controlled within the range of 0.92–2.15 dB. Additionally, this design supports independent control of the electromagnetic responses to transverse electric and transverse magnetic polarized incident waves in four modes. After verifying the design feasibility through an equivalent circuit model, a 100 mm × 100 mm FSS prototype was fabricated, and its performance was verified through experimental tests.

A patch-slot-patch frequency selective surface (FSS) exhibiting bandpass response is proposed in this paper. The FSS unit cell is composed of two cross-dipole shaped patches and a cross-dipole shaped slot in between. The proposed FSS exhibits bandpass response with two closely spaced transmission poles around 3 GHz and 3.5 GHz. A 3-dB transmission bandwidth of upto 20% is achieved for the FSS. The proposed design has a thickness on the order of 0.032 λ <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</inf> . The paper presents with parametric studies how the poles can be tuned individually. The transmission phase of the FSS varies linearly towards zero with frequency that makes the FSS useful in designing Filter-Antenna when placed as a superstrate above a broadside radiator.

CPW-fed dual-element metamaterial inspired multiband antenna using simple FSS for gain enhancement

In this paper, frequency selective surface (FSS) with relative zero-phase shift is realized and characterized. The FSS unit cell is designed by adding cross aperture to the cross loop slot at its center, and the four ends are placed by Y-shaped slots. Compared with normal FSS, the FSS with the relative zero-phase shift avoid the aberrance of the main lobe direction on the radiation pattern of antenna, when the FSS is designed for the radome. Numerical simulations are carried out to validate wider bandwidth and better angle stability. It is a miracle that the epsilon-near-one(ENO) metamaterial properties are presented, so the screen of these FSSs have many properties similar to air. The near-field and far-field are almost not affected by the screen for the cylindrical wave.

A novel miniaturised frequency selective surface (FSS) unit cell is proposed based on the Hilbert fractal and 2.5-dimensional (2.5-D) metallic loop. For the increase of current path, both sides of substrate are used which are connected together by four vias. The dimension of FSS unit cell is obtained as 0.041λ0 × 0.041λ0 at the frequency of 2.47 GHz. The transmission coefficient is insensitive and independent of angle and polarisation of incident wave. A prototype model of the proposed FSS is fabricated and measured for the proof of concept. It is shown that it achieves better miniaturisation relative to the FSS reported in the literature.

This paper presents the first additively manufactured, single-layer, polarization-insensitive, fluidically switchable band stop frequency selective surface (FSS) for electromagnetic (EM) shielding applications. The unit cell features a meandered square-loop metallic pattern with a fluidic cavity embedded within the substrate via fused deposition modeling. Frequency tuning is realized by controlling the fluidic state of the cavity through an automated pump system. In the absence of water, the structure resonates at 3.5 GHz; introducing distilled water into the cavity lowers the resonance to 2.6 GHz, achieving a 29.5% tuning range. The proposed FSS provides a minimum shielding effectiveness of 30 dB for both cases. An equivalent circuit model is developed to interpret the tuning mechanism, and both simulation and experimental results confirm the effectiveness of the design. The proposed FSS maintains stable performance up to 60 ∘ under both TE and TM polarizations. With its simple, low-profile, single-layered design, this FSS offers strong potential for EM shielding and interference mitigation applications.

The design, analysis, and measurement of a 90° polarization converting frequency selective surface (FSS) are implemented for incident wave polarized in arbitrary azimuth angle. The FSS unit cell is achieved by two orthogonally arranged monopole-coupler-monopole (MCM) module pairs. The incident linearly polarized (LP) wave is polarized along $u$ -axis of the local orthogonal coordinate system ( $\boldsymbol{u}, \boldsymbol{v}, \boldsymbol{z}$ ), it can be effectively converted into a wave polarized along $\boldsymbol{v}$ -axis, where the azimuth angle between ${\boldsymbol{u}-}$ and $\boldsymbol{x}$ -axis of ( $\boldsymbol{x}, \boldsymbol{y}, \boldsymbol{z}$ ) coordinate system is arbitrary. The polarization conversion mechanism is explained by the fact that the decomposed components along ${\boldsymbol{x}-}$ and $\boldsymbol{y}$ -axis of the incident wave are independently converted into ${\boldsymbol{y}-}$ and $\boldsymbol{x}$ -axis, respectively, by the co-orientated MCM modules, and then combined into a wave polarized along $\boldsymbol{v}$ -axis in the outgoing wave space. A prototype of the polarization FSS is fabricated and measured. The measured results have good agreement with the simulated ones.

Wave concept iterative procedure (WCIP) is used for the determination of scattering characteristics of single and double metallic ring frequency selective surfaces (FSS). In the WCIP method, the analysis of the FSS is reduced to that of the unit cell. Basically, the WCIP method consists of scattering the incident waves at the interface and the reflection of the scattered waves by the closing ends of the periodic wall box containing the FSS unit cell. The use of the two dimensional fast Fourier transform algorithm results in a reduction of the running time and memory space. FSSs based on single and double metallic rings are analyzed by the WCIP method. A good agreement is observed between the WCIP results and the experimental results. © 2008 Wiley Periodicals, Inc. Microwave Opt Technol Lett 50: 1324–1328, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.23383

This work proposes a frequency‐selective surface‐based radome structure to selectively allow the radar altimeter operating band and also minimize the interference from Sub‐6GHz 5 G frequency bands. A Jerusalem cross and combinational loop (JCL) based frequency selective surfaces (FSS) unit cell of overall dimension 0.37λ was synthesized using a thin flexible substrate for use in fuselage of airborne systems. The proposed FSS unit cell exhibits transmission zero at 3.61 and 6.025 GHz and highly selective pass band at 4.3 GHz. The effect of bends and angle of incidence on the frequency response of the synthesized FSS was studied in transverse electric and transverse magnetic modes. A lumped element‐based equivalent circuit model for the proposed design was also presented. Finally, the fabricated prototype was experimentally validated in an Anechoic chamber setup and the suitability of the proposed FSS layers for aviation radar systems was confirmed.

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.

In this paper, a graphene-based frequency selective surface (FSS) is proposed. The proposed FSS exhibits a tunable bandpass filtering characteristic due to the alterable conductivity of the graphene strips which is controlled by chemical potential. Based on the reconfigurable bandpass property of the proposed FSS, a cylindrical antenna radome is designed using the FSS unit cells. A conventional omnidirectional dipole can realize a two-beam directional pattern when it is placed into the proposed antenna radome. Forward and backward endfire radiations of the dipole loaded with the radome is realized by properly adjusting the chemical potential. The proposed antenna radome is extremely promising for beam-scanning in terahertz and mid-infrared plasmonic devices and systems when the gain of a conventional antenna needs to be enhanced.

A wideband frequency selective surface (FSS) absorber implementing lossy ink layer for electromagnetic absorption is presented in this paper. The absorber is designed on a metallic ground plane which acts as reflector. A thin layer of lossy ink material is used in the form of double square loops which serves as an absorber at the top surface of the substrate. The FSS unit cell has an overall size of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$18\mathbf{mm x }18\mathbf{mm x }0.8\mathbf{mm}$</tex> with outer and inner square loops whose dimensions are 15mm and 10 mm, respectively. The results show that the proposed absorber operates between 8 GHz to 14 GHz covering X and lower Ku bands. It is shown that this design provides a fractional bandwidth of 54.5% and 98% absorptivity in the whole band. The wideband and high absorptivity of the proposed FSS absorber demonstrates its potential for electronic warfare and other strategic applications.

A frequency selective surface (FSS) that is electronically switchable between reflective and transparent states is tested. It can be used to provide a spatial filter solution to reconfigure the electromagnetic architecture of buildings. The FSS measurements show that the frequency response of the filter does not change significantly when the wave polarization changes or the angle of incidence changes up to ±45° from normal. The FSS is based on square loop aperture geometry, with each unit cell having four PIN diodes across the aperture at 90 degree intervals. Experiments demonstrated that almost 10 dB additional transmission loss can be introduced on average at the resonance frequency, for both polarizations, by switching PIN diodes to on from off state.

In this paper, a rectangular metallic ring FSS (Frequency Selective Surfaces) with parallel metallic strips for multi band applications are characterized by the WCIP (Wave Concept Iterative Procedure) method. The proposed FSS structures are made of metallic elements that are simultaneously independent in the spatial domain and uncoupled in the spectral domain. Thus the FSS multiband behavior can be achieved only by a building process based on independent metallic elements in both domains. The resonant characteristics of the proposed FSS are determined. The structure shows three resonant frequencies when the incident wave is x-polarized and three resonant frequencies when the source is polarized in the orthogonal direction. A variation in the length of the strips and the rectangular ring allows a fine tuning in the FSS resonant frequencies to give raise to tuned bandwidths. The suppression of a resonant frequency can be insured either by the elimination of the responsible metallic strip or by the insertion of a PIN diode switch. Three FSS structures are manufactured and characterized. The FSS bandwidth can be enhanced in both perpendicular FSS exciting source directions by only varying the strips lengths and the ring dimensions to the corresponding resonant frequencies. A good agreement is obtained when the WCIP results are compared with the simulations of COMSOL Multiphysics software and experiment.