Interview

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 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.

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.

In this letter, a novel bandpass frequency selective surface (FSS) with miniaturized periodic element is presented. The proposed FSS is printed on one face of a single-layer substrate with a relative permittivity of 2.65. The novel FSS has promising miniaturization characteristics with the unit-cell dimension 0.058λ×0.058λ, where λ represents the free-space wavelength corresponding to the resonant frequency. The FSS exhibits excellent stability under different polarizations and incident angles. Both the simulation and measurement results validate the stable performance of this FSS.

We present an efficient method for modeling 3-D doubly periodic structures over a wide frequency range based on hybrid finite element/boundary integral (FEBI) methods. The 3-D periodic structures can be represented as non-orthogonal lattices composed of inhomogeneous bianisotropic media with arbitrary metallic patches. Triangular prismatic volume elements were utilized to mesh the unit cell, which provided a great deal of flexibility in modeling complex planar geometries of arbitrary shape. The adaptive integral method (AIM) was applied to speed up the calculation of the matrix-vector product for the BI part within the iterative solver. Furthermore, a model-based parameter estimation (MBPE) technique was proposed for the wide-band interpolation of sparse impedance matrix elements in the BI portion for near field components that were used in the AIM. The accuracy and efficiency of the proposed algorithm is demonstrated by the presented numerical results. To demonstrate the flexibility of the proposed method in modeling arbitrarily shaped elements of frequency selective surfaces (FSS), we included simulation results for an FSS with a non-orthogonal lattice and two elliptical shaped patches embedded in bianisotropic media.

The frequency selective surface (FSS) is a periodic structure with filtering performance for optical and microwave signals. The general periodic arrays made with patterned metallic elements can act as an aperture or patch on a substrate. In this work, two kinds of materials were used to produce unit cells with various patterns. Gold nanoparticles of 25 nm diameter were used to form periodic monolayer arrays by a confined photocatalytic oxidation-based surface modification method. As the other material, silver gel was used to create multiple layers of silver. Due to the ultra-thin nature of the self-assembled gold nanoparticle monolayer, it is very easy to penetrate the FSS with terahertz radiation. However, the isolated silver islands made from silver gel form thicker multiple layers and contribute to much higher reflectance. This work demonstrated that multiple silver layers are more suitable than gold nanoparticles for use in the fabrication of FSS structures.

In order to analyze the effects of changing the value of PIN diodes and variodes on the resonant frequency and the depth of the resonance, we firstly use electromagnetic simulation solver to simulate frequency selective surface (FSS) loaded with PIN diodes and variodes respectively in this paper. Then, the equivalent circuit method (ECM) is used to validate correctness and prove rule of influence. Finally, active FSS loaded with DC bias line is also analyzed as a whole, and the influences of the DC bias line on the active FSS's electromagnetic property are illustrated.

The field of radiation emission from electron beams is reviewed with special reference to work related to free-electron lasers. Different schemes of interaction in periodic structures, electromagnetic slow-wave structures, and in transverse confining force are distinguished. Various effects and devices such as traveling wave amplifiers, Smith-Purcell radiators, Cerenkov and bremsstrahlung-free electron lasers, cyclotron resonance masers, coherent bremsstrahlung and channeling radiation are discussed and the differences and relations among them are explained. A simple comprehensive model is developed to describe electron-beam interaction with an electromagnetic wave in periodic electromagnetic structures. The model is general enough to describe both collective and single-electron modes of interaction and quantum mechanical, classical and Fermi degenerate regimes. Simplified expressions are developed for the gain by stimulated emission of radiation and for gain conditions of the Smith-Purcell-Cerenkov type free-electron lasers under conditions of very thin electron beams and infinite interaction length.

Fractal geometries have been extensively implemented in the design of multiband antenna and frequency selective surface (FSS). For example, convoluted and Hilbert patch elements were first proposed to reduce the unit cell size of FSS (Parker and El Sheikh, IEE Proc.-H, 138, 19–22, 1991.) A fractal FSS with self-similar Sierpinski dipole array was shown to have two stop bands (J. Romeu and Y. Rahmat-Samii, IEEE Trans., AP-48, 1097–1104, 2000). However, its resonant frequencies are rather sensitive to incident angles and polarizations. Minkowski patch FSS and Sierpinski carpet fractals have been shown to provide two or three stop bands with dual polarization capability (J.P. Gianvittorio, et. al., IEEE Trans., AP-51, 3088–3096, 2003.) Recently miniature band-pass FSS with fractal antenna-filter-antenna arrays was designed and demonstrated with good frequency selectivity (S. Zheng, et. al., IEEE AWPL, 11, 240–243, 2012.) A dual band fractal FSS with a 3:1 reflection to pass band ratio and significantly improved resonant frequency stability was proposed by the author (T.K. Wu, Microwave Optical Tech. Letters, 54, 833–835, 2012).

A highly selective tri-band bandpass frequency-selective surface (FSS) is presented by cascading three layers of periodic arrays. The middle layer is composed of double square loops (DSLs) while the two exterior layers are composed of gridded-double square loops (G-DSLs) structure. The proposed FSS can provide multitransmission zeros which lead to a wide out-of-band rejection between each two adjacent passbands and sharp rejection behavior on both sides of each passband. Furthermore, the FSS exhibits stable response over a wide range of incident angles for both TE and TM polarizations. The design procedure, simulation, and experiment of the FSS are presented. The measured results are in good agreement with the simulations.

Periodic structures have attracted much attention in the past decades due to numerous applications such as metamaterials, antenna arrays, frequency selective surface (FSS), photonic band gap (PBG) or electromagnetic band gap (EBG) structures, etc. Infinite periodic structures are impossible in a realistic fabrication. When truncating infinite periodic structures, discontinuities occur at the interface and surface waves will be excited according to mode conversion, which leads to the edge effect of finite periodic structures. The prediction of wave natures in the edge cells of finite periodic structures is an indispensable procedure during the design of finite periodic structures. The conventional element-by-element method is rigorous and suitable for small finite arrays. However, it requires heavy computational resources for very large arrays. The well-established infinite periodic approximation approach neglecting the edge effect, although reduces the computational domain to a unit cell, is not sufficiently accurate for engineering applications.

In this paper, a complementary split ring resonator (CSRR) loaded compact CPW-fed circular shaped monopole UWB antenna with dual band notch characteristics is designed on an FR-4 substrate. The propounded band-notched UWB antenna is integrated with two frequency selective surface layers referred as bandpass frequency selective surface (FSS-1) and reflector (FSS-2) to offer enhanced radiation performance. Bandpass FSS-1 enhances the gain of the antenna by improving directivity and reflector FSS-2 reflects back the backside radiation of the proposed antenna towards the forward direction thereby increasing gain and directivity of the proposed antenna. Furthermore, a two-element orthogonal MIMO antenna is proposed with improved gain using two FSS layers. The complete configuration of the proposed two-element MIMO antenna consists of CSRR-based radiating patches, monopole ground planes and two integrated FSS layers. The proposed MIMO antenna operates in UWB range from 3.46[Formula: see text]GHz to 16.02[Formula: see text]GHz with dual notches at 3.9[Formula: see text]GHz and 7.2[Formula: see text]GHz. Isolation between MIMO elements is greater than 23 dB. Both antennas are fabricated and investigated at 5.9[Formula: see text]GHz for vehicular communications. Maximum gain improvements of 8 dB and 6.4 dB are observed in case of the presented single element and MIMO antenna within the entire operating band due to the integration of dual FSS layers. The diversity performance of the suggested MIMO antenna is investigated by analyzing mean effective gain, envelope correlation coefficient, channel capacity loss and diversity gain parameters. The proposed MIMO antenna is a suitable candidate for vehicular and UWB wireless communication applications.

Miniaturized frequency selective surfaces (FSSs) have been intensively investigated because of the space limitation in practical applications, such as wireless communication system working at low frequencies. Loading lumped capacitor is an effective miniaturization method to achieve the unit cell size as compact as λ 0 /15 (λ 0 refers to the resonant wavelength in free space) [1, 2]. But this approach requires bulk components which increase the complexity and cost of fabrication. Also, the tolerance of lumped elements makes it a challenge to maintain a stable resonant performance. Another effective method of reducing the unit cell size is to adopt convoluted elements which perform well in conforming to curved surface [3–5]. This method not only improves the angular stability of the frequency responses, but also moves the operating band away from the grating region which is determined by the periodicity of the array. The convoluted structures can reduce the FSS dimension because of their space filling property [5]. Therefore, their miniaturization potential will be restricted by the element area determined by the periodicity. To address this problem, the double layered convoluted structures, which make use of the element areas on both sides of substrate, are presented in this work to further miniaturize the FSS unit cell. Fig. 1 shows the geometry of the proposed bandpass FSS, which consists of a convoluted square slot loop and a convoluted capacitive element arranged on each side of the substrate. The top layer acts as a parallel LC resonator and the bottom layer acts as a capacitor. The LC resonator and the capacitor are connected in parallel. Therefore, the proposed bandpass FSS operating at 2.5 GHz provides the unit cell dimension as small as λ 0 /23 because of the increased capacitance of the unit cell. On the other hand, a double layered convoluted structure for the bandstop FSS is also proposed as shown in Fig. 2. It is seen that the top and bottom layers are connected by metallic via to construct a closed loop, which results in an increased inductance of the unit cell. Consequently, the element size of the presented bandstop FSS resonating at 2.5 GHz is only λ 0 /22. In addition, all the presented FSSs exhibit good performance of resonance stability with respect to different polarizations and incident angles.

In this paper, a numerical analysis of a periodic planar structure using a uniaxial substrate is shown. The paper means to explore the possibility to use non-conventional substrate to reduce the size of planar radiating structure. The encouraging numerical results set the ground for a further experimental analysis. The presented results can be applied easily to the design of planar antennas, arrays and Frequency Selective Surfaces (FSS). A FSS resonating at 20.75 GHz has been analyzed. Pyrolytic boron nitrite (PBN), which has an anisotropic dielectric with a relative electric permittivity represented by a diagonal tensor, is adopted as the substrate of the designed FSS to achieve the size reduction while maintaining the FSS performance. The design is then compared with the one that uses isotropic substrate (i.e., RO4003). Comparison shows that the introduction of such a non-conventional medium allows the patch size to be reduced approximately by 20% while maintaining almost the same electromagnetic performance. Issues about size reduction and frequency shifting are further presented and discussed in the paper.

In this paper, a multifunctional switchable frequency selective rasorber (FSR) incorporating water-based active thermal control devices is proposed. Compared with other FSRs, the proposed design, on the basis of reconfigurability, exploits the thermochromic dispersion and fluidity characteristics of water-based materials to achieve enhanced functional integration. The proposed design consists of a water-filled chamber absorber, a bandpass frequency selective surface (FSS), and integrated micropumps and thermal control devices. The proposed absorber is fabricated using 3D printing technology, whereas the bandpass FSS is manufactured using printed circular board (PCB) technology. Experimental results demonstrate that, under the FSS transmission mode, the proposed design achieves a -3 dB bandwidth of 10.3-11.6 GHz with a minimum insertion loss of 0.74 dB within the passband. Under the integrated absorption-attenuation mode, the in-band attenuation exceeds 5.0 dB, while the absorption rate is greater than 80% in the out-of-band frequency ranges of 11.9-13.6 GHz and 20.9-21.4 GHz. The two operating modes can be reversibly switched by loading and unloading water via the micropump. Additionally, the proposed design incorporates a water-cooling circulation system with thermal control devices to enable effective thermal management and tunable absorption performance. In the cooling mode, the temperature can be rapidly reduced from 53.2 °C to 32.3 °C within 60 s. By activating the temperature control mechanism, adjustable absorption levels can be achieved. The proposed multifunctional switchable FSR not only reduces manufacturing costs but also satisfies the requirements for thermal stability in complex communication and detection environments, demonstrating significant potential for radome applications.