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In this letter, a miniaturized-element dual-polarized frequency selective rasorber (FSR with independently regulated dual-transmission response has been presented. The proposed FSR is made of two cascaded layers separated by an air spacer, where the top lossy layer provides wideband absorption along with in-band transmission and the bottom lossless layer exhibits a dual bandpass filter. The novelty of the proposed design lies in the use of lumped capacitors in the top layer to realize the independently controlled highly selective transmission frequencies as well as attain a miniaturized unit cell topology (having dimensions of 0.08L 0.08L, L being the lowest operating frequency . In addition, convoluted slot geometries are patterned in the bottom layer to obtain the transmission bands appearing at the same frequencies as that of the top layer. The proposed structure has an operating range of 1.87 - 7.97 GHz, whereas two distinct transmission responses are located at 3.52 GHz and 5 GHz having insertion losses of 1.2 dB and 0.82 dB, respectively. Further, the geometry exhibits an angularly stable performance upto 45o for both TE and TM polarizations. A prototype of the proposed FSR was fabricated and measured to validate the simulated responses under normal incidence

A simple optically transparent absorber based on a double complementary structured layer of indium tin oxide (ITO) deposited polyethylene terephthalate (PET) film is presented. The structure showed more than 90% absorption from 2-4.5 GHz of the frequency with a thickness of 0.0847L (wavelength corresponding to the lowest frequency of absorption band). The computed absorptivity of the proposed structure is validated using the transmission line theory and measured experimentally as well. The structure has polarization insensitive response and is highly stable for oblique wave incidence over a wide angular range. This absorber can be conformally wrapped on a curved surface and reduces the radar cross-section (RCS) by more than 10 dB of a curve cylinder. This 10 dB RCS reduction covers almost the entire S-band of microwave frequencies. Optical transparency and flexibility of the proposed absorber make it suitable for a wide range of applications.

Usually, Radar signature of an object is reduced using microwave absorber surface. Recently, another approach based on time‐modulated frequency‐selective surfaces has been developed to deceive Radar detection by selectively generating harmonics/sidebands. Here, a polarization‐insensitive time‐modulated frequency‐selective surface is proposed that works over a broad frequency range. The design employs a PIN diode‐based switchable frequency‐selective surface to switch between the two states of reflection coefficient. Monochromatic incident electromagnetic (EM) wave, upon reflection from the surface is modulated by a periodic time‐varying reflection coefficient to generate several harmonics/sidebands along with the incident frequency. In the frequency spectrum of the reflected EM wave, the power of the incident frequency component is suppressed below that of the sidebands. The time‐modulated frequency selective surface has the capability to suppress the incident frequency component for any incident EM wave in 2–4 GHz that is S band, as well as generate spurious reflection sidebands.

In this letter, three different wrapping methods i.e., circle having incisions (CI), pentagon and hexagon (P&H) and petals wrapping are proposed for covering of spherical shaped surfaces with flat microwave metamaterial absorber (MMA) without defects. To achieve the better performance, an extensive analytical study was carried for counting the number of unit cells and packing fraction in the aforementioned techniques. It was found that maximum number of unit cell can be wrapped over a spherical ball by using CI method. The maximum packing fraction was found to be in P&H method. Further, we studied variation in the number of unit cells and packing fraction for variation in the shapes (circle, square, and hexagon) and size (2–4 mm) of the unit cell. For experimental purpose, an indium tin oxide coated over polyethylene terephtalate (ITO/PET)-based conformal MMA was designed and then fabricated using excimer laser micromachining. The proposed conformal absorber gives broadband absorption from 6.21 to 15 GHz. Finally, the fabricated absorber was wrapped on a spherical ball and microwave response was measured. The measured microwave response was compared with the simulated response and found to be in good agreement with each other.

A compact five-port multi-input multi-output (MIMO) antenna for n79 (4.4–5 GHz) 5G band exhibiting pattern and polarization diversity with good isolation is proposed in this paper. This MIMO antenn...

In this paper, a compact triple-band printed monopole antenna with circular polarization (CP) has been presented for C-band wireless applications. To achieve the triple-band CP response, the top la...

Cylindrical waveguides filled with anisotropic metamaterials enable novel wave propagation phenomena. A nanoporous alumina microtube is a manifestation of such an anisotropic waveguide with an effective permittivity tensor ( $\varepsilon _{r} \neq \varepsilon _\phi = \varepsilon _{z}$ ). The metamaterial would be considered to be filled homogeneously or inhomogeneously depending on the variation of the size of the nanopores and density of the nanopores. The electromagnetic fields in such metamaterial waveguide are highly sensitive to the refractive index of the material embedded in the nanopores and the variation of the density of the inclusion in the radial direction due to the modal field distributions. We show that there is a large variation in the reflection coefficient ( $S_{11}$ ) of the inhomogeneously filled nanoporous waveguide with an embedded liquid with small variations in the material properties of the liquid. This variation in the $S_{11}$ parameter can use to determine the purity of liquid. Our results show that the permittivity variations of the inclusion material can be monitored up to the sixth decimal places theoretically. The analysis also indicates that the inhomogeneous metamaterial waveguide is more sensitive compared to both the homogeneously filled metamaterial waveguide and an isotropic waveguide covered with metamaterial clad. This waveguide has potential applications in biomedical and chemical sensing applications.

A polarization-insensitive broadband absorber with a notch band characteristic has been presented for radar cross section (RCS) reduction applications. The proposed design is a three-layered structure, consisting of metal-printed substrate layers separated by air spacers. The top layer is a resistor-loaded convoluted cross-dipole geometry that results in a wideband absorption. The band-notch response is achieved by careful design of a square loop in the middle layer, and the bottom layer is a complete metal ground. The overall structure exhibits two discrete absorption bands (having absorptivity above 90%) over the frequency ranges of 2.12 to 4.15 GHz and 6.08 to 9.58 GHz, whereas a reflective behavior is obtained from 4.3 to 5.65 GHz. The proposed geometry has several novel features, such as compact topology (with unit cell size of 0.137 λL, λL being the lowest operating wavelength), low profile, polarization-insensitivity, and wide operating bandwidth of 127.52%, unlike the existing band-notched absorbers. By deriving an equivalent circuit model and several parametric variations, the performance of the proposed absorber has been qualitatively analyzed. A sample prototype has also been fabricated and reasonable agreement among the circuit analysis, simulation response, and measured result is obtained under normal incidence.

Based on the scattering cancellation technique, an approach is proposed to improve the mantle cloak’s performance for electrically large cylinders. The procedure is based on the observation that th...

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.

An electromagnetic (EM) wave modulation technique using actively tuned frequency-selective surfaces (FSSs) to shift the frequency of EM waves reflected from the surface is presented. The reflected EM wave contains significant components at multiple sideband frequencies along with the original incident frequency. The time-modulated complex-valued reflection coefficient can be appropriately designed to highly suppress the original incident frequency component and significantly boost the sidebands. Such a time-modulated surface can help in evading Doppler radar where the object velocity is detected from the Doppler shifted frequency of received signal. The presence of multiple frequency sidebands and suppression of the original frequency component can make Doppler radar imaging very difficult or even impossible when the modulation frequencies are dynamically managed. Here, the reflected EM wave from an FSS is modulated by applying a time-varying periodic biasing voltage across varactor diodes in the FSS. Variation in the bias voltage changes the reflection coefficient of the FSS and thereby modulates the reflected EM wave. A prototype of the designed modulator surface is fabricated to verify the modulation performance experimentally. Here, two modulation schemes, sinusoidal and saw-tooth biasing, have been demonstrated to show symmetrical and asymmetrical power distributions in the sidebands.

An approach for improving the invisibility performance of mantle cloak is proposed. It is based on an interesting observation that the cloaking performance of mantle cloak can be significantly enha...

A general computational paradigm, the time-domain infinitesimal dipole model (IDM) method, is developed and applied to compute reactive energy and radiated power for arbitrary shaped antennas with different time-domain excitations. The proposed technique first utilizes finite-difference time-domain method to compute the time-domain radiating antenna currents from near-zone electric/magnetic fields. Further, the far-zone time-domain radiated power is analytically estimated using a suitable IDM of the antenna spacetime current distributions. The time-domain reactive energy is calculated by adopting a rigorous energy subtraction approach. The proposed time-domain IDM technique is first validated by reproducing the standard antenna-Q-factor results of half-wavelength thin-wire dipoles. Second, time-domain reactive energy signatures are obtained for pulse-excited single and multiband dipole antennas, both in isolated condition and two-element multi-in multi-out (MIMO) topology. Third, the effects of parasitic reflector and director elements on the spatio-temporal energy dynamics of thin-wire Yagi–Uda antennas are demonstrated. The proposed algorithm is very general, applicable to arbitrary antenna shape and spacetime excitations, and hence is expected to enable engineers to look beyond traditional single frequency Q-factors of electrically small antennas to probe into the more fundamental spatio-temporal energy dynamics of general radiating structures, especially future generation 5G MIMO antennas.

In this letter, an optically transparent broadband microwave absorber has been developed. The proposed geometry is constructed of the patterned resistive films of an indium-tin-oxide on flexible substrates, such that the structure exhibits broadband absorptivity (above 90%) from 4 to 17.20 GHz (covering C, X, and Ku bands). The novelty of the design lies in leveraging the properties of interdigital capacitance to achieve wide absorption (fractional bandwidth of 124.53%), angular stability, as well as optical transparency compared to the earlier reported broadband absorbers. Moreover, the structure has been extensively investigated through deriving an equivalent circuit model and studying surface current distributions. Experimental validation of the fabricated prototype has also confirmed the potential use of resistive film based broadband absorbers in several applications.

Design and implementation of a broadband polarization-insensitive multilayered microwave absorber using resistive ink has been presented here. The proposed absorber is constructed using layers of resistive surface where each layer consists of periodic arrangement of resistive patterns printed on a low-cost FR4 sheet using resistive ink. Due to the multilayered nature, the proposed absorber shows broadband absorption having simulated reflection coefficient less than $-$ 10 dB in the frequency range from 2.00 to 18.50 GHz under normal incidence, which covers the entire S, C, X, and Ku bands with the smallest thickness of 0.082 $\lambda _L$ (at lowest operating frequency of the absorption band) as compared to the state-of the-art absorbers. In order to validate the proposed absorber, the equivalent circuit model has also been presented. The equivalent circuit model shows good match with the proposed absorber. The proposed absorber is experimentally verified and the experimental results shows good agreement with the simulated results.

This review discusses the evolution of the various radar cross-section (RCS) reduction techniques, with an emphasis on metasurfaces. The paper first introduces the terms RCS and RCS reduction and then discusses conventional and modern techniques to reduce RCS. The two main strategies used are scattering and absorption. The traditional methods of shaping and Radar Absorbing Material (RAM) are first briefly reviewed, followed by an extensive review of metasurface-based RCS reduction. RCS-reducing metasurfaces have the unique characteristics of acting as scatterers and absorbers. They are also described with regard to their passive and active configurations. The paper discusses RCS reduction techniques with respect to profile, bandwidth, angular stability, polarization sensitivity, design complexity, and cost-effectiveness. A comprehensive comparison chart based on the performance parameters such as bandwidth, size and angular stability is tabulated for the different types of metasurfaces. The review also details areas that require further investigation. Published version

A finite-difference time-domain-based technique for evaluating envelope correlation coefficient (ECC) and mean effective gain (MEG) of multiple-input multiple-output (MIMO) antennas operating in real-life nonuniform propagation environment is introduced. The proposed algorithm is based on the formulation of a modified cross correlation Green’s function (CGF), which directly incorporates the specific angular distribution functions and cross-polarization discrimination parameters. Thus, the proposed technique based on modified CGF is able to calculate ECC/MEG for MIMO antennas in nonuniform propagation scenario, directly from the time-domain antenna currents. This avoids the troublesome determination of far-field antenna pattern at each working frequency and the consequent postprocessing, as required in the conventional approach. Numerical examples using thin-wire half-wavelength dipole antennas and printed microstrip-fed slot antennas are provided to show the effectiveness of the proposed technique for characterizing MIMO antennas in nonuniform propagation environment.

In this paper, a low-profile broadband frequency selective surface (FSS) is presented for wideband shielding. The FSS consists of periodic patterns of circular loops connected among themselves through varactor diodes, which exhibit tunable operation. The novelty of the proposed design lies in its wideband tuning, where the bandstop response can be varied from 0.54 to 2.50 GHz under the reverse bias voltage, thereby exhibiting a tunable range of 363%. Moreover, the structure exhibits a wide 1.28 GHz stopband (fractional bandwidth of 152%), which is maximum as compared to other wideband FSSs. Additionally, the design is polarization-insensitive and angularly stable for both TE and TM polarizations. A prototype of the proposed structure has been fabricated where the varactor diodes are biased through a novel biasing technique. The sample is measured using parallel-plate waveguide setup as well as the free-space technique, where the measured results show good agreement with the simulated responses.

A cross polarization conversion (CPC) structure using metasurface over wide bandwidth has been presented for practical applications in this article. The unit cell of the proposed metasurface is made of metallic patch of single circular split-ring imprinted on top surface of a metal-backed single layer dielectric substrate. Full width half maxima bandwidth of polarization conversion ratio (PCR) of 11.12 GHz is realized along with wide bandwidth of 9.92 GHz extending from 6.06 GHz to 15.98 GHz with more than 0.8 PCR magnitude is realized using suitable optimization of the geometrical dimensions. Four distinct PCR peaks are observed at 6.56 GHz, 10.38 GHz, 15.12 GHz and 16.68 GHz. The polarization conversion phenomena at these four frequencies have been analyzed in the light of electromagnetic resonances. The roles of several geometrical parameters of the design are simultaneously investigated. The proposed structure has been studied under oblique incidence, both for TE and TM polarizations. Wideband polarization conversion characteristics upto 45° incident angles have been observed for both cases. A prototype of the proposed metasurface is fabricated and experimental results are in good agreement with the simulated responses. The proposed structure is ultra-thin; ~ λ/11.8 with respect to the centre frequency of the CPC bandwidth.

In this communication, polarization-insensitive tunable absorbers are presented for single- and dual-band applications with wide tuning range. The proposed designs consist of periodic patterns of split square loops connected with cross-dipoles through varactor diodes which exhibit tunable performances. Due to four-fold symmetry, the structures have the novel characteristic of polarization insensitivity unlike the earlier reported tunable absorbers. Moreover, the designs are comprised of vias to provide required biasing to the diodes, which also result wide frequency tuning. Two single-band tunable absorbers are initially designed, which are combined to exhibit a dual-band tunable absorber with independent tuning of absorption frequencies. A novel biasing methodology has been developed to provide bias voltages to the semiconductor diodes without disturbing the original frequency response. The fabricated sample, while measured, shows good agreement with the simulated responses under normal incidence.

Non-linear shape external cloak using a parabolic transformation function is proposed with finite material properties. A simple one directional folding operation is utilised that provides homogeneous material properties in the complementary region, and also eases the creation of anti-object in the complementary region from an object present in the cloaked region. In the core region, the axial material parameter is spatially invariant and other parameters are spatially varying but possess finite value. Due to finite small material range and its one directional spatial variation, the proposed cloak even with discrete material properties in core region shows nearly zero scattering in contrast to earlier reported cloak. Further, off-diagonal elements of the core constitutive tensors in the Cartesian coordinate system are removed by converting them to diagonal elements in the principal coordinate system. Thus, only three diagonal elements are needed in the principal coordinate system to create the core medium, which are finite and realisable using two-dimensional metamaterials.

In this communication, a parabolic shaped complementary cloak is proposed which makes the object invisible by reducing its scattering under different incidence angles of electromagnetic wave. The proposed design is based on transforming a line segment into an enclosed space that is surrounded by the core region. The enclosed space is symmetrically divided into two regions, namely, cloaked region and complementary region. The shifted parabolic coordinate system introduced in the present work enhances the area of cloaked region and provides diagonally anisotropic but optimized homogeneous material properties in the core region. The cloaked region is mapped onto the complementary region using a simple one directional folding transformation, which yields homogeneous and isotropic material properties in the complementary region and also eases the creation of antiobject. Numerical results confirm the cloaking effect of the proposed approximate complementary cloak. The metamaterial unit cell designs are also presented for realizing the proposed material properties of core and complementary regions.

Higher order locally one-dimensional finite-difference time-domain (HO-LOD-FDTD) methods are proposed here to improve the accuracy of the four-step LOD-FDTD, five-step LOD-FDTD, and six-step LOD-FDTD methods. In the proposed HO-LOD-FDTD methods, higher order finite-difference approximations are used for all the three space derivatives; therefore, they give improved accuracy in all the three directions. In addition to HO-LOD-FDTD methods, hybrid-HO LOD-FDTD (HHO-LOD-FDTD) methods are also proposed here. In the HHO-LOD-FDTD methods, higher order finite-difference approximations are used for the desired direction(s) only. As a result, they give improved accuracy in the desired direction(s), and take less computational time than the HO-LOD-FDTD methods. Moreover, to validate the analytical results, simulation results of the fourth-order (FO) and hybrid-FO LOD-FDTD methods are also discussed here.

In this paper, a three-element compact multiple-input-multiple-output (MIMO) antenna system having both pattern and polarization (linear/circular) diversity is proposed. The proposed MIMO system consists of a chamfered-edge square patch antenna with an offset feed that provides circular polarization in broadside direction. Furthermore, two printed dipole antennas are placed adjacent to it for providing linearly polarized endfire radiation. The three-element MIMO antenna system exhibits good isolation (>15 dB) without the use of any separate decoupling structure. The match between the simulated and measured results on fabricated antenna prototype suggests that the proposed antenna can be a good candidate for pattern-and polarization-diversity MIMO applications in the 5.8-GHz WLAN frequency range.

A dual-band switchable absorber is presented with independent switching of absorption frequencies. The unit cell of the proposed structure is composed of an annular ring and a cross-dipole, where switchable p-i-n diodes are embedded. By controlling the bias voltage of the diodes, both the absorption frequencies can be switched between two frequency bands independently, thereby resulting in a dual-band/bandwidth-enhanced absorber. Moreover, the proposed design is polarization-independent due to fourfold symmetry unlike the earlier reported switchable absorbers. A novel biasing network has also been developed for measuring the fabricated structure, which is experimentally verified under normal incidence.

In this letter, a polarization-insensitive broadband switchable absorber has been presented, which exhibits wide absorption bandwidth switching between two frequency bands ( S -band in one biasing state, and C - and X -bands in other state). The structure utilizes the circuit analog absorber concept to achieve broadband absorption, whereas switchable mechanism is obtained by implementing active p-i-n diodes in multilayer geometry. The novelty of the proposed design lies in its four-fold symmetrical configuration and simplified biasing network as compared to the earlier reported switchable absorbers. Several parametric variations have also been investigated to explain the switching and the broadband absorption mechanisms of the structure. Furthermore, a prototype has been fabricated and experimentally demonstrated under normal incidence.

A novel polarization rotator design is demonstrated theoretically as well as experimentally based on frequency-selective surface (FSS). The polarization rotator selectively allows the linearly polarized incident wave to pass through the structure and rotates it by $\text{90}^\circ$ in a given frequency band. The periodic element of the FSS design consists of orthogonally oriented V-shaped slots on two metal sheets with the dielectric substrate separating the sheets. The proposed structure exhibits a relative bandwidth of $\text{8}\%$ (9.76–10.57 GHz) with a maximum insertion loss of 0.5 dB. Copolarized reflection coefficient is less than −10 dB in the same frequency band. The novelties of the structure lie in its reduced thickness ( $\text{0.053}\lambda _o$ ) as well as compact design ( $\text{0.28}\lambda _o$ ) as compared to the existing polarization rotators. A prototype has also been fabricated, and good agreement between the simulated and experimental results is observed.