Frequency Selective Structure Research Articles

A miniaturized dual band pass frequency selective surface insensitive to oblique angles and polarizations of wave

Abstract A Dualband Pass Frequency Selective Structure (DPSFSS) with miniaturized elements is proposed in this article. The miniaturized elements induce high inductance and capacitance values. The meandered line concept is used which extensively increases inductance of the dipole element. Very large capacitance is obtained between two meandered elements which have wide common area perpendicular to electric field for wave propagating in Z-direction. Two pass band poles and two zeros are obtained by perpendicular arrangement of meandered patterns on two sides of the substrate. Orthogonal arrangement of patterns ensure polarization symmetry (polarization insensitive) for Transverse Electric (TE) and Transverse Magnetic (TM) modes of exciting wave. Two pass band pole frequencies are positioned at 1.84 GHz and 4.53 GHz with low Insertion Losses (IL) of 0.67 dB and 1 dB, respectively. The lower and higher frequency bands have factional bandwidths of 41 % and 46 %, respectively. The transmission nulls occupy 2.8 GHz and 6.58 GHz frequencies. The structure is miniaturized to 0.032λ 0 × 0.032λ 0 dimensions with respect to lowest resonance residing at 1.83 GHz. The low IL at both pass bands is realized despite of using a lossy FR-4 material that can be improved to less than 0.5 dB at both the bands using less lossy materials such as F4BME or RO4003C. The proposed DPSFSS design is elaborated using Equivalent Circuit Model (ECM) and Surface Current Distribution (SCD). Measurements are performed on fabricated DPSFSS prototype and it is observed that the measured results coincide well with the simulated response. Angle stability of the DPSFSS is excellent and no unwanted resonances are observed at oblique angles upto 80° for dual polarizations of wave.

Implementation of 3-D Bandstop Frequency-Selective Structures With Ultralarge Angular Stability Utilizing Narrow L-Shaped Strip Lines

Implementation of 3-D Bandstop Frequency-Selective Structures With Ultralarge Angular Stability Utilizing Narrow L-Shaped Strip Lines

Absorption-transmission-diffusion-type radar cross section reduction metasurfaces based on frequency selective absorber and polarization conversion chessboard structure

Absorption-transmission-diffusion-type radar cross section (RCS) reduction metasurfaces based on the frequency selective absorber and the polarization conversion chessboard structure are proposed in this paper. The RCS reduction metasurfaces consist of three parts, i.e. the top layer realized by a cross-shaped meander line loaded with lumped resistors for low-frequency absorption and high-frequency transmission, the mid-layer constructed by angular ring vias frequency selective surface (FSS) sandwiched with two square patches FSS for broadband transmission in mid-frequency zone, and the bottom layer employed chessboard metasurfaces composed of dual-arrow type polarization conversion units for scattering cancellation in high frequency zone. The physical mechanisms of the RCS reduction metasurfaces with multifunctional electromagnetic controlling including wave absorption, bandpass transmission, polarization reversal, and phase cancellation are also discussed and analyzed. Simulation and experimental results demonstrate a reflection of lower than −10 dB for normal incident linearly polarized waves over a frequency range of 2.5 GHz–18 GHz, with a relative bandwidth of 151.22%; transmission passband properties with insertion loss better to −3 dB in the frequency range of 5.62 GHz–11.0 GHz, with a relative bandwidth of 64.74%. Due to the resonance-coupling-resonance principle, the broadband transmission with low insertion loss was obtained. The RCS properties of the proposed multi-functional metasurfaces were further analyzed by numerical simulation and experimental measurement, and the results showed that RCS reduction of over 10 dB was achieved for normal incident different polarized waves at the frequency range of 2.5 GHz to 17 GHz. Even in the case of normal incidence, there was a deviation between the reflection characteristics and RCS reduction. The RCS reduction metasurfaces are expected to be employed in applications where multi mechanism collaborative control of electromagnetic waves is of extreme importance.

Wideband design and modeling of efficient multi-resonance reflective linear-to-circular polarization converter

Abstract A reflective linear-to-circular polarization converter based on dual frequency-selective structures (FSSs) is proposed and modeled to exhibit efficient wideband performance. The design utilizes a diagonal array of two connected circular patches as an effective anisotropy with regular current distribution in several successive resonances, resulting in orthogonal reflections with a 90° phase difference. The relevant upper-part characteristic is improved by using two separate square patches as a high-frequency resonator. This design with distinct key parameters leads to high overlapping and then excellent bandwidth and efficiency over 105% and 96%, respectively, with an axial ratio below 1.7 dB. A sophisticated equivalent circuit-admittance model including effective mutual coupling between two FSSs is extracted, featuring closed-form equations for the physical design. Different dielectric constants are studied on the converter, which offer controllable coverage in the range of 3–24 GHz (S, C, X, Ku, and K bands), variably. For actual validation, a very thin (0.04λ0 at 3.65 GHz) 8 × 8 array prototype was built and measured at different incident angles, showing angular stability up to 45° in 78% (6–14 GHz) bandwidth. This converter has potential applications in communication, spectroscopy, detection, and imaging in micro-, mm-, and THz-wave regions.

BDCDTA-based Reconfigurable Frequency Selective Filter Structures for Medium Wave Communication and Biomedical Signal Processing Applications

BDCDTA-based Reconfigurable Frequency Selective Filter Structures for Medium Wave Communication and Biomedical Signal Processing Applications

High-Precision Modeling for Arbitrary Curved Frequency Selective Structures Based on Perfect Mapping Between Thick Surface and Its Minimum Distortion Flat-Unfolding Solution

High-Precision Modeling for Arbitrary Curved Frequency Selective Structures Based on Perfect Mapping Between Thick Surface and Its Minimum Distortion Flat-Unfolding Solution

Calculation and Measurement of Dielectric Parameters of Ceramic and Ferroelectric Materials in the Microwave Range

Introduction. Determination of the electrophysical parameters of linear and nonlinear dielectric materials for use in microwave technology represents an important direction. Linear dielectrics are used as a basis for the substrates of microwave circuits, as well as volumetric elements for the construction of frequency selective or resonant structures that operate across a wide temperature range. Therefore, the issue of stabilizing the electrical parameters of such structures from temperature influences appears relevant. A possible solution lies in the use of a multilayer combination of dielectrics, both with linear and nonlinear properties. Due to their nonlinear properties, ferroelectrics find application in functional units with an electrical rearrangement of frequency and phase characteristics. Therefore, it is important to determine not only the relative permittivity of the material, but also the control coefficient in the RF– microwave wavelength ranges.Aim. Construction of computational mathematical models for layered bulk and film structures to determine the relative permittivity of linear and nonlinear dielectrics in the ultrahigh frequency range.Materials and methods. The construction of computational mathematical models for the analysis of complex layered structures was carried out on the basis of Maxwell’s equations and the Galerkin method using boundary conditions for electromagnetic field components.Results. An electrodynamic analysis of a two-layer volumetric disk resonator was performed, and numerical results of calculating the resonant frequency were obtained. A numerical analysis of a multielectrode half-wave resonator with a ferroelectric film (FEF) was carried out.Conclusion. The mathematical models created and the experiment performed made it possible to numerically evaluate the properties of linear and nonlinear dielectric bulk and film materials in the microwave range.

A New 3D Frequency-Selective Structure for 5G Communication

Background: In this paper, a new frequency-selective structure (FSS) for 3 to 4 GHz frequency band of fifth generation (5G) is proposed as a result of an analytical mode-matching method. Methods: A new periodic structure with stepped rods is designed using a closed-form equation derived by the analytical mode-matching method. Performance of the structure is simulated by different numerical packages. Results: The analytical and simulation results demonstrate that the designed structure transmits incident waves in 3.4 to 3.9 GHz frequency range with return loss lower than 10 dB and insertion loss of about 0.5 dB. The structure reflects the frequencies out of this range, especially wireless local area network (WLAN) 5 GHz, which is adjacent to this band. Furthermore, the performance of the proposed structure is independent of the TE and TM polarization of the incident wave and relative to the angle of the incident wave up to 60 degrees from perpendicular to the FSS surface, it has minor variations of about 8% in the transmitted frequency bandwidth. In addition, the average value of maximum field enhancement factor (MFEF) as the ratio of maximum field magnitude on the FSS surface to the magnitude of the incident field, used for assessing power handling capability of the structure, is about 4.5. Conclusion: Therefore, these features make the proposed structure suitable for 5G communication and high power systems.

Bandpass Absorptive Frequency-Selective Structures With Wide Absorption Bands Based on Hybrid 2-D and 3-D Structures

This article proposes a bandpass absorptive frequency-selective structure (AFSS) with wide absorption bands based on the hybrid two-dimensional (2-D) and three-dimensional (3-D) structures. It consists of a 3-D lossy structure, a 2-D lossy layer, and a 2-D bandpass frequency-selective surface (FSS). The 3-D lossy structure functions in its fundamental mode ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$f_{0}$ </tex-math></inline-formula> ) and third-order mode ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$3f_{0}$ </tex-math></inline-formula> ) to provide two broad absorption bands. It transmits electromagnetic (EM) waves when the open-ended slotline operates in half-wavelength mode ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2f_{0}$ </tex-math></inline-formula> ). The 2-D lossy layer absorbs EM waves between the frequencies of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$f_{0}$ </tex-math></inline-formula> and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$3f_{0}$ </tex-math></inline-formula> and provides a transmission window at <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2f_{0}$ </tex-math></inline-formula> . The 2-D bandpass FSS has a broad transmission band at <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2f_{0}$ </tex-math></inline-formula> and a broad rejection band outside of the band. The operating principle of AFSS is investigated in depth based on an accurate equivalent circuit model (ECM). A prototype is fabricated and assembled to demonstrate the viability of the principle. The experimental results indicate a wide passband with low insertion loss. The bandwidth of lower and upper wide absorption bands is 120.3% and 43.5%, respectively. In addition, both the simulated and measured outcomes exhibit that the proposed AFSS has stable performance under oblique incidence.

Four-Port 38 GHz MIMO Antenna with High Gain and Isolation for 5G Wireless Networks

In this paper, a 38 GHz 4-port multiple-input multiple-output (MIMO) antenna with considerable isolation and gain enhancement for 5G applications is introduced. The suggested antenna element is a monopole antenna composed of a circular patch with a rectangular slot etched from it and a partial ground plane is used to extend the desired frequency to operate from 36.6 GHz to 39.5 GHz with a center frequency of 38 GHz. The high isolation is achieved by arranging the four elements orthogonally and adding four stubs to reduce mutual coupling between elements at the desired frequency bands. The gain improvement is also introduced by placing a frequency selective structure (FSS) which is designed at the same frequency bands of the antenna under the suggested MIMO antenna to act as a reflector. The proposed four-element MIMO with the FSS prototype is built and tested in order to confirm the simulated results. The suggested antenna operated from 37.2 GHz to 39.2 GHz with an isolation of less than 25 dB across the obtained frequency range. The peak gain of the antenna is enhanced from 5.5 dBi to around 10 dBi by utilizing the FSS structure; furthermore, the back radiation is enhanced. The MIMO performance is validated by extracting its parameters and comparing with the simulated results. The results extracted from the simulation and the measurement show satisfactory matching along with the target band, indicating that the proposed structure could be used for 5G communications.

Design of compact dual‐band shared‐aperture antenna system based on frequency selective surface

AbstractA miniaturized dual‐band dual‐polarization base station antenna array operating at 2.4–2.8 and 4.6–5.0 GHz is proposed in this paper. The upper‐band unit is embedded in the lower‐band unit to miniature the array size. The size of the proposed shared‐aperture antenna element is 0.28 × 0.28 . To ensure the normal radiation of dual‐band under the small frequency ratio, a meander slot frequency selective surface structure is introduced, which is electromagnetically transparent to the lower‐band unit and function as metal ground for the upper‐band unit to ensure the normal radiation of the upper and lower‐band units, respectively. Simulation and measured results show that the antenna array meets the following design specifications: dual‐band operating (voltage standing wave ratio [VSWR] &lt; 2), mutual coupling less than −20 dB basically, satisfactory gain and radiation characteristics.

A 2.5D low-profile wideband frequency selective structure with high-selectivity performance

In this paper, a low-profile high-selectivity 2.5D FSS based on the dislocation patch structure is proposed and discussed. The dislocation patch structure is used to construct the low-profile transmission line, and a second-order band- pass FSS with a controllable transmission zero is realized by introducing a series LC structure to the dislocation patch structure. The effects of the geometrical parameters on the frequency response of the proposed second-order bandpass FSS are analyzed based on the transmission matrix method and the equivalent circuit simulation. Then, two proposed second-order bandpass FSSs with different transmission zeros are combined and placed crossly to construct two propagation paths, and a high-selectivity third-order FSS is obtained. Finally, the prototype is fabricated and measured to prove the effectiveness of the proposed FSS. The measured results show that the proposed high-selectivity FSS has a wide working bandwidth, proper selective performance, and a thinner profile than other high-selectivity 2D/3D FSSs.

Dual-Polarization Multibandstop 3-D Frequency-Selective Structures Based on Cross-Coupling Method and Subcell–Cell Structure

In this article, based on cross-coupling way and subcell–cell structure, a methodology to design the low-profile dual-polarization multibandstop 3-D frequency-selective structure (FSS) is proposed. Using the 3-D periodic vertical placing printed circuit board pieces (PVPCB) structure, a first-order multibandstop FSS is proposed and analyzed first. For this first-order multibandstop FSS, the number of working stopbands is determined by the number of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$LC$ </tex-math></inline-formula> resonators in the edge of PVPCB structure. Then, this first-order multibandstop FSS is used as the basic resonator and crossly placed in the substrate of the PVPCB. This cross placement achieves the coupling between the basic resonators, and a single-polarization second-order bandstop FSS is obtained. Subsequently, this single-polarization second-order multibandstop FSS is used as the subcell, and a novel placement of this subcell ensures that the proposed second-order bandstop FSS can operate in dual-polarization with cross-polarizations of ≤−15 dB. In the whole modeling process, the equivalent circuit model is established to explain the working principle. Finally, to verify the proposed methodology, two examples with two and three working stopbands are designed, fabricated, and measured. The measured results show that the proposed dual-polarization FSS has stable filtering performance under different polarizations and oblique incident angle up to 30°.

Battery-Free, Artificial Neural Network-Assisted Microwave Resonator Array for Ice Detection

Ice detection has developed into an integral part of aerospace and wind turbine sectors with the aim of preventing hazards and component breakdown. This article presents an ice detection system consisting of a battery-free, chip-less wirelessly interrogated resonator array, and an artificial neural network for enhanced detection robustness. The designed array of split-ring resonators (SRRs), operating at 3.05 GHz, was a narrowband frequency-selective structure with a ground plane reflector which shielded resonance from the effect of installation material. For an incident wave on the array’s surface, the reflection coefficient changed with the electrical properties of the ice and water, consequently affecting the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$S_{11}$ </tex-math></inline-formula> parameter of an interrogator antenna. The array had a surface area of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$13\times6.5$ </tex-math></inline-formula> cm2 and a substrate thickness of 0.79 mm and was wirelessly interrogated from a distance of 33 cm by a standard gain horn antenna. A custom LabView program was utilized for time-based data acquisition of the antenna’s reflection coefficient [ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$S_{11}$ </tex-math></inline-formula> (dB)], with results demonstrating a resonant frequency shift of 150 MHz when 30 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{L}$ </tex-math></inline-formula> ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim$ </tex-math></inline-formula> ) of ice was formed on the main split of SRRs. The artificial neural network then classified the reflection coefficients of the interrogator antenna to enhance ice/water differentiation. The neural network improved the classification of the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$S_{11}$ </tex-math></inline-formula> (dB) raw data to 94.67% while achieving an accuracy of 93.33% for a noisy simulated data set. The proposed battery-free artificial neural network-assisted ice sensor can be implemented in various sizes (depending on the footprint requirements of installation) with applications in aerospace and wind turbine industry.

Synthesis of 3-D Polarization Rotation Transmissive Surface by Arbitrary Angle Based on Orthogonally Inserted Parallel-Couple Slotlines

In this article, a polarization rotation transmissive surface (PRTS) of arbitrary angle based on a three-dimensional (3-D) frequency selective structure is proposed. The PRTS unit cell is composed of three parallel-coupled slotline sections with open-short ends etched on vertically and horizontally inserted singlelayer printed circuit boards (PCBs). Under the incidence of vertically polarized waves, the spatial waves are converted into guided waves by the open-circuited end of one slotline section, and then the guided waves are coupled to two other slotline sections that can act as two independent coupling transmission paths in the orthogonal planes. This can be observed as a power divider, and the power division ratio can be fully controlled by virtue of the coupling strengths of the two coupling transmission paths. Thus, the polarization of incidence waves can be rotated at an arbitrary angle at the output port. To simplify the design procedure, a filtering power division theory and an equivalent circuit model are first used to obtain the theoretical response. Subsequently, a coupling matrix is employed to synthesize and design the proposed PRTS. To confirm the effectiveness of the proposed PRTS, two examples with different polarization rotation angles and fractional bandwidths at the same center frequency of 5 GHz and return loss of 20 dB are designed, fabricated, and measured. The measured results confirm our theoretical predictions and agree with the simulated results, thus demonstrating that the proposed PRTS exhibits stable frequency response under a large oblique incidence, in addition to their easiness in fabrication.

A Dual Wideband Compact Shared-Aperture Microstrip Patch/Fabry–Perot Resonator Cavity Antenna

In this article, a dual wideband compact shared-aperture microstrip patch/Fabry–Perot resonator cavity antenna is proposed. A reflective meshed patch is designed to conduct as the shared surface, which works as the radiator at the lower band but the partial reflective sheet (PRS) at the upper band. A transmissive meshed patch is designed as the parasitic element to broaden the lower bandwidth without influence on the upper band radiation. The shaped ground is used to improve the front-to-back ratio at the lower band while maintaining a compact size. Moreover, the reflective meshed patch, which is built by two complementary frequency selective structures (FSSs), shows a positive reflection phase gradient to broaden the bandwidth of the antenna at the upper band without influence on the lower band radiation. The measurements reveal that a common bandwidth of ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\vert \text{S}_{11(22)}\vert &lt; -10$ </tex-math></inline-formula> dB, gain reduction < 3 dB) 25.4% and 21.3%, an isolation higher than 40 and 22 dB, a peak gain of 9.01 and 16.6 dBi at UHF band and C-band are achieved, respectively. The above properties make the proposed antenna suitable for dual-band point-to-point line-of-sight (LOS)/non-LOS (NLOS) communication systems.

Synthesis Design of Inductively-Coupled 3-D Bandpass Frequency-Selective Structures Based on Equivalent SOL-Extracted Circuit Model

In this article, an accurate and unified synthesis method for a class of 3-D bandpass frequency-selective structures (FSSs) is proposed. Each element of the presented 3-D FSSs is composed of stacked planar slotline sections cascaded with inductively coupled structures. Here, each inductively coupled structure is modeled as an equivalent <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${K}$ </tex-math></inline-formula> -inverter, and each stacked slotline section is regarded as an equivalent transmission-line resonator. By defining three ideal calibration standards, i.e., short circuit, open circuit, and matched load in a commercial electromagnetic full-wave simulator, a numerical short-open-load calibration technique is utilized to effectively extract all the equivalent circuit parameters involved in the presented 3-D FSS. Furthermore, a generalized synthesis design procedure is presented by mapping the proposed 3-D FSS topology onto an <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${n}$ </tex-math></inline-formula> th-order bandpass filtering network. To verify the feasibility of this design concept, two groups of design examples, including a single-polarized third-order Chebyshev bandpass FSS and two dual-polarized second-order ones, are, respectively, synthesized and designed. Finally, two prototypes of the designed 3-D FSS examples are fabricated, assembled, and then tested by using a free-space measurement method. The measured results exhibit good agreement with the predicted ones. The proposed 3-D FSSs feature simple topologies with high-order bandpass filtering responses and stable filtering responses under a large variation of incident angles of spatial waves.

Direct Synthesis and Design of Wideband Linear-to-Circular Polarizers on 3-D Frequency Selective Structures

A novel direct synthesis method for wideband linear-to-circular polarizers on 3-D frequency selective structures is presented. The proposed linear-to-circular polarizer consists of two orthogonal linearly polarized (LP) transmission paths, both of which are comprised by three cascaded sections of slotline structures. Under the incidence of an LP wave with its electric-field ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$E$ </tex-math></inline-formula> -field) vector of a titled angle 45°, each path can achieve a transmission passband for one of the two orthogonal components decomposed by the incident <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$E$ </tex-math></inline-formula> -field vector. To obtain a transmitted wave with circularly polarized (CP) property, the well-known conditions on magnitude and phase difference of these two paths are presented. Then, to develop a unified synthesis method, two new conditions of phase difference and phase slope at the center frequency are introduced to replace the typical phase condition. The closed-form equations are derived, and a synthesis procedure is given. It can be obtained that all the involved circuit parameters can be derived from the specifications of these two paths, such as in-band insertion loss (IL) ripples and fractional bandwidths. Moreover, the phase difference condition of these two paths is further modified to obtain an arbitrary axial ratio (AR) at the center frequency and enhance the AR bandwidth of the polarizer. A group of design examples are theoretically investigated to demonstrate and verify the design concept. An alternative design example is finally selected and fabricated. Good agreement between simulated and measured results features its advantages of wide AR bandwidth and low IL. Therefore, the proposed synthesis method can provide a promising solution to design many other types of wideband linear-to-circular polarizers.

Don’t Forget to Water Your Antennas! [Editor’s Comments

Everyone with just a bit of a green thumb will tell you that water can do miracles for a well-maintained lawn. Sometimes, however, water can also do great things for antennas. Please check out the article by Kong et al. focusing on a frequency-selective structure that can be controlled using water channels. The article starts with an introduction that reviews the concept of frequency-selective surfaces. Then it turns to the new design concept, providing both analytic and numerical evidence in support of the new idea. Finally, the contribution is enriched with experimental results confirming the performance of the technology, showing its practical interest. This issue of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">IEEE Antennas and Propagation Magazine</i> also features many other interesting articles. Surender et al. presents a nice review of rectennas for applications in wireless technologies. The work starts with a rich bibliographic analysis. It then reviews the primary elements of a rectenna system, examining the main solutions available in literature. Finally, attention is turned to applications, with a detailed list of relevant scenarios analyzed in large detail. The article by Notaros <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ˇ</small> is an excellent review of polarimetric weather radars. The article starts by illustrating the principles of radar polarimetry, then it proceeds with technical analyses of specific devices, and finally the contribution turns to applications, showing the data richness that can be provided by polarimetry and its applicability to weather analysis and forecasts. Finally, the article by Loghmannia and Manteghi presents a broadband strategy for small-antenna matching by an amplifier. After a bibliographic review and a section summarizing the relevant background material, the article reviews the principles of broadband parametric matching and presents an antenna design enriched with validating simulations. Finally, a prototype with experimental validations is presented showing achievable performance of the technology.

A New Miniaturized Low-Profile and Stable Dual-Band FSS with 2.5D Structure for ISM Bands

Based on 2.5-Dimensional (2.5D) frequency selective surface (FSS) structures, a new miniaturized frequency selective surface structure is proposed in this work to operate as a bandreject filter in the 2.4 GHz (2400 to 2483.5 MHz) and 5 GHz (5150 to 5350 MHz and 5470 to 5725 MHz) ISM (Industrial, Scientific, and Medical) bands. The FSS has the metallization layer formed by a square loop with an internal cross loop, with each geometry being responsible for a rejection frequency range. The main characteristics of the structure are discussed, emphasizing the resonance frequency variation as a function of the increasing number of vias. Numerical results are obtained through simulations using the commercial software ANSYS HFSS. In addition, by analyzing numerical results for structures with different numbers of vias, it was possible to obtain expressions for calculating the first resonance frequency for each geometry. The cell size of the 2.5D FSS of the proposed cell is only 0.0883 λ0, where λ0 corresponds to the free-space wavelength of the lower resonance frequency. The FSS presents dual polarization, angular stability for angles up to 50° and its simulated response presented two resonant frequencies, the first at 2.65 GHz and the second at 5.34 GHz, with relative bandwidths of 38.11% and 35.96%, respectively. For comparison and validation purposes, the prototype is fabricated and measured. A very good agreement between the simulation and measurement results is observed.