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The 32 papers in this special section focus on synthesis and optimization techniques in electromagnetics and antenna system design. The papers are briefly summarized here. [ABSTRACT FROM PUBLISHER]

A general method, based on susceptibility tensors, is proposed for the synthesis of metasurfaces transforming arbitrary incident waves into arbitrary reflected and transmitted waves. The proposed method exhibits two advantages: 1) it is inherently vectorial, and therefore better suited for full vectorial (beyond paraxial) electromagnetic problems; 2) it provides closed-form solutions, and is therefore extremely fast. Incidentally, the method reveals that a metasurface is fundamentally capable to transform up to four independent wave triplets (incident, reflected, and refracted waves). In addition, this paper provides the closed-form expressions relating the synthesized susceptibilities and the scattering parameters simulated within periodic boundary conditions, which allows one to design the scattering particles realizing the desired susceptibilities. The versatility of the method is illustrated by examples of metasurfaces achieving the following transformations: generalized refraction, reciprocal and nonreciprocal polarization rotation, Bessel vortex beam generation, and orbital angular momentum multiplexing. [ABSTRACT FROM PUBLISHER]

This paper presents design and analysis methods for planar antennas based on modulated metasurfaces (MTSs). These antennas operate on an interaction between a cylindrical surface-wave (SW) excited by an isotropic TM radiator, and an MTS having a spatially modulated equivalent impedance. The MTS is realized by using sub-wavelength patches printed on a grounded slab, thus resulting in a structure with light weight and compact volume. Both features are appealing characteristics for space applications. This paper introduces for the first time an impedance-based amplitude synthesis of the aperture field distribution and shows several new examples of antennas for space applications obtained in recent research projects financed by the European Space Agency. [ABSTRACT FROM PUBLISHER]

A new architecture for a two element biomimetic antenna array (BMAA) is presented. This two-element BMAA is in the form of a coupled antenna array with an external coupling network that is designed to efficiently extract power from both linearly-independent modes of excitation of the array. These include the common mode and the differential modes of excitation. This new array offers an enhanced phase response compared to the regular antenna array that occupies the same aperture. However, unlike previous BMAA designs, the output power of the array does not need to be sacrificed to achieve this phase enhancement factor. Compared to a regular array, this two-element coupled array extracts the same amount of power from an incoming electromagnetic wave but provides a considerably larger phase sensitivity. The paper describes the theory of operation and the design process of this antenna. Using the design procedure presented in this paper, a prototype of such a two-element BMAA is designed and experimentally characterized in the lab. Measurement results confirm the advantages of this new design compared to the previous BMAAs reported in the literature. [ABSTRACT FROM PUBLISHER]

The aim of this paper is to discuss the effectiveness of random sampling of the electromagnetic field in problems involving sparse radiating sources. The method adopted in this paper uses an intuitive geometrical approach based on the Johnson-Linderstrauss lemma applied to the \epsilon-net of the set of the fields radiated by the sparse source. The paper shows that a proper Non Uniform Random Sampling (NURS) using some a-priori information on the problem allows a significant reduction in the cardinality of the set of the measured data compared to uniform random sampling and to random sampling from a \lambda/2 equispaced set of data. It is also shown that there is a limiting number of data after which random sampling is not convenient. In this case a deterministic sampling strategy, called Modified Non Uniform Sampling (MNUS), is proposed. Numerical results on linear sparse arrays confirm the effectiveness of NURS and MNUS. [ABSTRACT FROM PUBLISHER]

This paper revisit and extend the interesting case of bounds on the $Q$ -factor for a given directivity for a small antenna of arbitrary shape. A higher directivity in a small antenna is closely connected with a narrow impedance bandwidth. The relation between bandwidth and a desired directivity is still not fully understood, not even for small antennas. Initial investigations in this direction have related the radius of a circumscribing sphere to the directivity, and bounds on the $Q$ -factor have also been derived for a partial directivity in a given direction. In this paper, we derive lower bounds on the $Q$ -factor for a total desired directivity for an arbitrarily shaped antenna in a given direction as a convex problem using semidefinite relaxation (SDR) techniques. We also show that the relaxed solution is also a solution of the original problem of determining the lower $Q$ -factor bound for a total desired directivity. SDR can also be used to relax a class of other interesting nonconvex constraints in antenna optimization, such as tuning, losses, and front-to-back ratio. We compare two different new methods to determine the lowest $Q$ -factor for arbitrary-shaped antennas for a given total directivity. We also compare our results with full electromagnetic simulations of a parasitic element antenna with high directivity. [ABSTRACT FROM AUTHOR]

In this paper, we examine sparse array quiescent beamforming for multiple sources in interference-free environment. To maximize the output signal-to-noise ratio (SNR), the beamformer design comprises two intertwined stages, the determination of beamforming weights and the reconfiguration of array structure. The SNR maximization may produce high sidelobe levels, making the receiver vulnerable to interferences. We consider the problem of achieving maximum SNR beamforming subject to specified quiescent pattern constraints and, as such, combine both adaptive and deterministic approaches for sparse array configurations. We employ two convex relaxation methods and an iterative linear fractional programming algorithm to solve the nonconvex antenna selection problem for sparse array beamformers. Simulation examples demonstrate that the array configuration plays a vital role in determining the beamforming performance in interference-free scenarios. [ABSTRACT FROM AUTHOR]

In this paper, we have investigated the effects of vehicular traffic on interpersonal wearable-to-wearable (W2W) communications channels in an urban environment at 2.45 GHz. In particular, we have studied the perturbations in the received signal caused by different types of vehicles as they passed through a channel between two persons who maintained various relative orientations while positioned at the opposite sides of a road. As the channel underwent different fading mechanisms depending on whether the vehicle was approaching, transitioning (i.e., intersecting the direct signal path), or receding from the persons, the overall disturbance was appropriately segmented depending on the journey stage. The results have shown that relative body orientation was a significant factor when considering the impact that a vehicle can have on a W2W link. When both persons faced the oncoming traffic, the link was particularly susceptible to significant fading events with variations in the received signal power from the unperturbed state as great as 44.1 dB observed to occur. For all of the journey stages, irrespective of the relative orientation of the persons, the logarithmically transformed long-term fading process was found to be multimodal and well described by a Gaussian mixture model. During the transitioning phase, shadowing caused by the passing automobile obstructing the line-of-sight signal path was found to be the main contributor to the signal fading. However, probably the most remarkable result of the channel characterization work conducted in this paper was the severity of the short-term fading often observed. Such was the intensity of the measured envelope fluctuation in many of the scenarios, we have been able to utilize the recently proposed $\kappa -\mu $ extreme distribution with great success and in the process, provide a further important empirical validation of this new fading model. Moreover, we have used the resistor-average distance, which is derived from the Kullback–Leibler distance to show the improved fit that the $\kappa -\mu $ extreme distribution offers compared with the $\kappa -\mu $ distribution when used to model the W2W channel in this fading environment. [ABSTRACT FROM PUBLISHER]

This paper presents an on-site calibration technique and significant results obtained from the compensation of errors and mutual coupling. The proposed algorithm deals with the problem of calibration of active antenna arrays at reception and transmission, proposing an on-site calibration technique. The proposed on-site calibration technique starts with the application of an off-line calibration algorithm of which two different techniques are proposed. These techniques are based on the direct estimation of the phase center from the active element pattern measured, and the phase center estimation based on the active element pattern suggested. The expansion of equations of the proposed off-line calibration algorithms is also presented. These algorithms deal with the compensation of mutual coupling effect and together with gain, phase, and location errors. [ABSTRACT FROM PUBLISHER]

Numerical analysis of a large-scale finite periodic array is accurate but quite costly when the array is analyzed as a finite array. The computational cost for the numerical analysis can be reduced greatly when the array is approximated as an infinite periodic structure. However, the edge effect that strongly affects the active impedance or current distribution of the array elements near the edge is neglected. In this paper, a macro block-characteristic basis function method (MB-CBFM) for numerical analysis of a large-scale finite periodic array with a uniform amplitude and linear phased excitation is proposed. The MB-CBFM utilizes blocks and macro blocks to group the elements in order to reduce its computational cost without degrading accuracy. Numerical simulations demonstrate that the CPU time and computer memory of the MB-CBFM are $O(N)$ when the size of array is sufficiently large. [ABSTRACT FROM PUBLISHER]

In this paper, the physical meanings and various computing methods of characteristic modes (CMs) of dielectric and magnetic bodies are investigated. Two types of surface integral equation (SIE) formulations based on the dependent relationships between the equivalent surface electric and magnetic currents are proposed. Following the two types of SIE formulations, two generalized eigenvalue equations are established to determine the CMs of dielectric or magnetic bodies. This is in contrast to the approach using induced volume currents. It is found that the eigenvalues possess clear physical meanings similar to those of perfectly electric conducting bodies. The CMs can be used to describe the resonant features, including resonant frequencies and resonant field distributions. Numerical results are presented to validate the proposed formulations. [ABSTRACT FROM AUTHOR]

A microstrip planar array antenna is presented in this paper, which is designed to have near-field focused beams, and it is capable of steering its focus on the designed focal plane. The array is composed of eight series-fed linear arrays, which are able to scan the focus by frequency around 10 GHz in the H-plane, whereas, E-plane focus scan can be achieved by feeding the linear arrays with different phases. The proposed array can be used in near-field scan systems, e.g., microwave imaging system, with a faster scan rate than mechanically scanned antennas and a lower cost than array antennas with full phase control elements. Theoretical analysis is presented in this paper, and is verified by simulations and experiments. [ABSTRACT FROM AUTHOR]

In this paper, the maximum “recoverable” energy of a radiator is determined. This energy is defined as the maximum energy that can be recovered from the electromagnetic field distribution in entire space generated by the radiator up to the time point considered. The calculation procedure is based on minimizing the future radiated energy. It is shown that the concept of recoverable energy has no problems with so-called “negative energies,” or with any dependence on coordinate system, issues that compromise many definitions found in literature of the concept of “stored” energy for a radiator. The question can be raised whether “recoverable” energy can be identified with stored energy for a radiator. [ABSTRACT FROM AUTHOR]

This paper discusses the fabrication, operation, and physical reconfiguration of a structurally embedded vascular antenna. The antenna is a thin-wire dipole that meanders with rotational symmetry according to a sinusoid with power-series growth. It is formed from two curvilinear vascular networks embedded into a structural composite panel and connected to a conductive vertical interconnect structure and SubMiniature Version A-connected parallel-strip transmission line for excitation and measurement. The pressure-driven flow of a liquid metal alloy within the vascular network enables the physical reconfiguration of the meandering dipole with the primary goal of dynamically tuning the matched impedance bandwidth of the fundamental dipole mode by symmetrically lengthening the conductive pathways forming the dipole. Details regarding the design, fabrication, simulation, and measurement of the meandering dipole antenna are provided along with a linearly expanding dipole for comparison. [ABSTRACT FROM PUBLISHER]

In the time-domain simulation of electromagnetic and multiphysics problems, the distributions of physical quantities of interest vary in both space and time. To achieve a good spatial resolution, high-order basis functions can be used to expand the unknown quantities, which is known as the $p$ -refinement. However, a global and static $p$ -refinement will increase the computational cost significantly. In this paper, a dynamic $p$ -adaptation algorithm is proposed based on the discontinuous Galerkin time-domain method, which is able to determine and adjust the basis order in a given discretization element in real time of the simulation. Based on the relation between the nodal and modal approximations defined on unstructured tetrahedral elements, an error estimator, which is very cheap to compute, is developed to determine the proper basis order to achieve a desired numerical accuracy. The dynamic $p$ -adaptation algorithm proposed in this paper is able to capture the fast varying physics by changing the order of basis functions wherever and whenever needed. Several numerical examples adopted from multiple physical disciplines are presented to demonstrate the accuracy, efficiency, and flexibility of the proposed algorithm in the simulation of electromagnetic and multiphysics problems. [ABSTRACT FROM PUBLISHER]

In this paper, a four-port ultrawideband antenna design is proposed. The antenna is suitable for installation in pavements. The main application of this antenna is in the reuse of the existing manholes for radio equipment and antenna installation. Equally important is its application in dense network deployment as a result of its capability to alleviate intercell interference. The antenna can support up to $4\times 4$ multiple-input and multiple-output (MIMO) systems. The isolation between different ports exceed 28 dB over the commercial long term evolution bands 3 and 7. The multiport matching bandwidth of the antenna (>-0.3 dB) extends over 1700–3000 MHz range. The antenna is mechanically robust and can withstand a weight of 40 ton. MIMO performance of this antenna is verified by virtue of field measurements. A comparative study between the proposed antenna and a classic half-wavelength dipole antenna is presented highlighting the advantages of the proposed antenna. Finally, its compliance with the exposure limits for nonionizing radiation is also demonstrated. [ABSTRACT FROM PUBLISHER]

In this paper, we present an analysis of the edge effects in wideband connected arrays of slots and dipoles. Due to the strong mutual coupling between the elements, these arrays can support the propagation of guided waves along their surface. Such waves arise from the edges of the finite array and can be especially detrimental to the performance, since they can travel within the array without geometrical spreading. In this paper, we introduce Green’s function-based equivalent transmission line models to describe the propagation of the guided waves. The elements’ active impedances are represented as periodic loads on these transmission lines. The equivalent models can be used as a simple and convenient tool to control and minimize the edge effects. Finite array simulations of relevant array structures are discussed. The evidence is that the active impedance and the interelement capacitance can be tuned to attenuate and reflect the edge-born waves. [ABSTRACT FROM PUBLISHER]

The emission of radiators can be shaped by means of electromagnetic bandgap (EBG) materials exploiting proper Bloch waves supported by the lattice rather than its bandgaps. So far, such a method has relied only on the dispersion diagram of the periodic structure, i.e., on the eigenvalues of the lattice, neglecting the particular configuration of the associated eigenfunction. This paper explores the radiation mechanism under a novel viewpoint, which is mostly focused on the electric field pattern of lattice modes, allowing a deeper understanding of the underlying physics. Such an approach is profitably used here to improve the performance of antennas based on different Bloch waves by reducing spurious lattice modes. Geometrical configurations coupled to a line source are provided with little adjustments, getting EBGs to work at a frequency where they had no bandgap originally. As a proof-of-concept of the proposed perspectives, a compact antenna using a cheap low-permittivity dielectric is conceived, fabricated, and successfully tested. [ABSTRACT FROM PUBLISHER]

In certain radar and wireless communication applications, omnidirectional circularly polarized (CP) antennas are desired but existing solutions have complex 3-D structures and are large in size. In this paper, a compact and low profile CP antenna having omnidirectional pattern on azimuthal plane is demonstrated. The CP waveform is synthesized by combining two linearly polarized and omnidirectional antennas with 90° phase difference on their terminals. The most challenging part of the design is the realization of the horizontally polarized antenna with omnidirectional radiation pattern. This antenna is designed using a novel two-faced slot antenna structure capable of emitting in-phase radiation of equal magnitude on both sides of the antenna. The two faces of the low-profile slot antenna are connected by a thin meander parallel plate waveguide to provide antiparallel field distributions over the two faces of the slot radiators. This special design removes radiation nulls on E-plane that exists on conventional slot antennas, the mechanism of which has been carefully discussed in this paper. The other antenna is a compatible pair of planar inverted-F antennas constructed on the sides of the finite-ground plane two-faced slot antenna for the provision of vertically polarized omnidirectional radiation. A feed network with power splitter and phase shifter is also accommodated on the compact CP antenna. The antenna is designed and fabricated, and its performance is measured against numerical simulations. It is shown that the fabricated antenna at 2.4 GHz can provide measured axial ratio of lower than 1.8 and gain variations of less than ±1.5 dB over all azimuth angles. [ABSTRACT FROM PUBLISHER]

In this paper, we propose a new method for designing dual polarized beam switching antennas by using reconfigurable radial waveguides excited by central probes and feeding multiple radiating elements. The power distribution of the radiating elements is controlled by cylindrical electromagnetic bandgap structures made of metallic wires loaded with PIN diodes. The proposed technique allows azimuth beam switching and multiple beams, with arbitrary and well controlled polarizations, using low dc voltage. First, a new rectangular periodic structure is used as a model for analyzing the transmission and reflection coefficients for different diode states. Then, numerical and experimental results are presented for a reconfigurable radial waveguide power divider and for the radiating elements. Finally, numerical and experimental results are presented and discussed for a complete dual polarized antenna operating at wireless local area network (WLAN) 5 GHz band (5.18–5.825 GHz). [ABSTRACT FROM PUBLISHER]

This paper presents a wideband circularly polarized millimeter-wave (mmw) antenna design. We introduce a novel 3-D-printed polarizer, which consists of several air and dielectric slabs to transform the polarization of the antenna radiation from linear to circular. The proposed polarizer is placed above a radiating aperture operating at the center frequency of 60 GHz. An electric field, E , radiated from the aperture generates two components of electric fields, E \mathrm {x} and E \mathrm {y} . After passing through the polarizer, both E \mathrm {x} and E \mathrm {y} fields can be degenerated with an orthogonal phase difference which results in having a wide axial ratio bandwidth. The phase difference between E \mathrm {x} and E \mathrm {y} is determined by the incident angle $\phi $ , of the polarization of the electric field to the polarizer as well as the thickness, {h} , of the dielectric slabs. With the help of the thickness of the polarizer, the directivity of the radiation pattern is increased so as to devote high-gain and wideband characteristics to the antenna. To verify our concept, an intensive parametric study and an experiment were carried out. Three antenna sources, including dipole, patch, and aperture antennas, were investigated with the proposed 3-D-printed polarizer. All measured results agree with the theoretical analysis. The proposed antenna with the polarizer achieves a wide impedance bandwidth of 50% from 45 to 75 GHz for the reflection coefficient less than or equal −10 dB, and yields an overlapped axial ratio bandwidth of 30% from 49 to 67 GHz for the axial ratio ≤ 3 dB. The maximum gain of the antenna reaches to 15 dBic. The proposed methodology of this design can apply to applications related to mmw wireless communication systems. The ultimate goal of this paper is to develop a wideband, high-gain, and low-cost antenna for the mmw frequency band. [ABSTRACT FROM AUTHOR]

Electromagnetic scattering characteristics highly depend on the geometry and material property of the scatterers. In particular, electromagnetic wave scattering occurs at the discontinuity of the propagation path, such as the edge of a scatterer. In this paper, the edge scattering from a triangular object is largely reduced by patterning the triangular surface with inhomogeneous and anisotropic impedance surface. Because surface waves propagate toward the direction with the lower surface refractive index on an anisotropic surface, the direction of the wave propagation can be controlled by designing the distribution of the refractive index. Consequently, the proposed anisotropic impedance surfaces can make the current flows toward or away from the edge so that the edge scattering can be reduced or redirected to a different angle. For demonstration, anisotropic unit cells are analyzed and designed. Also, the anisotropic impedance surfaces are simulated, fabricated, and measured. Here we propose two types of impedance profiles, which are capable of changing the surface current direction either toward or away from the scattering edge. The experimental measurement demonstrates a 7–10 dB edge scattering reduction. [ABSTRACT FROM AUTHOR]

We present a mixer-duplexer-antenna leaky-wave system based on periodic space-time modulation. This system operates as a full transceiver, where the upconversion and downconversion mixing operations are accomplished via space-time transitions; the duplexing operation is induced by the nonreciprocal nature of the structure, and the radiation operation is provided by the leaky-wave nature of the wave. A rigorous electromagnetic solution is derived for the dispersion relation and field distributions. The system is implemented in the form of a spatio-temporally modulated microstrip leaky-wave structure incorporating an array of subwavelengthly spaced varactors modulated by a harmonic wave. In addition to the overall mixer-duplexer-antenna operation, frequency beam scanning at fixed input frequency is demonstrated as one of the interesting features of the system. A prototype is realized and demonstrated by full-wave and experimental results. [ABSTRACT FROM PUBLISHER]

In this paper, we propose a new 3-D conservative FDTD(2, 4) scheme exhibiting properties close to the 2-D-M24 scheme. This scheme is designed to retain a 2-D stencil in the field update equations and has high-phase accuracy at low-grid resolutions. Moreover, an accurate Courant–Friedrichs–Lewy condition has been validated for this scheme. Some experiments are conducted to assess the higher accuracy of the proposed scheme. [ABSTRACT FROM AUTHOR]

This paper proposes a compact wideband and low-profile antenna mountable on large metallic surfaces. Six rows of coupled microstrip resonators with different lengths are printed on a Teflon block. The lengths of the microstrip resonators in different rows are gradually reduced along the endfire direction. The first four rows of resonators provide four resonances at different frequencies for bandwidth enhancement. The last two rows of resonators are used as two directors to suppress sidelobe levels at high frequencies. A trapezoidal launcher is applied to serve as a reflector and provide one resonance at the lowest frequency. A trapezoid-shaped capacitive-feed strip is utilized and also printed on the Teflon block to globally optimize the wideband impedance matching. The proposed antenna covers a relative bandwidth of 109% for VSWR < 2 with a height of 0.044\lambda L and a volume of only 0.03\lambda L^{3} ( \lambda L being the free-space wavelength at the lowest operating frequency). Within the operating band, the realized gain is better than 6.5 dBi and the radiation efficiency is higher than 95%. [ABSTRACT FROM AUTHOR]

A slot helix antenna working in Global Positioning System (GPS) L1 and L2 bands is proposed in this paper. Adopting sequential rotation technique, the proposed antenna is equipped with six helix radiation arms and fed by a microstrip line with folded shape. The whole antenna can be built on a full printed circuit board; so, it is easy to process the proposed antenna. The circularly polarized radiation mechanism of a circular array is analyzed in a new perspective to guide the design. A double-faced slot radiation structure is also established for dual-band operation. Additionally, a traveling wave feeding network suitable for the proposed antenna is presented. The experimental results show that the proposed antenna has good properties in GPS L1 and L2 bands. [ABSTRACT FROM PUBLISHER]

Starting from measurements performed at 1140 MHz in an urban environment (Rio de Janeiro, Brazil), the signal strength measured is compared with the usual predictions calculated using the methods of International Telecommunication Union-Radiocommunication (ITU-R) Rec. 526-11/Cascade Knife Edge, and ITU-R Rec. 526-12/Delta-Bullington. The first model presented better adjustment to the data and, by using artificial neural networks (ANNs), this paper deals with the signal coverage prediction in a mobile channel calculated by the ITU-R model, which is used to train the ANN in order to reduce the average deviations. This hybrid application (ITU-R/ANN) showed efficiency, resulting in higher accuracy for the signal strength prediction, and an average improvement of 8 dB was achieved. [ABSTRACT FROM PUBLISHER]

A simple, low-cost, linearly polarized reconfigurable reflectarray surface (RAS) is presented that can provide beam steering over a wide range of angles in one plane. The results for a prototype design demonstrated beam steering from 35° to 60° at 5.5 GHz. Simple equations describing the beam steering angle and overall size for a minimum radar cross-sectional area (RCS) are presented that aid the system designer. The concertina RAS can be manufactured using screen-printing techniques making this potentially a low-cost product that can meet the needs of widespread deployment needed by wireless systems. A sample design using an A2-sized sheet of paper increased a 5.5 GHz signal by 20 dB, eliminating a “dead spot” in the laboratory. [ABSTRACT FROM AUTHOR]

Presents a listing of the editorial board, board of governors, current staff, committee members, and/or society editors for this issue of the publication. [ABSTRACT FROM AUTHOR]

Presents a listing of the editorial board, board of governors, current staff, committee members, and/or society editors for this issue of the publication. [ABSTRACT FROM AUTHOR]

In this paper, a design of reflector antennas to produce near-field focused (NFF) radiation is presented. Instead of focusing the radiation at a single focal point, which results in an ellipsoidal reflector, this new reflector antenna application concept designates a finite zone, especially a finite curved surface, for NFF illumination. This finite zone may consist of several subzones for multi-NFF radiation. The design concept employs equal path lengths of wave propagation from the feed’s phase center toward the focal zone via reflections from the reflector’s surface in a one-to-one correspondence, which results in a confluence of reflection points found from a set of ellipsoidal surfaces. Based on a definition of the mapping mechanism to extract the reflection points to form the reflector, the field can be properly focused in the target zone by the concept of geometric optics (GO). In this paper, the basic theoretical concept is first summarized with a design equation developed for easy implementation. The focus property is demonstrated by a time-domain physical optics (TD-PO) analysis of spherical wave illumination with a transient-step field behavior to show the focusing responses of electromagnetic scattering from the reflector. The focusing characteristics of the frequency domain (FD) are also shown by PO simulation at 20 GHz. [ABSTRACT FROM AUTHOR]

This paper presents a second higher-order-mode microstrip leaky-wave antenna (LWA) for single main beam radiation. Different from its conventional second higher-order leaky-mode counterpart with two symmetric radiation beams, the proposed antenna with an array of I-shaped slots radiates only one main radiation beam at the cross-sectional center plane. By introducing a transversally loaded slot array along the microstrip line, the cross-polarization of the antenna can be largely enhanced, while a longitudinally loaded slot array interrupts the current flow and thus effectively reduces the original two radiation beams. Subsequently, an array of I-shaped slots, including transversal- and longitudinal-slot portions, is etched on the strip conductor and its unique characteristics are then analyzed to obtain a single main radiation beam in the center plane. To validate this proposal, a prototype of single main beam LWA with second higher-order mode is designed, implemented, and fabricated. Owing to larger leakage constant, the LWA presented in this paper radiates higher power within a shorter length in terms of guided wavelength, resulting in a higher gain. The measured results show a beam-scanning capability from 20° to 55° as the frequency varies from 5.1 to 4.6 GHz, while the maximum gain of 14.6 dBi is achieved at 4.9 GHz. [ABSTRACT FROM AUTHOR]

This paper presents the spectral Green’s function (GF) to analyze Fabry–Perot antennas to generate circular polarization (CP). The structure consists of a single cavity created between a partially reflective surface and a high impedance surface, fed by a low directive, linearly polarized source. Elements in the antenna are characterized by tensor impedances. It allows conceiving a circularly polarized antenna supporting a single hybrid leaky-wave mode. Residue analysis is used to demonstrate that a single hybrid leaky-wave mode is sufficient to generate CP. The tool is validated by an antenna design matched over a relative bandwidth of 28.5% with an axial ratio lower than 3 dB. [ABSTRACT FROM AUTHOR]

For high-power electromagnetic (HPEM) applications at high-repetition rate, conventional microwave inverters based on pulse transformers are not practical. At high-repetition rates, the magnetic materials used in the inductive coupling saturate. In this paper, we present an alternate concept for an inverter based on the technology used for ultrawideband baluns which provides isolation through capacitive coupling. Design of the device is discussed through numerical modeling, and two implementations are built and demonstrated: a circuit element version and a transmission line version. The devices are demonstrated at charge voltages of 6 kV and repetition rates up to 100 Hz, with excellent performance between 20 MHz and 2.5 GHz. The circuit element device has a rise time of 445 ps with an insertion loss of less than 3 dB, while the transmission line device has a rise time of 240 ps with an insertion loss of less than 2 dB for most frequencies. The inverter is demonstrated as part of the feed network for a hyperband HPEM antenna with a differential input. [ABSTRACT FROM AUTHOR]

In this paper, a wideband, low-profile, 1-D tightly coupled dipole array (1-D TCDA) antenna with vertical metal strips (VMSs) is proposed. The 1-D TCDA has a limited scanangle due to variations in vertical wave numbers according to the scanangle. Floquet theory and the electric field profile can be used to analyze the theory of a TCDA, obtaining scan-independent characteristic. Removing some metal strips from the side walls also improved the radiation efficiency. The overlapped impedance bandwidth (IBW) for dual polarization was 0.8–2.5 GHz (3.1:1) for VSWR < 3, and the maximum scanangle was ±50°. The radiation efficiency was over 70% within the IBW and maximum scanangle. The measurement results show that the gain was 7.0–12.4 dB for vertical polarization and 7.3–15.4 dB for horizontal polarization within the IBW and maximum scanangle. Furthermore, the height of the antenna was $0.13\lambda _{\mathrm {low}}$ at the lowest operating frequency, and the overall dimension of the full array structure was $1.69\lambda _{\mathrm {low}}\times 0.59\lambda _{\mathrm {low}}\times 0.13\lambda _{\mathrm {low}}$. [ABSTRACT FROM AUTHOR]

The theory of inner resonant modes (IRMs) is very useful for the analysis and design of dielectric resonator antennas (DRAs), but it suffers from several limitations including canonical DRA shapes, high permittivity substrate, and large errors. Characteristic modes (CMs) contain similar physical insights but without the limitations owing to its full-wave nature. In this paper, a new approach is proposed for the codesign of DRAs and feeding structures using CMs. The volume-surface formulation combining substructure treatment is adopted to calculate CMs of DRAs. To detailedly demonstrate the proposed method, electric and magnetic feeding structures are utilized to excite a cubic DRA as examples. Compared with the results obtained by an isolated design of DRAs and feeding structures, characteristic mode analysis (CMA) of the whole structure reveals several new modes, in which the lower order modes correspond to much lower resonant frequencies. Proper modifications of the feeding structures can effectively excite these modes and result in dual- and triple-band operation. Several prototypes were fabricated and measured for verification. For a cubic DRA with a side length of 25.4 mm, it achieves a triple-band operation at 1.6, 3.1, and 4.6 GHz. Therefore, this method is ideal for the codesign of arbitrary DRAs with feeding structures. [ABSTRACT FROM AUTHOR]

This paper presents a new array response control algorithm named complex-coefficient weight vector orthogonal decomposition (C2-WORD), and its application to robust sidelobe control and synthesis in the presence of steering vector mismatch. The proposed C2-WORD algorithm is a modified version of the existing WORD approach. We extend WORD by allowing a complex-valued combining coefficient in C2-WORD, and then determine the optimal combining coefficient by maximizing the white noise gain. Moreover, assuming that the steering vector uncertainty is norm-bounded, we further devise a robust C2-WORD algorithm, which is able to precisely control the upper boundary response level of a sidelobe point as desired. To enhance the practicality of the proposed robust C2-WORD algorithm, we also study how to determine the upper norm boundary of steering vector uncertainty under various mismatch circumstances. By applying the robust C2-WORD algorithm iteratively, a robust sidelobe synthesis approach is developed. Contrary to the existing approaches, the devised robust C2-WORD algorithm offers an analytical expression of weight vector updating and can work starting from an arbitrarily specified weight. Simulation results are presented to validate the effectiveness and good performance of the robust C2-WORD algorithm on sidelobe control and synthesis in the presence of steering vector uncertainties. [ABSTRACT FROM AUTHOR]

In this paper, we propose an antenna design procedure based on the eigenmode decomposition technique, which avoids the use of the external matching networks that are typically required at the input ports of the mode decomposition network (MDN). We illustrate the technique by examining the properties of a circularly symmetrical four-port antenna configuration, which can achieve four orthogonal radiation patterns or eigenpatterns that allow both polarization and pattern diversity. We then use the proposed theory to design an actual antenna structure with eigenmodes operating around 2.6 GHz and whose eigenpatterns are directed in one hemisphere. The antenna and an example MDN are manufactured and tested showing that the four eigenmodes can simultaneously operate within 9.5% of bandwidth (RL > 10 dB) with a worst case total radiation efficiency of 72%, correlation coefficient of 0.04, and port coupling of −24 dB. Finally, we use the measured results to assess the antenna in a multiple-input-multiple-output (MIMO) communication system and demonstrate its beam-tilting capability. [ABSTRACT FROM AUTHOR]

This paper studies the rain-induced propagation characteristics over a tropical location Kolkata, India, using the data collected over a 7 year period, i.e., 2004–2010. This location experiences substantial rainfall during the Indian Summer Monsoon (ISM) season which is preceded by premonsoon rainfall accompanied by strong convection. Accumulations, event duration, and fade duration statistics are presented. Long-term observations for three propagation effects, namely rain attenuation, scintillations, and depolarization, are analyzed in this paper. The seasonal patterns of these phenomena are examined to reveal the characteristic features of Kolkata. The interrelations among rain rate, rain attenuation, scintillations, and depolarization are also investigated. Propagation detriments for identical rain rates are more severe in premonsoon compared to monsoon period, mainly because of the different microphysical properties of precipitation. However, the percentage occurrences of various propagation effects are higher in monsoon than premonsoon. [ABSTRACT FROM AUTHOR]

Wireless power transfer is a practical and widely used method to power various implantable devices. Commonly, the implanted receiver (RX) must be small, often on the order of millimeter, which poses significant design challenges. In this paper, a technique to improve the performance of systems with a size (inductance)-limited implanted RX coil is explored. Conventionally, only mutual coupling between coils is used to optimize the performance, which constrains the layout/geometry and choice of coils. In the proposed system, mutual coupling, mutual capacitance, and relative polarity can all be used, thereby reducing the constraints on coil layouts. Performance is further enhanced by two additional techniques that maximize the reflected impedance between the RX coils. Three designs (two-coil, conventional three-coil, and the proposed three-coil) are implemented with a 35 mm diameter RX coil, and their performance is measured at 5 MHz with a 1 $\text{k}\Omega $ load resistance. The efficiency of these designs is measured at varying distances (20–60 mm) between the transmitter and RX. The efficiency of the proposed three-coil system at 50 mm separation is 40%, while the implemented conventional three-coil system and the two-coil system are each less than 10%. Finally, the advantage of the proposed strategy in terms of RX coil current and load tolerance is discussed. [ABSTRACT FROM AUTHOR]

Currently, medical devices such as the wireless capsule endoscopy (WCE) are used for data transmission from inside to outside the body. However, for certain applications such as WCE, the data rates offered by the current medical frequency bands can be insufficient. Ultrawideband (UWB) frequency band has become an interesting solution for this. However, to date, there is no formal channel path loss (PL) model for the UWB frequency band in the gastrointestinal (GI) scenario due to the huge differences between the proposed studies. There are three main methodologies to characterize the propagation channel, software simulations, and experimental measurements either in phantom or in in vivo animals. Previous works do not compare all the methodologies or present some disagreements with the literature. In this paper, a dedicated study of the PL using the three aforementioned methodologies (simulations, phantoms, and in vivo measurements) and a comparison with previous studies in the literature is performed. Moreover, numerical values for a PL model which agrees with the three methodologies and the literature are proposed. This paper aims at being the starting point for a formal PL model in the UWB frequency band for wireless body area networks in the GI scenario. [ABSTRACT FROM AUTHOR]

This paper presents a general quasi-analytical analysis of time domain (TD) electromagnetic (EM) scattering from a perfecting conducting object when it is illuminated by the radiation of a relatively arbitrary EM source with a transient-step TD response. In particular, the EM scattering mechanism is interpreted by using TD physical optic (PO) approximation to result in a contributing (integration) contour for the radiation integral. A fast numerical algorithm is also developed to find the EM scattering field. This TD-PO solution can be further applied via a time convolution to derive the early-time transient fields scattered from the same scatterer when it is illuminated by a realistic astigmatic finite-energy pulse. Practical numerical examples are presented to demonstrate the physical phenomenon of scattering mechanisms. [ABSTRACT FROM AUTHOR]

In this paper, we derive an analytical model for plane-wave transmission through a periodic array of dielectric-loaded slots in an infinite metallic screen of finite thickness (a metagrating). We show the existence of (Fano) resonances associated with the presence of higher order modes in the slots, in addition to the well-known (Fabry–Perot) resonances associated with the dominant mode in the slots. The Fano resonances arise from a physical mechanism distinct from that which is responsible for so-called extraordinary transmission through a grating, which is strongly dependent on the coupling among the slots. Comparison of the analytical model with the results of full-wave simulations shows excellent agreement when the characteristic dimensions of the grating are small compared with a free-space wavelength. [ABSTRACT FROM AUTHOR]

This paper proposes that ring slot antennas, loaded with shorting strips, can achieve broadside gain enhancement while maintaining a low profile and bidirectional radiation. Both circularly polarized (CP) and linearly polarized (LP) ring slot antennas are used to demonstrate this method. With infinite ground plane (GP), CP and LP ring slot antennas are observed to increase by at least 0.8 and 1.7 dBi in realized gains, respectively. Combined characteristic modes (CMs) are used to analyze and explain the working mechanisms of these antennas. It is shown that shorting strips could help to enlarge the radiation apertures of ring slots, while the center frequency remains constant; hence, broadside gains are enhanced successfully. It is also discussed that by replacing the infinite GP with a finite one of about $1.5\lambda \times 1.5\lambda $ , gains of both CP and LP ring slot antennas change little at center frequencies. Prototypes of these two antennas are then fabricated and measured to experimentally validate the analysis and designing approach. [ABSTRACT FROM AUTHOR]

In this paper, we present a novel method that utilizes amplitude-only data of the electric field over a completely closed surface for the reconstruction of the spherical electric field, which represents a radiation source or an antenna under test (AUT) in the near-field (NF) region. The electric field exploiting genetic algorithm (GA) for global optimization is originally derived from a random initial estimate of field distribution and will gradually approximate to the exact value during iterations. When GA reaches the convergence criterion, we can calculate the far field (FF) by spherical NF-to-FF (NF-FF) transformation. The simulated and numerical results are in good agreement with each other, which shows that this method is effective over one or two closed surfaces. Furthermore, the related discussions corresponding to GA are given, confirming that the improved GA is feasible and efficient. [ABSTRACT FROM AUTHOR]

A two-dimensionally (2-D) scanning multibeam array with planar structure and low sidelobe level (SLL) is realized in this paper. It is for the first time that the SLL of 2-D scanning multibeam array is properly addressed. To suppress the SLL, a $4 \times 16$ beam-forming network (BFN) is proposed in this paper. Different from the existing schemes, such BFN can provide tapered illuminations in two orthogonal directions and thus can be applied to realize low-SLL 2-D scanning beams. The proposed $4 \times 16$ BFN is formed by interconnecting two sets of sub-BFNs, each of which is stacked by several $2 \times 4$ Butler matrices (BMs). Like other 2-D scanning schemes, the preliminary $4 \times 16$ BFN is of 3-D topology so that it is challenging to be integrated into planar substrate integrated waveguide (SIW) structure. Facing this problem, some modifications of the proposed topology are conducted. Operation principle, simulation, and experiment of a planar, low-SLL 2-D scanning multibeam array operating at 28 GHz are illustrated, which can help to demonstrate its feasibility for millimeter-wave applications. [ABSTRACT FROM AUTHOR]

This paper presents a 2-D near-field focusing (NFF) substrate-integrated-waveguide (SIW) leaky-wave antenna (LWA) array with high radiation efficiency. In most of the existing 2-D NFF antenna arrays, the required quadratic phase and balanced amplitude distribution are achieved by the use of the transverse topology with a complex feeding network or the simple radial topology with high polarization loss. Here, a folded reversely fed LWA with transverse polarization is proposed to achieve natural balanced radiation with a sample series-fed network. Meanwhile, the width of the single linear LWA is complementary with the reversely fed topology. In this case, the SIW with a stepped width is applied to achieve a quasi-uniform slot distribution and to avoid the spurious radiation. Then, all reversely fed linear LWAs are connected by the meander-line structure, which can simplify the feeding network and enhance the radiation efficiency. The limitation of the NFF LWA in radiation efficiency is broken. The 2-D focusing principle and the design process of the proposed antenna are presented in this paper. As an example, a 2-D focusing SIW LWA array operated at 35 GHz with an 80% radiation efficiency is fabricated and measured. [ABSTRACT FROM AUTHOR]