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The feasibility of electrically conductive paper as a low-cost and eco-friendly alternative to copper is presented for application as an antenna conductor in UHF radio-frequency identification (RFID) systems. The conductivity of the paper is determined to be 50 S/m and the effect of such a low conductivity on the radiation performance of a half-wavelength dipole is investigated through simulation and experimental measurements. The radiation efficiency of a dipole cut from a 0.4 mm thick conductive paper sheet is measured to be 5.0%, which translates to an RFID tag read range of 22% of that obtained using copper. This performance is deemed unacceptable for most antenna applications. However, through simulation a conductivity of 500 S/m is identified as a reasonable target for the developers of conductive paper as the current distribution and terminal properties of the resulting antenna become similar to those observed when using copper. The radiation efficiency is projected at 56 % in this case, resulting in an RFID read range of 73% of that obtained using copper. [ABSTRACT FROM PUBLISHER]

A complete characterization of the inkjet printing process using metallic nanoparticle inks on a paper substrate for microwave frequencies up to 12.5 GHz as well as its application to low-cost, high gain and wideband antenna design are demonstrated in this work. Laser and heat sintering of metallic nanoparticles are compared on paper substrate for the first time which demonstrate immense cost and time benefits of laser sintering. The antennas fabricated using the characterized process include a Vivaldi for the UWB band which exhibits a significantly higher gain of up to 8 dBi as compared to the currently published inkjet printed antennas, and a novel slow-wave log periodic dipole array which employs a new miniaturization technique to show 20% width reduction. [ABSTRACT FROM PUBLISHER]

In this paper, the B-scan technique is applied to a reverberation chamber (RC) for the first time to characterize the time-domain behavior of the chamber. Based on B-scan measurement results, three things are studied in this paper. 1) The statistical behavior of the fields in the time domain is investigated, and it is found that the received power of the impulse response follows chi-square distribution with one degree of freedom. 2) The stirrer efficiency is quantified based on the equivalent total scattering cross section (TSCS) of stirrers, and this definition is not sensitive to the antenna position and load in the RC. It is shown that the stirrer efficiency defined in this paper provides, for the first time, a quantitative way to evaluate the stirrer design and the chamber performance. 3) A time-gating technique is proposed which provides an alternative method to eliminate the early time response and obtain the chamber transfer function of the RC accurately. This could be potentially used to increase the stirrer efficiency without changing the stirrers physically. It is demonstrated that the B-scan technique is a very useful approach for the study and evaluation of an RC. [ABSTRACT FROM PUBLISHER]

This paper compares three modern and two classical multiobjective optimizers (MOOs) as applied to real-world problems in electromagnetics. The behavior of sophisticated optimizers on simple test functions has been studied exhaustively. In contrast, the algorithms here are tested on practical applications, where the function evaluations are computationally expensive, making the convergence rate a crucial factor. The examples considered include the optimization of a narrowband slot antenna, a mushroom-type electromagnetic bandgap structure, and an ultrawideband Vivaldi antenna. Another popular topic in the literature is in comparing classical MOOs on electromagnetics problems. The modern optimizers chosen in this paper are state of the art and each has a distinct design philosophy. This paper introduces two unique MOOs to the electromagnetics community: BORG, an auto-adaptive genetic algorithm and the Multi-Objective Covariance Matrix Adaptation Evolutionary Strategy (MO-CMA-ES), an extension of the popular single-objective CMA-ES. These algorithms are compared to the Multi-objective Evolutionary Algorithm based on Decomposition (MOEA/D), a Chebysheff scalarization algorithm, and two classical MOOs. This paper will study the behavior of these algorithms on problems in electromagnetics with a limited number of function evaluations using five distinct metrics and will provide useful guidelines and recommended optimizer settings. [ABSTRACT FROM AUTHOR]

In this paper, a quad-mode endfire planar phased antenna array with wide scan angle and 1.2 mm clearance is proposed for 5G mobile terminals. The proposed antenna can obtain over 8 GHz of impedance bandwidth. In this paper, it is suggested to efficiently combine a multimode array element with different radiation patterns for each mode into a phased antenna array. In the array, similar and wide embedded radiation patterns are obtained for the all four modes. Furthermore, a coaxial to differential stripline transition is designed in this paper. The differential feeding structure is very compact and utilizes only MMPX connector and vias. The total scan pattern and coverage efficiency of the measured and simulated phased array antenna are calculated in the range from 25 to 33 GHz, and good agreement between measured and simulated results is observed. The mean coverage efficiency along the frequency range is very similar, but minor difference in variance of coverage efficiency is observed in the measurements. The coverage efficiency of around 50% for the threshold gain of 5 dBi is achieved in the chosen frequency range. [ABSTRACT FROM AUTHOR]

This paper discusses a method to regularize linear problems in presence of incomplete and noisy data generated by partially or totally unknown sources. The idea is to identify the “simplest” solution, looking for the system having the minimum number of degrees of freedom (NDF) compatible with the available data. It is shown that a normalized version of the trace norm (known as 1-Schatten norm in functional analysis) is a good convex approximation of the NDF of the field. This allows to use the trace norm as a convex surrogate of the NDF in the minimization process, taking advantage of the recent literature on rank minimization. Examples regarding MIMO beamforming from incomplete channel matrix, field interpolation with no a priori information on the sources, and land-mine identification using the total field are discussed. [ABSTRACT FROM PUBLISHER]

This paper deals with the efficient computation of scattering problems by fast oscillating periodic surfaces using two-scale expansion homogenization techniques. The proposed method splits the field between an effective field and a boundary layer corrector and applies an asymptotic expansion. Then, the evaluation of these boundary layers leads to the extraction of effective parameters, modeling the effects of the highly oscillating surface on the scattered field. A previous paper has already presented such a technique to deal with TM polarization. First, this paper extends the approach to TE polarization and completes it by pointing out all the key points. Especially it shows the map of the split fields underlining the physical contribution of each term. Second, this paper exposes and compares two different uses of these computed effective parameters. Numerical examples show the validity of the method, especially by justifying the asymptotic expansion ansatz. Computational efficiency is discussed. [ABSTRACT FROM AUTHOR]

This paper discusses the design and implementation of a biomimetic, flexible antenna system consisting of flexible dipoles on Pyralux AP. They are implemented into a novel, airborne atmospheric sensor system. The antennas and substrate serve a secondary function as a rotor system, forcing the sensor nodes to autorotate when falling to maximize drag and decrease their fall speed. The designed antenna system works both in the industrial, scientific, and medical band and the GPS L1 band with the designed antennas having bandwidths of 100 and 130 MHz, respectively. This paper also presents a test procedure to characterize the antenna performance at different bend angles. [ABSTRACT FROM AUTHOR]

This paper presents the Greene second-order parabolic equation (PE) solution to model radio propagation over irregular terrain. This solution is implemented by finite-difference and shift-map technique. The second-order finite-difference approach performs well for slopes up to about 15°, and discontinuous slope changes up to about 30°, which is better than can be achieved using the split-step/Fourier approach. The results prove that the Greene PE solution has greater propagation angles than does the Claerbout PE solution. Thus, the Greene second-order solution can certainly give better results for large angle propagation over complicated terrain boundaries. Besides, we treat the important problem of accuracy of the different approximations of the PE-based propagation models, and derive the general PE solutions with respect to the terrain slope for the different approximations. It is useful to have collected in one paper, the most important PE approximations for radio propagation. [ABSTRACT FROM AUTHOR]

This paper is concerned with obtaining closed-form exact solution for the 2-D canonical problem of the reflected field from a moving planar discontinuity of a dielectric–magnetic medium that is excited by an impulsive line current. The spectral theory of transient (STT) that originally deals with nondispersive frequency-domain plane-wave spectra is used in this paper for addressing a time-variant scattering problem. The scattering from a moving object yields dispersive and anisotropic PW spectra. Nevertheless, we demonstrate that the STT is capable of dealing with these types of spectra and obtain the desired exact time-dependent solutions. The unique wave phenomena that are associated with the medium and scatterer velocity are explored. [ABSTRACT FROM PUBLISHER]

This paper presents a novel surface-wave (SW)-based high-impedance surface (HIS) multibeam antenna with 360° scanning capability in azimuth. The proposed antenna consists of four rotationally symmetric beamforming networks (BFNs) and a microstrip patch array-based HIS. Regarding to the BFN, it is formed by a planar offset parabolic reflector fed by multiple substrate integrated waveguides. Through proper design, the proposed BFN can efficiently launch guided SWs with TM0 mode. With the perturbation of the HIS structure, the bounded SWs can be transformed to $m=-1$ space harmonic mode leaky-wave radiation. Combining the feature of the leaky-wave radiation in the elevation plane and the manipulation of the propagation direction of the SWs by switching the feeding ports, beam scanning in the azimuth direction with a tilted elevation angle can be realized. As the HIS is shared by the four BFNs, the size of the antenna can be reduced and a broadened antenna scanning range is achieved. In this paper, a prototype of the proposed antenna with 28 feeding ports, 7 in each BFN, is numerically analyzed and experimentally demonstrated. The proposed antenna features low-cost, low-profile, and single layer configuration. [ABSTRACT FROM PUBLISHER]

Singular divergence-conforming bases have been proposed for the solution of integral equations although they have seen only occasional use in practical applications. The existing singular bases are not hierarchical, which prevents their use in adaptive $ {p}$ -refinement applications. In this paper, a new family of singular hierarchical basis functions is proposed for quadrilateral cells. These functions model the singularities associated with current and charge density at edges and are more convenient for modeling such singularities than triangular bases of the same kind. The basis functions are of the additive kind and combine a hierarchical polynomial representation on quadrilaterals with linearly independent singular terms that incorporate general exponents that may be adjusted for the specific wedge angle of interest. Moreover, the added singular basis functions are computed on the fly. On the basis of various reported numerical results, this paper also illustrates the difficulties, the advantages, the accuracy, and the cost of using such bases in the method of moment solutions of integral equations. [ABSTRACT FROM AUTHOR]

This paper describes a multifunctional microwave device that can be reconfigured to behave either as a filter or an antenna. The device is based around a single-resonant element, referred to as a building block element that is connected to four ports. The resonant element is a microstrip square patch working in the 2.45 GHz unlicensed Industrial Scientific and Medical band used for IEEE 802.11 WiFi. The proposed building block is simple in its design and construction. This paper also presents devices composed of three building blocks, which can realize more complex filtering and antenna mode performance. In addition, p-i-n diodes have also been incorporated into the devices in order to facilitate electronic control. Measured results, pertaining to fabricated prototypes, show that filter and antenna operation can be achieved in the same device at a frequency of around 2.45 GHz with acceptable insertion loss and efficiency. In the future, it is anticipated that this could lead to a new paradigm, namely, a device that is analogous to a field programmable gate array operating in the microwave frequency regime. [ABSTRACT FROM AUTHOR]

In this paper, we present modeling of very low frequency (3–30 kHz) through-the-Earth (TTE) radio transmissions in four multistory buildings using the finite-difference time-domain method. These structures are generally made of reinforced concrete, whose steel content is the most important control on bulk conductivity. The floors and walls are represented as sheets of a given conductance, based on the fact that electromagnetic geophysical methods will only resolve conductive sheets in terms of their conductance as opposed to both conductivity and thickness. We validate this approach first using synthetic data and second by comparison with field data. Conductance models were derived from observed data for a variety of propagation environments, including reinforced concrete, steel floors, and basements, and for transmission into, out of, or within a building. The models were able to account for nonintuitive observations, such as increase in signal strength near the edges of buildings and in localized areas in between buildings. Conductances obtained for the floors and walls could be used as representative values in future forward modeling to assess the viability or optimize a TTE radio link in a multistory building. [ABSTRACT FROM PUBLISHER]

This paper presents a new theory for analyzing the leaky-wave (LW) mechanism supported by planar metasurfaces (MTSs) described by continuous, anisotropic, nonuniform, locally periodic boundary conditions (BCs), and excited by a vertical dipole at the interface. These BCs well represent a dense distribution of subwavelength metal patches of infinitesimal thickness, printed on a grounded slab. We denote succinctly this theoretical formulation as flat optics for LWs. This formulation is based on an adiabatic, asymptotic form of the Floquet theorem, introduced here for the first time. The adiabatic Floquet-wave analysis allows for: 1) describing the mechanism of global interaction between the cylindrical surface wave (SW) launched by the dipole and the modulated MTS; 2) introducing a generalized curvilinear-wavefront LW field that can be used for controlling the radiation-pattern shaping; 3) describing the local and global transfer of energy from SW to LW; 4) establishing an analytical relationship between the aperture field polarization and the anisotropic parameters of the MTS and 5) between the leakage parameter $\alpha $ and the MTS modulation index. Concerning the latter point, a closed-form formula for $\alpha $ is introduced, which allows for unprecedented control of the aperture field amplitude. The theoretical results are successfully validated by comparison with a full-wave analysis, showing impressive accuracy. In a companion paper published in this Journal issue, the present theory is used for the synthesis of a large class of planar aperture antennas. [ABSTRACT FROM AUTHOR]

Integration of the finite-difference time-domain (FDTD) method into the iterative multiregion (IMR) technique, an iterative approach used to solve large-scale electromagnetic scattering and radiation problems, is presented in this paper. The idea of the IMR technique is to divide a large problem domain into smaller subregions, solve each subregion separately, and combine the solutions of subregions after introducing the effect of interaction to obtain solutions at multiple frequencies for the large domain. Solutions of the subregions using frequency domain solvers has been the preferred approach as such; solutions using time domain solvers require computationally expensive bookkeeping of time signals. In this paper, we present an algorithm that makes it feasible to use the FDTD method, a time domain numerical technique, in the IMR technique to obtain solutions at a prespecified number of frequencies in a single simulation. A hybrid method integrated into the IMR technique is also presented in this paper. This hybrid method combines the desirable features of the FDTD method and the method of moments (MoM) to solve radiation problems more efficiently. As a result, a considerable reduction in memory storage requirements and computation time is achieved. [ABSTRACT FROM PUBLISHER]

In this paper, we present a new methodology for a nonoverlapping domain decomposition method using boundary integral equations and adaptive cross approximation in the frequency domain (IE-ACA-DDM). This paper proposes two main novelties. First, the IE-DDM condition number is improved with the development of a formulation based on electric field integral equation (EFIE) implementation over the entire subdomains and on a simplification of the magnetic field integral equation (MFIE) on the interfaces. As a consequence, the number of matrix–vector products is significantly reduced during the iterative resolution. Then, this paper proves that the ACA can be applied and efficiently implemented in IE-DDM methods. Moreover, a hierarchical LU (HLU) decomposition is proposed to accelerate the ACA solver. The IE-ACA-DDM is very suitable for the resolution of multiple excitations and to bypass the low-frequency breakdown. The flexibility, accuracy, and efficiency of IE-ACA-DDM are demonstrated by comparing with reference integral equation solutions and scaled mock-up measurements. [ABSTRACT FROM PUBLISHER]

The mutual reactance between two elements of a lineal array is formulated using Fourier transform, autocorrelation, convolution, and Hilbert transform concepts, with the invocation of the Hilbert transform being the novel contribution of this paper. It is shown herein that the mutual reactance of an array can be determined from the radiation pattern, if one considers the entire time-averaged power delivered to both the visible and the nonvisible far-field regions. Power to the nonvisible region corresponds to stored energy in the Poynting volume; stored energy corresponds to reactive power, which is in quadrature with the nonvisible, time-averaged power; and quadrature suggests the invocation of the Hilbert transform. The end result of the Hilbert transform process is a straightforward, closed-form analytical expression for the mutual reactance. Since the closed-form mutual resistance has been determined previously, both closed-form expressions can be combined to provide an elegant, but easy-to-use formulation for the mutual impedance. For a given current distribution, the results are exact and validated against data obtained numerically. Cases associated with the half-wave dipole, generalized dipole distribution, and the uniform distribution are provided herein as examples of the robustness of the presented theory. [ABSTRACT FROM AUTHOR]

It is found that for very low-frequency (VLF) signals, the mode-conversion interference produces a cycle slip during the sunrise or sunset transition at certain distances far away from the transmitter. In this paper, an anisotropic terrestrial waveguide with abrupt effective height change of 20 km geometry has been adopted to investigate the diurnal behaviors of amplitude and phase caused by the night-to-day transition. At the waveguide junction, sequential determinations of transmission and reflection coefficients are determined by the resulting linear equations from field matching. Field calculations across the day-to-night transition are carried out by using a reciprocity theorem correspondingly. This paper illustrates that VLF wave propagation experiences multimode interference in periodic variations in amplitude during sunrise and sunset over a long propagation path. [ABSTRACT FROM AUTHOR]

The electric field integral equation is a well-known workhorse for obtaining fields scattered by a perfect electric conducting object. As a result, the nuances and challenges of solving this equation have been examined for a while. Two recent papers motivate the effort presented in this paper. Unlike traditional work that uses equivalent currents defined on surfaces, recent research proposes a technique that results in well-conditioned systems by employing generalized Debye sources (GDS) as unknowns. In a complementary effort, some of us developed a method that exploits the same representation for both the geometry (subdivision surface representations) and functions defined on the geometry, also known as isogeometric analysis (IGA). The challenge in generalizing GDS method to a discretized geometry is the complexity of the intermediate operators. However, thanks to our earlier work on subdivision surfaces, the additional smoothness of geometric representation permits discretizing these intermediate operations. In this paper, we employ both ideas to present a well-conditioned GDS-electric field integral equation. Here, the intermediate surface Laplacian is well discretized by using subdivision basis. Likewise, using subdivision basis to represent the sources results in an efficient and accurate IGA framework. Numerous results are presented to demonstrate the efficacy of the approach. [ABSTRACT FROM PUBLISHER]

We give an overview of recent developments in the modeling of radiowave propagation, based on machine learning (ML) algorithms. We identify the input and output specification and the architecture of the model as the main challenges associated with ML-driven propagation models. Relevant papers are discussed and categorized based on their approach to each of these challenges. Emphasis is given on presenting the prospects and open problems in this promising and rapidly evolving area. [ABSTRACT FROM AUTHOR]

This paper presents the design of a modulated metasurface (MTS) antenna capable to provide both right-hand (RH) and left-hand (LH) circularly polarized (CP) boresight radiation at Ku-band (13.5 GHz). This antenna is based on the interaction of two cylindrical-wavefront surface wave (SW) modes of transverse electric (TE) and transverse magnetic (TM) types with a rotationally symmetric, anisotropic-modulated MTS placed on top of a grounded slab. A properly designed centered circular waveguide feed excites the two orthogonal (decoupled) SW modes and guarantees the balance of the power associated with each of them. By a proper selection of the anisotropy and modulation of the MTS pattern, the phase velocities of the two modes are synchronized, and leakage is generated in broadside direction with two orthogonal linear polarizations. When the circular waveguide is excited with two mutually orthogonal TE11 modes in phase–quadrature, an LHCP or RHCP antenna is obtained. This paper explains the feeding system and the MTS requirements that guarantee the balanced conditions of the TM/TE SWs and consequent generation of dual CP boresight radiation. [ABSTRACT FROM AUTHOR]

In a previous publication, the problem of 2-D beam diffraction by a wedge has been solved via the complex-source (CS) approach. However, the straightforward CS formulation may be applied only when the incident beam is diverging as it hits the edge, but not when it is converging as it hits the wedge. In this paper, we generalize the CS setup, so that it can address both problems. The surprising result is that the CS approach can be applied for the converging beam case, but only if the CS coordinates are defined in a specific fashion. We then formulate the angular harmonics and the spectral integral representations for both cases, and also derive uniform asymptotic expressions for beam diffraction by a wedge. The validity of the results is verified by calculating the diffracted field via each one of these formulations, and comparing them with yet another approach, wherein the field of the incident diverging or converging beam is synthesized using a plane-wave integral, and the diffracted field is then calculated via multipole expansion. The overall goal of this paper is the derivation of techniques for the analysis of 3-D beam diffraction by a cone. [ABSTRACT FROM AUTHOR]

The reconstruction of the field radiated by a source from square amplitude-only data falls into the realm of phase retrieval. In this paper, we tackle the phase retrieval with two different approaches. The first one is based on a convex optimization problem called PhaseLift. The latter exploits the lifting technique to recast phase retrieval as a linear problem with an increased number of unknowns and then, because the linear problem is highly undetermined, it adds further constraints (based on the mathematical properties of the solution) to estimate it. The second approach formulates phase retrieval as a least squares problem and, therefore, it requires to tackle the minimization of a quartic functional which will be carried out by applying a gradient descent method. In the second part of this paper, in order to corroborate the effectiveness of both approaches, we present the numerical results. Afterward, we provide a comparison between the two methods and finally, we emphasize how the ratio between the number of independent data and the number of unknowns impacts on the performance in both approaches. [ABSTRACT FROM AUTHOR]

In this paper, a detailed investigation of the realization of conformal wearable transparent antennas by integrating conductive mesh with polymer has been presented. The proposed realization method is much simpler and more cost-effective than the existing realization methods of transparent antennas, and the prototype fabricated from the selected composite materials is more flexible and robust in bending operations than other transparent antennas. In this paper, the mechanical, electrical, and optical characteristics of the proposed composite material have been investigated to analyze its suitability for transparent flexible antenna realization. For concept demonstration, a prototype of a dual-band antenna operating at 2.33–2.53 GHz and 4.7–5.6 GHz has been fabricated and tested. These frequencies cover both the instrument, scientific, and measurement (ISM) and the wireless local area network (WLAN) bands. Full ground plane is utilized in the antenna design for on-body operations. The suitability of the antenna for wearable applications has been investigated by measuring its performance under physical deformation and testing its performance on phantom. Next, the RF performance of the antenna has been improved by using two layers of conductor to form the radiating element. Although transparency is slightly compromised, the double-layer element improves the gain and efficiency of the antenna. [ABSTRACT FROM AUTHOR]

One of the major challenges in wireless communication technologies is the antenna design. In this paper, a wideband flexible antenna made by covering a Kapton substrate with a flexible magnetodielectric polymer-based nanocomposite layer is proposed. This novel magnetodielectric polymer-based nanocomposite relies on carbon-coated cobalt (CCo) and is coated with a conjugated polymer, polyaniline (PANI). The nanocomposite (PANI/CCo) fabrication, whose morphology was studied via scanning electron microscopy, demonstrates a relative permeability, a dielectric permittivity, and a conductivity of 5.5, 4.3, and 7500 S/m, respectively. In this paper, two frequency bands are of interest [1.8–2.45 GHz (personal communication service and wireless local area network) and 5.15–5.825 GHz (wireless networks)]. The functioning of the antenna in free space and on body is numerically and experimentally investigated. The average specific absorption rate is also discussed. Thanks to its performance this magnetodielectric nanocomposite polymer (PANI/CCo) antenna has been found to be a good candidate for wearable applications where complex bending situations are encountered. [ABSTRACT FROM AUTHOR]

Electrically small loops have been the focus of the research literature on loop-antenna signal processing, but such electrically small loop-antennas are electromagnetically inefficient. Electrically large loop-antennas will instead be analyzed here in this paper, in the context of estimating an incident source’s direction-of-arrival or polarization. Specifically, three large loop-antennas here are collocated and are oriented orthogonally, in order to measure all three Cartesian components of the incident magnetic field simultaneously all at one specific spatial position. This orthogonal triad offers azimuth–elevation bivariate directivity despite the three loops’ spatial collocation. For such a triad of electrically large loops, this paper (first in the open literature) formulates the array manifold, develops the corresponding algorithms in closed form to estimate an incident source’s azimuth–elevation direction-of-arrival or polarization, and demonstrates these proposed estimators’ precision as close to the Cramér–Rao bounds. [ABSTRACT FROM AUTHOR]

This paper proposes a new modal approach for evaluating $Q$ -factors of antennas based on newly introduced XeR/XmR modes rather than characteristic modes (CMs). In our new approach, $Q$ -factors could be represented as direct linear superpositions of eigenvalues of XeR and XmR modes. We validate the accuracy of the new formula for $Q$ -factors by studying two canonical wire antennas. Moreover, the relations between XeR/XmR, TE/TM, and Xe/Xm modes are presented, which show interesting properties. It is found that eigenvalues of XeR/XmR modes reveal useful information about minimum $Q$ -factors. Compared with the CMs, some disadvantages of the proposed modes are also discussed. [ABSTRACT FROM AUTHOR]

Additional resonances are often introduced in an inverted-F antenna (IFA) by modifying the “F” structure, which typically makes the antenna both complex and bulky. To mitigate this problem, novel dual-/tri-band conformal capsule antennas with simple configuration are proposed in this paper by modifying the ground of the IFA and shifting the feed point. With these modifications, the IFA is miniaturized, and yet it is able to simultaneously cover the MedRadio band (401-406 MHz) and ISM bands (902-928 MHz and/or 2.4-2.5 GHz). The resulting configurations remain simple and compact despite the modifications, which is very desirable for conformal capsule type of applications. Furthermore, the impedance matching characteristics of the proposed IFAs remain stable even when a battery is added inside the capsule and the size or the position of the battery is modified. Moreover, the impedance matching of the antenna continues to remain stable even as its surrounding environment is changed. The operating principle of the proposed dual-/tri-band IFAs is discussed in detail and parametric study is carried out to investigate the characteristics of the proposed antennas. The results of this paper are expected to be helpful for designing multiband antennas in the future. [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]

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]

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]

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 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]

Before a new airport runway is built, the electromagnetic compatibility of the instrument landing system (ILS) must be fully taken into consideration. The electromagnetic compatibility requirements for airports are becoming more and more complex, so rather than using traditional ray-tracing (RT) simulation models, a more accurate approach such as the finite-difference time domain (FDTD) is needed. The application of FDTD to ILS signal modeling suffers from an intensively high computation burden. The computation burden is owing to: 1) the FDTD has to loop a large number of time steps and 2) for every step, a huge number of cells has to be updated. This paper solves the above-mentioned issues by twofold. First, this model allows a large Courant–Friedrich–Levy number for the FDTD to reduce the total time step. This is achieved by combining the locally one-dimension (LOD) FDTD and the complex-envelope (CE) technique. The combination of CE, LOD, and FDTD (CE-LOD-FDTD) makes the model update with larger time step and retain the same level of accuracy. Second, we propose a set of novel GPU-based acceleration algorithms to update the field in parallel for reducing the computation time. By comparing the measurement results, it is shown that the FDTD model is more accurate than the traditional RT-based method. Our GPU-based CE-LOD-FDTD model is 22 times faster than the conventional parallel FDTD model in a multicore machine. [ABSTRACT FROM AUTHOR]

This paper combines a study of human body morphology with physical layer characteristics to introduce a novel biometric identity feature for security in wearable communication applications. The physical layer characteristics of close fitting wearable devices are shown to vary in a unique manner through electromagnetic interactions between the tissue morphology and the antenna. Experimental measurement results demonstrate the new biometric concept using return loss characteristics to identify individuals on multiple body parts. An optimized directional coupler design is implemented with the antenna to optimize the characteristic feature detection range for human identification. Experiments conducted on human subjects using a prototype standalone test-bed and sensing circuitry at 2.45 GHz show that classification accuracies of over 98% are achieved for stationary subjects and 93% for mobile subjects. The new physical layer biometric has the potential to be used for authentication and authorization by using return loss as an indicator for secure user applications, using circuitry already implemented in wireless wearable communication systems. [ABSTRACT FROM AUTHOR]

This paper presents a finite-element method (FEM), based on a new set of vector basis functions, for an efficient solution of the electromagnetic scattering problem from a continuously varying inhomogeneous sphere. The new bases lead to a large number of similar terms in the entries of the finite-element matrix and make the interaction between the inner and outer field expansions’ diagonal. The method is compared with a tetrahedral-based FEM, and the efficiency of the method is investigated through some numerical examples. [ABSTRACT FROM AUTHOR]

In satellite communications, the adverse propagation effects produced by the atmosphere to the signal are compensated by the systems using propagation impairment mitigation techniques, protection by diversity being one of their main categories. Two slant-path propagation experiments have been carried out in Madrid in the Ka- and $Q$ -bands, with the Eutelsat KA-SAT and Alphasat satellites, respectively, with three concurrent years of measurements available: from March 2014 to February 2017. In this paper, results on time diversity and orbital diversity—the latter obtained from a frequency scaling approach— are presented, the first one for both frequency bands and the second for the $Q$ -band. An acceptable overall behavior is obtained by comparing the results with the available models. For both diversity techniques, possible gains are moderate but not negligible due to the climatological conditions of the evaluated site. [ABSTRACT FROM AUTHOR]

A geometrical theory of diffraction model for on-body propagation is developed in this paper. The exact solution to the canonical problem of a plane wave incident on an infinitely long cylinder, with arbitrary constitutive parameters, is found. The same is done for a magnetic and an electric infinitesimal dipole source of any orientation, located on the surface of the cylinder. The exact solutions are transformed with the Watson transformation to yield asymptotic expressions that are valid in the deep shadow region. These asymptotic expressions are validated by comparison to the numerically evaluated exact solution. It is found that the expressions are valid as long as the object is opaque, with a geometry down to the size of $({\kappa }/{\kappa ^{2}+\tau ^{2}}) > \lambda _{0}/2$ , and the rays not too torsional $\tau /\kappa < 2$ , where $\kappa $ and $\tau $ are the curvature and torsion of the local geometry, respectively. The asymptotic expressions are found to approximate the exact solution significantly better than the asymptotic expression of an equivalent perfect electric conductor geometry. The same is the case for the impedance boundary condition asymptotic approximation for low dielectric constant materials. Finally, the asymptotic expressions are generalized so they can be applied to any convex geometry of the human body or an opaque lossy dielectric of electrically large size. [ABSTRACT FROM AUTHOR]

This paper introduces an advanced wave engineering technique based on unidirectional frequency generation and spatial decomposition in general space–time-modulated (STM) slabs. The extraordinary wave engineering provided by such slabs is comprised of unidirectional wavefront shaping, nonreciprocal frequency generation, and spatial decomposition of ST harmonics (STHs). We first derive a rigorous analytical solution for the wave scattering from general ST permittivity- and permeability-modulated (STP&PM) slabs. The analytical solution considers the most general form of spatiotemporal variation and wave incidence, i.e., oblique incidence and scattering from time-periodic spatially aperiodic/periodic STP&PM slabs. The spatial aperiodicity may be utilized for further spectrum engineering and suppression of undesired STHs. Next, it will be shown that an equilibrated ST refractive-index-modulated slab with equal permittivity and permeability modulation strengths exhibits highly enhanced nonreciprocity. The FDTD numerical simulation of general STM slabs is presented, providing extra leverage for the analyses of wave propagation and scattering in such media. We show that oblique incidence to an STM slab yields nonreciprocal spatial decomposition of generated STHs. The explored peculiar properties of STM slabs under oblique incidence is expected to pave the way for the realization of new class of subharmonic generators, radars, illusion cloaks, and spatial mixers and isolators. [ABSTRACT FROM AUTHOR]

In this paper, a mixed finite-element time-domain (FETD) method is presented for the simulation of electrically complex materials, including general combinations of linear dispersion, instantaneous nonlinearity, and dispersive nonlinearity. Using both edge and face elements, the presented method offers greater geometric flexibility than existing finite-difference time-domain (FDTD) implementations, and in contrast to existing nonlinear FETD methods, also incorporates both linear and nonlinear material dispersions. Dielectric nonlinearity is incorporated into the Crank–Nicolson mixed FETD formulation via a straightforward Newton–Raphson approach, for which the associated Jacobian is derived. Moreover, the dispersion is modeled via the Möbius $z$ transform method, yielding a simpler more general algorithm. The method’s accuracy and convergence are verified, and its capability demonstrated via the simulation of several nonlinear phenomena, including temporal and spatial solitons in two spatial dimensions. [ABSTRACT FROM AUTHOR]

The ray-based method of geometrical optics can be used for wave propagation over a spherical earth with good results. A previous such a method also includes an exponential refraction index. This method gives good results for large terminal heights and distances, but for small distances, it has some shortcomings. In this paper, an improvement is proposed which removes these shortcomings. Also, as the method depends on numerical procedures, some improved numerical calculations are put forward. [ABSTRACT FROM AUTHOR]

In this paper, we propose an efficient procedure to obtain loop and star basis functions useful to the surface integral equation. They are of any order, for 3-D curved surfaces, and they keep the solenoidal/nonsolenoidal splitting, improving the performance for low frequencies (near field). These bases, introduced in a former work and applied to the electric field integral equation (EFIE), herein are used in both the magnetic field integral equation (MFIE) and the combined field integral equation (CFIE) providing a good performance even at the low-frequency regime. For addressing the low-frequency performance, scale factors and normalization of the basis are applied to EFIE, MFIE, and CFIE formulations. [ABSTRACT FROM AUTHOR]

A new technique is introduced for evaluating the vector potential of circular loop antennas (CLAs) with arbitrary current distribution and dimensions, which can be used to obtain the exact electromagnetic fields in near and far regions of the antenna. The technique is based on the spherical Bessel functions (SBFs) and associated Legendre polynomials (ALPs) expansion. This expansion is used to decouple the azimuth variables of the integrand of the vector potential integrals from the other variables in a separate functions that depend exponentially on azimuth variables. This technique reduces greatly the mathematical operations for evaluation of the vector potential to only one simple integration. The technique is used to obtain exact electromagnetic fields expressions for a thin-wire CLA of arbitrary current distribution and dimension, which is valid everywhere except on the loop. The well known far-fields for the CLA of arbitrary current distribution as well as near and far-fields for uniform current loop and small loop are obtained from the general field expressions presented in this paper as an special cases. [ABSTRACT FROM PUBLISHER]

In this paper, a stable and effective preconditioner for the method of moments-based electric field integral equation matrix, based on the null-field method, is presented to expedite the convergence of Krylov subspace-based fast iterative solution. The preconditioner is computed from all near-field blocks and selected far-field blocks in accordance with the null-field procedure. The final form of the preconditioner is block diagonal, and therefore generates no additional fill-ins in its inverse and is also amenable to parallelization. A complexity analysis is presented to show the near-linear cost of preconditioner construction and usage in terms of computation time and memory. The numerical experiments with a sequential implementation demonstrate on an average 1.5– $3\times $ speed-up in the iterative solution time over threshold-dependent incomplete LU-based preconditioners. [ABSTRACT FROM PUBLISHER]

This paper considers a distributed wave-based sensing system that probes a scene consisting of multiple interacting idealized targets. Each sensor is a collocated transmit–receive pair that is capable of transmitting arbitrary wideband waveforms. We address the problem of finding the space-time transmit waveform that provides the best target detection performance in the sense of maximizing the energy scattered back into the receivers. Our approach is based on earlier theoretical work that showed, for an idealized infinite half-space geometry, that the solution could be constructed by an iterative time-reversal (TR) process. In this paper, we give a more realistic example involving a two-sensor time-domain system. We show that for this system, the iterative TR process can be used to tune automatically to all the target resonances that are within the bandwidth of the interrogating radar system. We show that although obtaining eigenvalues (and hence resonances) of the scattering operator is in general unstable, using the iterative TR process to obtain the resonances is a stable process. Moreover, we show that these resonance frequencies are connected to the poles of the singularity expansion method. [ABSTRACT FROM PUBLISHER]

Optically transparent planar antennas made of transparent conductive oxide (TCO) have attracted considerable attention in research over the years, particularly for solar panel applications. However, their design has proved difficult due to their inherent conductor losses. This paper focuses on the design of an optically transparent reflectarray using indium tin oxide, which is a type of TCO. The design is composed of subwavelength rectangular patch elements that exhibit 272° of phase range and mitigate conductor losses in reflectarray operation. This paper examines the behavior and response of lossy reflectarray unit cells and investigates different techniques to reduce the impact of loss in TCO reflectarray designs, primarily using different patch geometries and subwavelength elements. The proposed element design is simulated and experimentally characterized. A full reflectarray prototype operating at 26 GHz is successfully demonstrated and fabricated, with the simulated and experimental results in good agreement. [ABSTRACT FROM AUTHOR]

This paper follows the time-honored analyses to provide a formal bridge connecting the PML and the well-understood (classical) Dirichlet and Neumann boundary value problems. The connection offers many theoretical and practical advantages not immediately obvious from any PML reconstructions. For example, the analysis shows that the reflection-free of PML boundaries is composed of the superposition of one Dirichlet and one Neumann solution acting out of phase constituents. In fact, it uncovers that the existing PML solutions are readily obtained from the projection of the well-understood 1-D Robin impedance-matched wave equations. The projection hence not only explains how the PML split fields fundamentally come about but also that the PML implicitly uses only an orthogonal projection. Naturally, we further extend the construction to include nonorthogonal decomposition. Local-to-global coordinate transformation is introduced to give a general nonreflecting boundary value solution valid for planar orientations not aligned with the Cartesian coordinates. Nonorthogonal generalizations also make it geometrically explicit as to what causes an incorrect PML problem that can arise from mediums lacking orthogonal symmetry invariant terminations. [ABSTRACT FROM AUTHOR]

In this paper, a contactless RFID system formed by chipless tags based on the magneto-inductive wave (MIW) delay lines is proposed. In order to carry out the reading process, two reader probes have been designed, manufactured, and measured: the first one based on an axial to planar MIW transition and the second one based on a printed monopole. Moreover, a study is presented to verify the probe with better performance in terms of delay and losses when a tag is interrogated. By using the probe based on an axial to planar MIW transition, it is possible to transmit a pulse with a distance between tag and probe higher than 1 cm. Finally, two chipless tags are designed and manufactured to verify the performance of the proposed system. All the designed and manufactured elements are printed on a low-cost substrate (FR4). The manufactured 2 bit tags codify the 01 and 10 patterns. A good decodification of the time-domain response associated with the tags is achieved when using the proposed whole system. [ABSTRACT FROM AUTHOR]