The numerical calculation of the Specific Absorption Rate (SAR) averaged over a certain tissue mass is a common practice when evaluating the potential health risk due to the human exposure to electromagnetic sources. Nevertheless, SAR values are strongly influenced by many factors such as, for instance, the shape of the volume containing the reference mass, the spatial discretization step, or the treatment of internal air, just to mention some of them: different choices can induce significant discrepancies. In this work an overview on some of the most commonly adopted SAR algorithms is firstly presented, and a discussion on their potential differences reported. Then, based on a spherical volume approach, some new algorithms are proposed. All the algorithms are then used to evaluate the SAR both in artificially generated test cases and in some practical human-antenna interaction problems. The result comparison highlights relevant discrepancies and enforces the necessity of a reasoned standardization of the techniques for the SAR calculation.
This paper mainly deals with the detection problem of the target with low Radar Cross-Section (RCS) in heavy sea clutter with unknown Power Spectral Density (PSD). Since the performance of traditional singlescan detectors degrades as the target of interest is smaller and weaker, three adaptive detectors, based upon a two-step design procedure, are proposed within the framework of the multiple-scan signal model. Firstly, the Multiple-Scan Detectors (MSDs) are derived according to the Generalized Likelihood Ratio Test (GLRT), Rao and Wald tests respectively under the assumption that the PSD of primary data is known. Secondly, three strategies are resorted to estimate the unknown PSD, and their Constant False Alarm Rate (CFAR) properties are assessed. Finally, numerical simulation results show that the adaptive MSDs outperform the traditional single-scan detector using Monte Carlo method.
In this paper, we present a one-dimensional (1-D) inversion algorithm for triaxial induction logging tools in multi-layered transverse isotropic (TI) formation. A non-linear least-square model based on Gauss-Newton algorithm is used in the inversion. Cholesky factorization is implemented to improve the stability and the reliability of the inversion. Zero-D inversion is conducted at the center of each layer to provide a reasonable initial guess for the best efficiency of the inversion procedure. Cross components are used to provide sufficient information for determining the boundaries in the initial guess. It will be illustrated that using all the nine components of the conductivity/resistivity yield more reliable inversion results and even faster convergence than using only the diagonal components. The resultant algorithm can be used to obtain various geophysical parameters such as layer boundaries, horizontal and vertical resistivity, dipping angle and rotation angle etc. from triaxial logging data automatically without any priori information. Several synthetic examples are presented to demonstrate the capability and reliability of the inversion algorithm.
Compressed sensing (CS) has attracted significant attention in the radar community. The better understanding of CS theory has led to substantial improvements over existing methods in CS radar. But there are also some challenges that should be resolved in order to benefit the most from CS radar, such as radar signal with low signal to noise ratio (Low-SNR). In this paper, we will focuses on monostatic chaotic multiple-input-multiple-output (MIMO) radar systems and analyze theoretically and numerically the performance of sparsity-exploiting algorithms for the parameter estimation of targets at Low-SNR. The novelty of this paper is that it capitalizes on chaotic coded waveform to construct measurement operator incoherent with noise and singular value decomposition (SVD) to suppress noise. In order to improve the robustness of azimuth estimation, interpolation method is applied to construction of sparse bases. The gradient pursuit (GP) algorithm for reconstruction is implemented at Low-SNR. Finally, the conclusions are all demonstrated by simulation experiments.
Design and measurement results of a beam-steering integrated lens antenna at 77 GHz are presented. An 8-element LTCC aperture coupled patch antenna feed array with a switching network is used to electrically steer the main beam in H-plane. A 100-mm diameter Rexolite (ε_{r} = 2.53) lens is simulated and tested. The eccentricity of the lens is optimized in an earlier work with ray-tracing simulations for improved beam-steering properties compared to the conventional extended hemispherical and elliptical lenses. The beamsteering properties including scan loss, main-beam width and direction, side-lobe levels, directivity, and cross-polarization are analyzed in detail with both simulations and radiation pattern measurements. As expected, the results show that the side-lobe and cross-polarization levels are not predicted accurately with large feed offsets using the ray-tracing simulations. Nevertheless, it is shown that the lens shape can be successfully optimized with the simple and fast ray-tracing simulations. The measured half-power beam-width at 77 GHz is 2.5°±0.2° up to the largest tested beam-steering angle of 30°. The optimized eccentricity low permittivity lens results in smaller scan loss than the conventional lenses.
The use of permanent magnets as bearings has gained attention of researchers nowadays. The characteristics of forces and moments have to be analysed thoroughly for the proper design of permanent magnet bearings. This paper presents a mathematical model of an axially magnetized permanent magnet bearing (ring magnets) using Coulombian model and a vector approach to estimate the force, moment and stiffness. A MATLAB code is developed for evaluating the parameters. Furthermore, it is extended to analyse stacked ring magnets with alternate axial polarization. The proposed model is validated with the available literature. Comparison of force and stiffness results of the present model with the results of three dimensional (3D) finite element analysis using ANSYS shows good agreement. Finally, the cross coupled stiffness values in addition to the principal stiffness values are presented for elementary structures and also for stacked structures with three ring permanent magnets.
Novel designs of probe-fed broadband shorted patch antennas for ultrawideband (UWB) applications are presented in this paper. In these designs, unequal resonance arms fed by a folded patch produce multi resonances to broaden the impedance bandwidth. In the first design, the antenna consists of an asymmetric Eshaped patch, a folded-patch feed and shorting pins. This antenna is achieved by four adjacent resonances with the measured -10 dB impedance bandwidth of 76.18%. The pins are utilized to miniaturize the size of the patch. By introducing a folded ramp-shaped feed in the similar structure with the first design, a wider bandwidth with the five resonances is obtained. This improved design introduces an antenna with an impedance bandwidth of more than 110% and a considerable size reduction compared to the first antenna. The antennas present resonance tuning ability within the impedance bandwidth by varying the length of unequal arms. In addition, parametric studies are performed by investigating the effects of different key parameters on obtaining optimal designs of the proposed antennas.
This paper extends the existing transmission and capacity analysis in order to investigate the broadband potential of overhead high-voltage/broadband over power lines (HV/BPL) connections where a single repeater is additively deployed between their existing transmitting and receiving ends (overhead HV/BPL connections with two-hop repeater system). The contribution of this paper is three-fold. First, the broadband performance of various overhead HV/BPL connections with two-hop repeater system has been studied with regard to their cumulative capacity. The analysis and relevant simulations validate the potentially excellent communications medium of overhead HV/BPL channels over a 25 km repeater span well beyond 100 MHz in terms of cumulative capacity. In addition, through the deployment of two-hop repeater systems, apart from the upsurge of cumulative capacity, overhead HV/BPL connections become more adaptive to different capacity requirements. Second, it is found that overhead HV/BPL network capacity performance depends drastically on factors such as the overhead HV grid topology and the noise characteristics. Through the deployment of two-hop repeater systems, capacity losses due to existing aggravated overhead HV/BPL topologies and high noise environments are significantly reduced. Third, the numerical results reveal the importance of considering as suitable mitigation technique the deployment of overhead HV/BPL connections with two-hop repeater system. Except for the low-cost and quick technology upgrade of existing overhead HV/BPL networks, this mitigation technique may permit the future broadband exploitation of overhead HV/BPL networks and their interoperability with other broadband technologies.
In this paper, a finite-difference based method is presented to simulate the electromagnetic field generated by arbitrarily-oriented coil antennas in three-dimensional (3-D) complex underground media. The media have multiple layers in both the vertical and horizontal direction and can be fully anisotropic. The developed finite-difference method uses a staggered grid to approximate a vector equation in terms of the scattered electric field. The resultant linear sparse matrix is solved iteratively using a generalized minimal residual (GMRES) algorithm and an incomplete LU precondition technique is applied to improve the convergence behavior of the linear equation, thus accelerate the solution. The developed algorithm is validated by numerical examples and then applied to the simulation and study of the popular triaxial induction tools in electrical well logging engineering for anisotropy detection.
Standard radar detection process requires that the sensor output is compared to a predetermined threshold. The threshold is selected based on a-priori knowledge available and/or certain assumptions. However, any knowledge and/or assumptions become inadequate due to the presence of multiple targets with varying signal return and usually non stationary background. Thus, any fixed predefined threshold may result in either increased false alarm rate or increased track loss. Even approaches where the threshold is adaptively varied will not perform well in situations when the signal return from the target of interest is too low compared to the average level of the background. Track-before-detect (TBD) techniques eliminate the need for a detection threshold and provide detecting and tracking targets with lower signal-to-noise ratios than standard methods. However, although TBD techniques eliminate the need for detection threshold at sensor's signal processing stage, they often use tuning thresholds at the output of the filtering stage. This paper presents a Hidden Markov Model (HMM) based target detection method that avoids any thresholding at any stage of the detection process. Moreover, since the proposed HMM method is based on the target motion models, the output of the detection process can easily be employed for manoeuvring target tracking.
Iris type waveguide to cavity couplers are used to couple power to particle accelerator cavities. Waveguide to cavity coupling for arbitrarily oriented rectangular iris is analyzed using Bethe's small hole coupling theory. Magnetic moment of rotated iris is obtained by defining its dyadic magnetic polarizability. Power radiated by magnetic moment into the incoming waveguide is used for coupling calculations at arbitrary angle. A close agreement is found between the proposed theory, simulations and microwave measurements.
In this paper, mathematical analysis supported by computer simulation is used to study cellular system information capacity change due to propagation loss and system parameters (such as path loss exponent, shadowing and antenna height) at microwave carrier frequencies greater than 2 GHz and smaller cell size radius. An improved co-channel interference model, which includes the second tier co-channel interfering cells is used for the analysis. The system performance is measured in terms of the uplink information capacity of a time-division multiple access (TDMA) based cellular wireless system. The analysis and simulation results show that the second tier co-channel interfering cells become active at higher microwave carrier frequencies and smaller cell size radius. The results show that for both distance-dependent: path loss, shadowing and effective road height the uplink information capacity of the cellular wireless system decreases as carrier frequency f_{c} increases and cell size radius R decreases. For example at a carrier frequency f_{c} = 15.75 GHz, basic path loss exponent α = 2 and cell size radius R = 100, 500 and 1000 m the decrease in information capacity was 20, 5.29 and 2.68%.
We show that by applying accidental degeneracy, we can obtain a triply-degenerate state at the zone center in the band diagram of two dimensional (2D) photonic crystal. The dispersion near the zone center comprisestwo linear bands and an additional flat band crossing at the same frequency. If this triply-degenerate state is formed by the degeneracy of monopole and dipole excitations, we show that the system can be mapped to an effective medium with permittivity and permeability equal to zero.While "Dirac cone" dispersions can only be meaningfully defined in 2D systems, the notion of a Dirac point can be extended to three dimensional (3D) classical wave systems. We show that a simple cubic photonic crystal composed of core-shell spheres exhibitsa 3D Dirac-like point at the center ofthe Brillouin zone at a finite frequency. Using effective medium theory, we can map our structure to an isotropiczero refractive index material inwhich the effective permittivity and permeability are simultaneously zero at the Dirac-like point frequency (ω_{D}). The Dirac-like point is six-fold degenerate and is formed by the accidental degeneracy of electric dipole and magnetic dipole excitations, each with three degrees of freedom. We found that 3D Dirac-like pointsat can also be found in simple cubic acoustic wave crystals.Different from the case in the photonic system,the 3D Dirac-like points at \overrightarrow{k}= 0 in acoustic wave systemis four-fold degenerate, and is formed by the accidental degeneracy of dipole and monopole excitations. Using effective medium theory, this acoustic wave system can also be described as a materialwhich hasboth effective mass density and reciprocal of bulk modulus equal to zero at ω_{D}. For both the photonic and phononic systems, a subset of the bands has linear dispersions near the zone center, and they give rise to equi-frequency surfaces that are spheres with radii proportional to (ω - ω_{D}).
Finite-Difference Time-Domain (FDTD) subgridding schemes can significantly improve efficiency of various electromagnetic circuit simulations. However, numerous subgridding schemes suffer from issues associated with stability, efficiency, and material traverse capability. These issues limit general applicability of FDTD subgridding schemes to realistic problems. Herein, a robust nonuniform subgridding scheme is presented that overcomes those weaknesses. The scheme improves simulation accuracy with the aid of greatly increased stability margin and an optimal interpolation technique. It also improves simulation efficiency by allowing the use of time step factors as close as the Courant-Friedrichs-Lewy (CFL) limit. In addition, latetime stability and general applicability are verified through practical microstrip circuit simulation examples.
The stability and numerical error of the extended four-stages split-step finite-difference time-domain (SS4-FDTD) method including lumped inductors are systematically studied. In particular, three different formulations for the lumped inductor are analyzed: the explicit, the semi-implicit, and the implicit schemes. Then, the numerical stability of the extended SS4-FDTD method is analyzed by using the von Neumann method, and the results show that the proposed method is unconditionally-stable in the semi-implicit and the implicit schemes, whereas it is conditionally stable in the explicit scheme, which its stability is related to both the mesh size and the values of the element. Moreover, the analysis of the numerical error of the extended SS4-FDTD is studied, which is based on the Norton equivalent circuit. Theoretical results show that: 1) the numerical impedance is a pure imaginary for the explicit scheme; 2) the numerical equivalent circuit of the lumped inductor is an inductor in parallel with a resistor for the semi-implicit and implicit schemes. Finally, a simple microstrip circuit including a lumped inductor is simulated to demonstrate the validity of the theoretical results.
Metasheet structures together with bulk composite dielectric layers can be used for antenna radomes, absorbers, and band gap structures. Transmission (T) and reflection (G) coefficients for a plane wave incident at any angle upon a metasheet embedded in a dielectric layer are considered. These metasheets are either patch-type or an aperture-type, and they can be either single-layered or multi-layered. To calculate T and Γ for a patch-type metasheet, a concise unified matrix approach is derived using the Generalized Sheet Transition Conditions (GSTC). The Babinet duality principle is utilized to get T and G for single-layered aperture-type metasheets (as complementary to the patch-type ones) at an arbitrary angle of incidence. The T-matrix approach is applied to calculate characteristics of multilayered metasheet structures containing a cascade of metasheets and dielectric slabs. In this paper, the minimum distance for neglecting higher-order evanescent mode interactions between the metasheets has been determined. Computed results based on the proposed analytical approach are compared with the fullwave numerical simulations. The analytical results are verified for satisfying the energy balance condition.
This paper describes a wideband double-dipole Yagi-Uda antenna fed by a microstrip-slot coplanar stripline transition. The conventional dipole driver of a Yagi-Uda antenna is replaced by two parallel dipoles with different lengths to achieve multi-resonances, and a small, tapered ground plane is used to allow flexibility in the placement of a pair of reflectors for effective reflection of back-radiated electromagnetic waves. The measured bandwidth of the antenna was 3.48-8.16 GHz for a -10 dB reflection coefficient, with a flat gain of 7.4 ± 0.4 dBi. A two-element array of these antennas was also constructed, with a center-to-center spacing of 36 mm (0.72 λo at 6 GHz) and a common reflector between the elements. The two-element array had a measured bandwidth of 3.56-7.92 GHz, a gain of 8.40-10.43 dBi, a cross-polarization level of <-15 dB, and a mutual coupling of <-16 dB within the impedance-matching bandwidth.
This study investigates the performance of a dipole antenna above electromagnetic bandgap (EBG) substrateswith different number of patchesto realize a low specific absorption rate (SAR) antenna for a 4G wireless communications system (3.5GHz band). A cubic head model is used fortheinitialanalysis toestimate the radiation characteristics and the SAR of the antenna. Computational results have shown that the antenna above anEBG substrate could provide a maximum reduction of 81% in the SAR and a radiation efficiency improvement of 10% when compared with theantenna above a perfect electric conductor (PEC) ground plane.However, the antenna above an EBG substrate with a lower number of patches results in a higher resonance frequency and cannot provide sufficient SAR reduction.In both the cubic and realistic head models,a similar tendency was observedin the SAR reduction capability ofthe antenna above the EBG substrates whencompared with the antenna above the PEC ground plane. For therealistic head model, the SARs of the dipole above EBGsubstrateswith 20 or 24 EBG patches can be reduced by 16% when compared to the casewith 12or16EBG patches.The variability of the SAR in the operating frequency band (|S_{1110 dB) of the antennais 5-35% for different EBG substrates. }
This paper presents comparative studies on different types of basis and testing functions used in Method of Moments (MoM) in terms of analytical complexity, convergence and condition number of the co-efficient matrix when applied to electrostatic problem of evaluating capacitance and charge distribution of conducting bodies. A thin conducting cylinder of finite length has been taken as a representative case study to evaluate the capacitance and charge distribution. The basis and testing functions which have been studied for this problem are pulse-delta, pulse-pulse, triangular-delta, triangular-pulse and triangular-triangular functions respectively. Numerical data on capacitance and charge distribution has been presented for each set of basis and testing functions in terms of condition number and convergence.
The design of resonators with degenerate magnetic and electric modes usually requires the ability to perturb one or both types of modes in order to induce alignment of magnetic and electric properties. In this paper perturbation theory is used to identify different types of inclusions that can be used to realize fundamental-mode degeneracy in a rectangular dielectric resonator and thus, can ultimately be used in the design of negative-index metamaterials. For reasons associated with fabrication in the infrared-frequency regime, rectangular resonator designs are of particular interest.