In this paper, we focus on target detection and system configuration optimization of Multiple-input Multiple-output (MIMO) radar in low-grazing angle, where the multipath effects are very abundant. The performance of detection can be improved via utilizing the multipath echoes. First, the reflection coefficient, considering the curved earth effect, is derived. Then, the general signal model for MIMO radar is introduced for low-grazing angle. Using the Neyman-Pearson sense, the detector of MIMO radar with multipath is analyzed. We use the deflection coefficient as a criterion of system configuration both for MIMO radar and phased-array radar. The simulation results show that the performance can be enhanced markedly when the multipath effects are considered, and the optimal configuration of phased-array radar is with the same number of transmitters as that of receivers, however, the optimal configuration of MIMO radar depends on the signal-to-noise ratio (SNR).

In this paper, a modified Wilkinson power divider structure with three order harmonics suppression is presented. The quarter-wavelength microstrip lines in the traditional Wilkinson power divider (WPD) are replaced by two transmission line segments with ends connected (TTLWEC). The TTLWEC performs the functions of impedance transformation and three order harmonics suppression. The design equations are deduced by odd- and even-mode theory. An example of power divider operating at 1 GHz is designed and fabricated based on the printed circuit board technology. The measured results of 3.13 dB insert loss (IL) and 35 dB return loss (RL) are obtained at the operating frequency, and the first, second and third harmonic harmonics suppressions are -38 dB, -44 dB and -39 dB, respectively, which agree well with the simulated results and validate the availability of the proposed structure.

A functional microstrip circuit module for annular slot antenna is proposed. This module consists of an annular microstrip line component, two PIN diodes and a DC bias circuit. Reconfigurable circular polarizations can be simply controlled by this functional module. Axial ratio is adjustable by changing the clip angle of the notch made by the annular microstrip line component. Simulated and experimental results have shown good impedance bandwidth for return loss and antenna gains in circularly polarized states.

Battlefield surveillance is a common application of synthetic aperture radar (SAR), in which minefield detection is a challenging task. In this paper, a novel minefield detection approach is proposed via the morphological diversities between targets and background. Firstly, SAR image speckle is suppressed effectively by total variation, and targets edges are preserved well. Secondly, a nonlinear transform is introduced to map the special distributed targets, e.g. landmines, into spot targets. Lastly, the modification of morphological component analysis is adopted to improve the signal-to-clutter ratio and separate the spot targets from image. The performance of the proposed approach is validated by using the data acquired over an airship mounted SAR system.

In this paper, we examine the imaging ability of a planar superlens in both the transverse and vertical dimension. By studying the field patterns of the image from different objects (points and scattering surfaces with subwavelength details) in front of a planar superlens, we show the relation between the transverse and vertical resolutions. We mainly discuss why we cannot get high subwavelength resolution for three dimensions at the same time, and there is a trade-off between the transverse and vertical resolution capabilities which is fundamental in nature for a planar superlens.

In this paper we develop a small-sized mode converter with high performance, high conversion efficiency and instantaneous bandwidth as high as 55%. This mode converter transforms energy from the TM01 first high-order mode towards the fundamental TE11 circular waveguide mode. The proposed structure increases the free spurious operating bandwidth in comparison with the existing results in literature. An X/Ku-bands experiment prototype unit was designed, ensuring practical return losses better than 28 dB and insertion losses less than 0.1 dB (conversion efficiency > 98.8%) within the entire frequency bandwidth ranging from 9.25 GHz to 16.25 GHz. The presented architecture offers useful features such as very wide bandwidth, small size, easy achievement as well as excellent performance, which makes it very suitable for High-power microwave (HPM) sources that generate the TM01 circular waveguide mode. In these cases the TE11 mode is needed since it has a convergent radiation pattern able to drive conventional antennas. Moreover, this compact concept is fully scalable to any millimeter frequency band.

This paper presents the antenna design parameter dependency on the impedance and radiation characteristics of a terahertz coplanar stripline dipole antenna. The antenna response is numerically investigated by applying a semi-infinite substrate and by generating a constant voltage source to drive a signal on the antenna. In this way, we can analyze the antenna characteristics without the photoconductive material response and the substrate lens geometrical effects. Further, we explain the mechanism underlying the preferable uses of several millimeter length DC bias striplines in a typical THz coplanar stripline dipole antenna design. The antenna, consisting of a center dipole connected to long bias striplines, has a traveling wave characteristic supporting an attenuated current, rather than a resonant characteristic supporting a standing wave of current. The traveling wave behavior produces stable antenna input impedances and minimal changes in the antenna radiation patterns. We also found that the length of the center dipole has a prominent effect on the antenna gain response.

In this paper, a method for increasing bandwidth of metamaterial structures is presented. The metamaterial structures used in this study are based on Split Ring Resonators (SRRs), the most recognized structures for realization of metamaterials with negative magnetic permeability coefficients. To increase the frequency bandwidth of such metamaterials, two different methods, 1) rotating the inner ring of SRR with different angles in a hybrid structure, which is herein called unit cell, 2) changing dimensions of SRR, are presented. Moreover, the effect of SRR arrangement in unit cell on bandwidth is investigated. The idea of bandwidth enhancement is verified via simulations, which are performed via full-wave method and measurements, which are done using a built strip-line setup.

One miniaturized multilayer dual-band bandpass filter (BPF) is developed using standard low temperature co-fired ceramic (LTCC) technology. The filter makes use of four double-folded substrate integrated waveguide (SIW) resonators. Two sets of coupling paths between the source and load are implemented to generate dual-band responses. Utilizing this method, the two passbands can operate at independent frequencies and the bandwidth can be easily controlled. High isolation is obtained between two passbands, and two pairs of transmission zeros close to the passband edges are generated by source-load coupling, resulting in high skirt-selectivity. Good agreement between the simulated and measured results of the filter sample is obtained, with its high electrical performance validated.

In this paper, we present a novel fast method to solve Poisson's equation in an arbitrary two dimensional region with Neumann boundary condition, which are frequently encountered in solving electrostatic boundary problems. The basic idea is to solve the original Poisson's equation by a two-step procedure. In the first stage, we expand the electric field of interest by a set of tree basis functions and solve it with a fast tree solver in O(N) operations. The field such obtained, however, fails to expand the exact field because the tree basis is not curl-free. Despite of this, we can retrieve the correct electric field by purging the divergence-free field. Next, for the second stage, we find the potential distribution rapidly with a same fast solution of O(N) complexity. As a result, the proposed method dramatically reduces solution time compared with traditional FEM with iterative method. In addition, it is the first time that the loop-tree decomposition technique has been introduced to develop fast Poisson solvers. Numerical examples including electrostatic simulations are presented to demonstrate the efficiency of the proposed method.

In passive millimeter-wave imaging systems used indoors, the radiometric temperature contrast is barely enough for coarse object detection, being usually insufficient for recognition due to the absence of cold sky. The image contrast results from a combination of emissivity and reflectivity which are dependent on the dielectric constant of objects, the angle of incidence, and the polarization direction. To improve the capability of target recognition, we proposed the linear polarization sum imaging method which is based on the combination of the different polarization images for increasing the intensity contrast between the target area and the background area. In order to capture the linear polarization sum images of a metal sphere, a metal and a ceramic cup, we designed W-band quasi-optical imaging system which can generate the polarization dependent images by manually changing the linear polarization direction of its radiometer receiver from 0 to π /2 by the step size of π/8. The theoretical and experimental results of the linear polarization sum imaging show that it is capable for achieving good image quality enough to recognize the target.

A a new compact ultrawideband (UWB) patch antenna based on the resonance mechanism of a composite right/left-handed (CRLH) transmission line (TL) is proposed. The radiating element of the antenna is made from three left-handed (LH) metamaterial (MTM) unit cells placed along one axis, where each unit cell combines a modified split-ring resonator (SRR) structure with capacitively loaded strips (CLS). An analysis of the eigenfrequencies of these unit cells yields one- and two-dimensional dispersion diagrams, which correspond to one-unit cell antenna and the three unit-cell antenna, respectively. A trident feed and a slotted-partial ground plane are used to match the right-and left-handed (RH and LH) modes of the antenna, respectively. In addition, an analysis of the surface current distribution of the antenna shows that, slots on the metallic area reduce the Q-factor. This recdution in the Q-factor results in a wide bandwidth of 189% at 3.7 GHz, which spans the UWB frequency range between 2.9-9.9 GHz. The total footprint of the antenna at the lowest frequency is 0.2λ₀ x 0.2λ₀ x 0.015λ₀, where λ₀ is the free space wavelength. The gain of the antenna ranges between -1 to 5 dB throughout the frequency band.

This paper deals with electromagnetic design and three-dimensional (3-D) magnetic field analysis of a novel configuration brushless DC (BLDC) field assisted machine based on a hybrid analytical and 3-D finite element method (FEM) analysis. Aid of this hybrid design method is improving the accuracy and computation time for complex magnetic structure like to presented machine structure. In this hybrid design methodology obtained primary magnetic and electric characteristics including magnetic flux density, flux linkage and induced Back-EMF profile for studied configuration are verified by 3-D FE computation. Comparison of the calculated magnetic field and terminal voltage characteristics by their requested values and obtained values form analytical analysis respectively illustrates the conformity of design parameters stage. In this study in order to determine the optimum operation, geometry parameter of proposed machine are optimized based on multi objective optimization design and genetic algorithm, and finally 3-D FEM verification coupled by boundary integral equation method (BIEM). Additionally the accuracy of 3-D FE analysis is verified by comparing the calculated results with the experimental measured values.

Two efficient unconditionally-stable four-stages split-step (SS) finite-difference time-domain (FDTD) methods based on controlling parameters are presented, which provide low numerical dispersion. Firstly, in the first proposed method, the Maxwell's matrix is split into four sub-matrices. Simultaneously, two controlling parameters are introduced to decrease the numerical dispersion error. Accordingly, the time step is divided into four sub-steps. The second proposed method is obtained by adjusting the sequence of the sub-matrices deduced in the first method. Secondly, the theoretical proofs of the unconditional stability and dispersion relations of the proposed methods are given. Furthermore, the processes of obtaining the controlling parameters for the proposed methods are shown. Thirdly, the dispersion characteristics of the proposed methods are also investigated, and numerical dispersion errors of the proposed methods can be decreased significantly. Finally, to substantiate the efficiency of the proposed methods, numerical experiments are presented.

In this paper, a new technique to realize lumped dual-band impedance transformers for arbitrary frequency-dependent complex loads is proposed. For the complex impedance transforming, closed-form design equations are presented for a series-shunt and a shunt-series type and a concept of combination is also presented. They use the proposed equation of input impedance. This equation can easily and exactly obtain the input impedance of any two-port network using the ABCD matrix. Then, in order to realize dual-band operation, four topologies comprising two types and a design method are presented. This technique is numerically demonstrated by various examples with excellent results and it has advantages of simplicity, intuitiveness and versatility because it is a general solution for complex impedance transforming. The proposed dual-band impedance transforming technique can be utilized for practical matching problems such as microwave amplifiers and other devices.

This paper introduces a theoretical analysis as well as a design example for bandpass filters (BPF) with a distinctive topology. Based on the analysis of simple two-port symmetrical lossless networks with a parallel structure, a method for obtaining normalized BPF prototypes with desired bandwidths was developed. These prototypes can be scaled to any central frequency and symmetrical real termination in the same way as conventional filters. It is also demonstrated that with a slight modification of the basic BPF prototypes, transmission zeros with controllable frequencies can be introduced in both the lower and the upper stopband region. Such modified prototypes are more convenient for the realization of printed filters than the basic BPF prototypes. The proposed filters have almost identical characteristics in the broad vicinity of the passband region either when composed of ideal lumped elements or of transmission lines (TLs). Due to its simplicity, the proposed concept could be applied for the realization of a printed BPF at a large variety of PCB types, substrates and practical design configurations. A microstrip BPF model is realized for the experimental verification of the presented theory. The measured and theoretical results show excellent agreement, confirming the proposed concept and the exactness of the methodology.

This paper presents a novel ultra wideband (UWB) channel model in the 3-10 GHz range for body-centric wireless communications. The tests are performed in both indoor anechoic chamber environments, addressing on-body and off-body propagation scenarios. The body channel model is extracted by using a single spatial grid over all the body, and by distinguishing between LOS and NLOS condition. The large number and the uniform placement of the receiver locations attempt a representation of the body propagation links more comprehensive than previously published models. The statistical reliability of the model is investigated by applying jointly the Kolmogorov-Smirnov and the Akaike criteria. The analysis suggested that the Lognormal model fits the channel amplitude distributions with a percentage ≥ 64%. The on-body indoor channel amplitudes are modeled with a stochastic terms of about 4-5 dB higher than previously published models. Finally, a Negative-Binomial and Inverse Gaussian distribution are used to model the expected number of paths and inter-arrival time, respectively. Based on the results presented in this paper, clear recommendations are given with regards to the optimum statistical distribution of an accurate UWB body-centric radio channel modeling.

The aim of this work is to asses the computational performance of a finite element formulation based on nodal elements and the regularized Maxwell equations. We analyze the memory requirements and the condition number of the matrix when the formulation is applied to electromagnetic engineering problems. As a reference, we solve the same problems with the best known finite element formulation based on edge elements and the double curl Maxwell equations. Finally, we compare and discuss the computational efficiency of both approaches.

In this paper, an improved equivalence principle algorithm is proposed to solve the radiation problems of large antenna arrays with periodic structures. This method is a hybridization in which the typical scheme of periodic Green's function is combined with the original equivalence principle algorithm. The repeated elements are changed from the original antenna units into the surfaces enclosing the original ones. The proposed approach is compared with periodic method of moments which is based on the integral equation and the periodic Green's function. Numerical results validate the feasibility of the improved method.

A novel design of a temple shaped printed slot antenna for circular polarization applications is presented in this paper. The slot, half trapezoidal and half semi-circular in shape, is excited by a 50 Ω CPW feed terminated with a tuning stub. Modifications of the initial design for improving return loss and circular polarization characteristics are proposed and discussed. The antenna is very compact (40 mm x 35 mm) in size and simple to design. The final version of the antenna offers an experimentally measured impedance bandwidth of 108% i.e., from 2.75 GHz to 9.25 GHz. The 3-dB axial ratio bandwidth achieved is 51.6% i.e., from 4.6 GHz to 7.8 GHz. The antenna is further characterized by a peak gain of about 5 dB and a relatively stable radiation pattern in the useful band.