Parametric interpolation of the moment matrix in surface integral equation formulation

In the analysis of electromagnetic scattering and radiation from objects of arbitrary shape using the method of moments (MoM), it is desirable to fill the impedance (or moment) matrix efficiently so that larger size problems can be solved. This article describes a general MoM technique in which the matrix is filled by spatial interpolation with respect to a parametrized electrical separation between source and test elements. The parametrization is accomplished such that the same algorithm also provides frequency interpolation, thus facilitating efficient computations over a wide frequency band. The spatial interpolation method is illustrated by application to the analysis of radiation from tunable microstrip patch antennas over multiple frequency bands. By specializing the interpolation scheme to a surface integral equation formulation that employs rooftop basis functions on a grid of rectangular cells, it is shown that the interpolation method results in considerable reduction of the storage and CPU time requirements. ©1999 John Wiley & Sons, Inc. Int J RF and Microwave CAE 9: 474–489, 1999.     

Modeling and optimization of cylindrical antennas using the mode‐expansion method and genetic algorithms

For monopole antennas with cylindrically symmetric structures, a mode‐expansion method is highly time efficient, which is a realistic approach for integrating function‐optimization tools, such as genetic algorithms (GAs), in order to extract the best bandwidth property. In this article, a mode‐expansion method is used to simulate the impedance characteristics of the cylindrical antennas. As examples, two new types of monopole antennas are presented, one of which possesses a two‐step top‐hat structure while the other has an annulus around the stem. After the modeling scheme is examined for convergence and data validity, the associated optimization problem, with dimensions as decision variables, structural limitations as linear constraints, and desired bandwidth performance as an objective function, is solved using GAs. The effects of the geometric parameters on the impedance characteristics are investigated in order to demonstrate the optimality of the calculated solutions. Two optimized practical antennas are designed based on our numerical studies. One has a broad bandwidth of 3 GHz while the other shows a dual‐band property, which can satisfy the bandwidth requirements for both Bluetooth (2.45‐GHz band) and WLAN (5‐GHz band) systems. © 2005 Wiley Periodicals, Inc. Int J RF and Microwave CAE, 2005.     

Design and analysis of multiband notched pitcher‐shaped UWB antenna

To mitigate the interference with coexisting wireless systems operating over 3.3–3.6 GHz, 5.15–5.825 GHz, and 7.725–8.5 GHz bands, a novel triple band notched coplanar waveguide fed pitcher‐shaped planar monopole antenna is presented for ultrawideband applications. Bands notched characteristics are achieved using a novel mushroom type electromagnetic band gap structure like resonator and a split ring slot. A conceptual equivalent RLC (Resistor‐Inductor‐Capacitor)‐resonant circuit is presented for the band notched characteristics . Furthermore, the input impedance and VSWR (voltage standing wave ratio) obtained from the equivalent circuit are validated with simulated and measured results. Performances of the antennas in both, the frequency domain and the time domain are investigated. The simulated and measured results demonstrate that the proposed antennas have wide impedance bandwidth, nearly stable radiation patterns, and suppression of gain and total radiation efficiency at notched bands. © 2015 Wiley Periodicals, Inc. Int J RF and Microwave CAE 25:795–806, 2015.     

Hybrid method of FG‐FFT and PO for radiation calculation from antennas around an electrically large conducting platform

An efficient hybrid scheme combining fitting Green's function (FG) fast Fourier transform (FFT) and physical optics (PO) is presented to investigate radiation from antennas around an electrically large conducting platform. The whole region is divided into full wave region and PO one. Similar to hybrid method of integral equation FFT (IE‐FFT) and PO, this hybrid method features acceleration by FFT, fewer unknowns, less computing time than traditional IE‐FFT. Differently, realization of FG‐FFT is established by fitting the Green's function onto the nodes of a uniform Cartesian grid, not by Lagrange interpolation. Several examples are given to prove the hybrid method of FG‐FFT and PO in this letter featuring higher accuracy and being not sensitive to both grid spacing and the expansion order compared to hybrid method of IE‐FFT and PO.     

Design of conical DRH antennas for K and Ka frequency bands

Two conical double‐ridged horn (DRH) antennas for K and Ka frequency bands are presented. Detailed simulation and experimental investigations are conducted to understand their behaviors and optimize for broadband operation. The designed antennas were fabricated with 0.01 mm accuracy and satisfactory agreement of computer simulations and experimental results was obtained. The designed conical DRH antennas have voltage standing wave ratio (VSWR) less than 2.1 and 2.2 for the frequency ranges of 18–26.5 GHz (K band) and 26.5–40 GHz (Ka band), respectively. Meanwhile, the proposed antennas exhibit low cross‐polarization, low sidelobe level, and simultaneously achieve slant polarization as well as symmetrical radiation patterns in the entire operating bandwidth. An exponential impedance tapering is used at the flare section of the horns. Moreover, a new cavity back with a conical structure is used to improve the VSWR. Numerous simulations via Ansoft HFSS and CST Microwave Studio CAD tools have been made to optimize the VSWR performance of the designed antennas. Simulation results show that the VSWR of the proposed antennas is sensitive to the probe spacing from the ridge edge and the cavity back structure. The designed conical DRH antennas are most suitable as a feed for the reflectors of radar systems and satellite applications. Results for VSWR, far‐field E‐plane and H‐plane radiation patterns, and gain of the designed antennas are presented and discussed. © 2011 Wiley Periodicals, Inc. Int J RF and Microwave CAE, 2011.     

An effective NURBS MOM‐PO method for fast analyzing antenna around electrically large platform

A novel hybrid method of moments and physical‐optics (MOM‐PO) method is presented to accurately analyze the performance of antennas placed closely around the electrically large platform modeled with nonuniform rational B‐spline (NURBS) surfaces. Ludwig integral combining with stationary phase method is used to calculate the integral of induced currents. In addition, the computational efficiency is significantly improved using grouping of the MOM region and interpolation of critical points. Two numerical examples are calculated. Compared with the results calculated by other algorithms, such as MOM and conventional NURBS MOM‐PO, the proposed method shows great capability to solve this particular problem precisely. The listed execution time consumed in different methods demonstrates the high efficiency of this approach. © 2015 Wiley Periodicals, Inc. Int J RF and Microwave CAE 25:463–467, 2015.     

A high capacity tunable retransmission type frequency coded chipless radio frequency identification system

This article presents a 12‐bit frequency coded chipless RFID system in the frequency range of 3 to 6 GHz. The system consists of a fully printable chipless tag and a pair of high‐gain reader antennas. The tag also incorporates its own antennas to improve the read range. Information is encoded into frequency spectrum using a multi‐resonant circuit. The circuit consists of multiple microstrip U and L‐shaped open stub resonators patterned in a unique configuration. The proposed configuration aids in capturing more data in a reduced space as well as tunable frequency operation. Tag and reader antennas utilize techniques such as stepped impedance feeding line, defective partial ground plane, and stair‐step patch structure to achieve wide‐band impedance bandwidth in miniature size. The results of the wireless measurements in the non‐anechoic environment show that the proposed system has a reading range of more than 20 cm. The presented system possesses great potential for low‐cost short‐range inventory tracking.     

Performance enhancement of multiband antennas through a two‐stage optimization technique

This paper presents an optimization technique that aims at multiband antenna design. The proposed method is based on the framework of multi‐objective evolutionary algorithms, and a two‐stage mechanism that balances the degree of optimizing impedance matching and the degree of providing a wide impedance bandwidth is incorporated. Conventionally, the design optimization of multiband antennas relies on minimizing the maximum reflection coefficient or maximizing the area under the return‐loss curve over targeted frequency bands. However, these widely used methods direct an optimization algorithm to improper solution sub‐domains in the multiband design problem. To overcome the limitation of these conventional methods, the general rule of objective functions is thoroughly investigated in this paper. Furthermore, a two‐stage optimizer is designed based on what the multiband optimization problem needs. With the use of the proposed method, two multiband antennas for mobile communication systems covering 824–960 MHz and 1710–2170 MHz are successfully developed. Simulated and measured results show that the proposed technique outperforms conventional optimization approaches significantly.     

Investigation of ultra‐wideband Bowtie antennas for phased array feed application

In this article, two new ultra‐wideband (UWB) dual‐polarized Bowtie antennas are investigated as the elements for a phased array feed for reflectors. In addition to its UWB impedance matching characteristic, the Bowtie antennas have stable large beam‐width and a low cross‐polar level over a wide frequency band with a compact size, which is an essence for phased array applications. The simulated and measured results state a low ohmic loss, good impedance matching (S₁₁ below ?15 dB) and good radiation performance, with a simple structure for easy manufacturing. The proposed antennas can be good candidates for phased array feed (PAF) in FAST and the SKA (square kilometer array) pathfinder PHAROS2 projects, and massive MIMO antennas in wireless communication systems.     

A singular cell-based smoothed radial point interpolation method for fracture problems

A cell-based smoothed radial point interpolation method (CS-RPIM) is developed for fracture problems. The strain smoothing is performed over background triangular cells. The stiffness matrix is calculated based on the weakened weak formulation, using shape functions obtained by radial point interpolation method. A layer of five-node singular elements are used to simulate the singularity around the crack tip. Different schemes are devised in the five-node elements to perform the strain smoothing. Several examples are presented to validate the newly developed method. The results are found in excellent agreement with the exact (or reference) solutions.     

An efficient method for analyzing nonuniformly coupled microstrip lines

This article presents an efficient method for analyzing nonuniformly coupled microstrip lines. By choosing a modal‐transformation matrix, the coupled nonlinear differential equations describing the symmetric nonuniformly coupled microstrip lines are decoupled using even‐ and odd‐mode parameters; the original problem is thus transformed into two single nonuniform transmission lines. A power‐law function of arbitrary order and having two adjustable parameters is chosen to better approximate the equation coefficients. Closed‐form ABCD matrix solutions are obtained and used to calculate the S‐parameters of nonuniformly coupled microstrip lines. Numerical results for two examples are compared with those from a full‐wave commercial package and experimental ones in the literature in order to demonstrate the accuracy and efficiency of this method. This highly efficient method is employed to optimize a cosine‐shape 10‐dB codirectional coupler, which has good return loss and high directivity performance over a wide frequency range. © 2005 Wiley Periodicals, Inc. Int J RF and Microwave CAE, 2005.     

Efficient analysis of in‐line waveguide filters and frequency‐selective surfaces with stepped holes

This paper presents a novel method for the analysis of large classes of microwave and mm‐wave passive components, including in‐line waveguide filters, single‐ and multi‐layer frequency selective surfaces, and open‐ended waveguide array antennas. This method is based on the segmentation technique, which permits us to reduce complex components to cascaded waveguide step discontinuities, which are separately characterized through their generalized impedance matrices, as calculated by the integral equation (IE) technique and the boundary integral‐resonant mode expansion (BI‐RME) method. Some examples demonstrate the flexibility and efficiency of the IE/BI‐RME method, and its utility in investigating novel structures not requiring costly fabrication techniques. © 2003 Wiley Periodicals, Inc. Int J RF and Microwave CAE 13: 306–315, 2003.     

Efficient exponential compact higher order difference scheme for convection dominated problems

Present work is the development of a finite difference scheme based on Richardson extrapolation technique. It gives an exponential compact higher order scheme (ECHOS) for two-dimensional linear convection-diffusion equations (CDE). It uses a compact nine point stencil, over which the governing equations are discretized for both fine and coarse grids. The resulting algebraic systems are solved using a line iterative approach with alternate direction implicit (ADI) procedure. Combining the solutions over fine and coarse grids, initially a sixth order solution over coarse grid points is obtained. The resultant solution is then extended to finer grid by interpolation derived from the difference operator. The convergence of the iterative procedure is guaranteed as the coefficient matrix of the developed scheme satisfies the conditions required to be monotone. The higher order accuracy and better rate of convergence of the developed algorithm have been demonstrated by solving numerous model problems.     

Step impedance resonator‐based tunable perfect metamaterial absorber with polarization insensitivity for solar cell applications

In this work, a step impedance resonator (SIR)‐based structure is proposed to develop a compact tunable metamaterial (MTM)‐based perfect absorber for solar cell applications. This MTM absorber is able to improve the absorption over a wide range of visible frequency range from 550 to 650 THz. The absorption is high around the frequency 600 THz. The proposed model is designed based on SIR technique to achieve miniaturization. The parametric study of overall size of the proposed MTM absorber analyzed over the frequency range 430‐750 THz. The thickness of dielectric spacer, and top most layer (MTM Structure) illustrates the tunable characteristics of the proposed model. A complete comparative analysis of proposed model with different dielectric spacers like AlGaAs, InAs, GaAs, and AlAs are presented with the help of absorption (S₁₁) and transmission (S₁₂). The proposed model is suitable for high efficiency solar cell energy harvesting applications.     

A Hermite‐Lobatto Pseudospectral Method for Optimal Control

A pseudospectral (PS) method based on Hermite interpolation and collocation at the Legendre‐Gauss‐Lobatto (LGL) points is presented for direct trajectory optimization and costate estimation of optimal control problems. A major characteristic of this method is that the state is approximated by the Hermite interpolation instead of the commonly used Lagrange interpolation. The derivatives of the state and its approximation at the terminal time are set to match up by using a Hermite interpolation. Since the terminal state derivative is determined from the dynamic, the state approximation can automatically satisfy the dynamic at the terminal time. When collocating the dynamic at the LGL points, the collocation equation for the terminal point can be omitted because it is constantly satisfied. By this approach, the proposed method avoids the issue of the Legendre PS method where the discrete state variables are over‐constrained by the collocation equations, hence achieving the same level of solution accuracy as the Gauss PS method and the Radau PS method, while retaining the ability to explicitly generate the control solution at the endpoints. A mapping relationship between the Karush‐Kuhn‐Tucker multipliers of the nonlinear programming problem and the costate of the optimal control problem is developed for this method. The numerical example illustrates that the use of the Hermite interpolation as described leads to the ability to produce both highly accurate primal and dual solutions for optimal control problems.     

应用ACA算法快速分析导体目标电磁散射特性

提出了一种分析导体目标电磁散射特性的有效数值方法,该方法以矩量法为基础,将自适应交叉近似算法应用于远场组阻抗矩阵的低秩压缩计算,并且结合等效偶极子法加速抽取阻抗矩阵元素的填充。与传统矩量法相比,计算时间和内存消耗都得到了有效缩减,数值结果证明了该方法的精确性和有效性。     

Analysis for scattering of conductive objects covered with anisotropic magnetized plasma by trapezoidal recursive convolution finite‐difference time‐domain method

The finite‐difference time‐domain method (FDTD) is extended to three‐dimensional (3D) anisotropic magnetized plasma based on the trapezoidal recursive convolution (TRC) technology. The TRC technique requires single convolution integral in the formulation as in the recursive convolution (RC) method, while maintaining the accuracy comparable to the piecewise linear recursive convolution (PLRC) method with two convolution integrals. In this article, the numerical results indicate that the TRC‐FDTD method not only improves accuracy over the RC‐FDTD with the same computational efficiency but also spends less computational time than the PLRC‐FDTD based on the same accuracy. The 3D TRC‐FDTD formula is provided and the bistatic radar scattering sections of conductive targets covered with anisotropic magnetized plasma are calculated. The results show that magnetized plasma cover layer can greatly reduce echo energy of radar targets, and the anisotropic magnetized plasma cover has better absorption effect than nonmagnetized. © 2010 Wiley Periodicals, Inc. Int J RF and Microwave CAE, 2010.     

Low‐profile UHF RFID reader antenna with CP radiation and coupled feeding technique

A low profile annular‐ring patch antenna with circularly polarized (CP) radiation for radio frequency identification (RFID) reader applications in the ultra‐high frequency (UHF) band (922‐928 MHz) is presented in this article. Perturbation method is applied by loading a pair of triangular open‐notch into the outer circumference of ring patch, and good impedance matching can be determined by using the coupled feeding technique. The overall size of this proposed antenna is 150 mm × 150 mm × 10.4 mm. The measured results show desirable 10‐dB impedance bandwidth and 3‐dB axial ratio (AR) bandwidth of 3.5% (908‐941 MHz) and 0.65% (922‐928 MHz), respectively. Stable antenna peak gain and efficiency of 7.2 dBic and 87% are also exhibited, respectively.     

Dual‐port MIMO dielectric resonator antenna for WLAN applications

A dual‐port multiple‐input multiple‐output (MIMO) dielectric resonator antenna (DRA) for 5 GHz IEEE (802.11a/h/j/n/ac/ax) is discussed in this article. Two prototypes of single feed DRA and dual feed MIMO DRA are fabricated and measured results are compared with the simulated data. The proposed single feed DRA and dual feed MIMO DRA exhibits wide impedance bandwidth (IBW). Antennas have been fabricated on Rogers RT Duroid substrate with Eccostock made DRA placed over the substrate. DRAs are excited by aperture coupled feed to achieve wide bandwidth and high efficiency. The measured IBW of uniport DRA and dual‐port MIMO DRA are 26.6% (4.75‐6.21 GHz) and 27.5% (4.7‐6.2 GHz) respectively. Maximum gain of the antenna is 7.4 dBi. The results of the antennas are in good agreement with simulated data and they are suitable for WLAN applications. These antennas are also compact with area of substrate 32.8 cm².     

Antenna optimization by hybrid differential evolution

To solve complex antenna design problems, this article proposes a hybrid differential evolution algorithm (DE). The proposed method combines the DE with the simplified quadratic interpolation (SQI) to optimize the performance of the antenna. The DE is the global optimizer, and the SQI is used to fine tune. The hybrid DE is demonstrated on optimizing designing Yagi‐Uda antennas and wideband patch antennas. Numerical results confirm that the proposed method is superior to or at least competitive with the original DE and other evolutionary algorithms in terms of convergence speed and solution quality. © 2009 Wiley Periodicals, Inc. Int J RF and Microwave CAE 2010.