Full‐wave analysis of circular guiding structures using the finite difference frequency domain method

In this article, a general full‐wave two dimensional finite difference frequency domain (2D‐FDFD) method is presented that could be used to analyze general circular multi‐layered multi‐conductor guiding structures. The FDFD method is mainly used to get the dispersion curves for these structures. The results which are obtained using the FDFD equations come through solving an eigen‐value problem, where the obtained eigen‐values and eigen‐vectors are used to produce the propagation constants, distribution of the fields and the characteristic impedances for these structures. Several examples ranging from simple coaxial lines to coupled circular microstrip lines are presented. The FDFD results are compared with those obtained through other analytical and numerical techniques. © 2010 Wiley Periodicals, Inc. Int J RF and Microwave CAE, 2010.     

Synthesis of slow‐wave structures based on capacitive‐loaded lines through aggressive space mapping (ASM)

This article is focused on the automated synthesis of slow‐wave structures based on microstrip lines loaded with patch capacitors. Thanks to the presence of the shunt capacitors, the effective capacitance of the line is enhanced, and the phase velocity of the structure can be made significantly smaller than the one of the unloaded line. The target is to achieve the layout of the slow‐wave structure able to provide the required slow‐wave ratio, characteristic (Bloch) impedance and electrical length (i.e., the usual specifications in the design of slow‐wave transmission lines). To this end, a two‐step synthesis method, based on the aggressive space mapping (ASM) algorithm, is proposed for the first time. Through the first ASM algorithm, the circuit schematic providing the target specifications is determined. Then, the second ASM optimizer is used to generate the layout of the structure. To illustrate the potential of the proposed synthesis method, three application examples are successfully reported. The two‐step ASM algorithm is able to provide the layout of the considered structures from the required specifications, without the need of an external aid in the process. © 2015 Wiley Periodicals, Inc. Int J RF and Microwave CAE 25:629–638, 2015.     

Rapid surrogate‐assisted design optimization of minimum‐size broadband branch‐line couplers with variable topology

This work discusses simulation‐driven design of miniaturized wideband branch‐line couplers with a variable topology. Size reduction is enabled here by replacing uniform transmission lines of the original coupler with slow‐wave structures in the form of cascaded compact cells and meander lines. The primary goal is to determine a number of cells in the cascade and particular cell dimensions for which the minimum size of the coupler as well as its required operating conditions are ensured. To this end, we employ a surrogate‐assisted technique involving a trust‐region gradient search framework. Computational efficiency of the design process stems from estimating the Jacobian of circuit responses at the level of a low‐fidelity model of the cascade. The latter is composed in a circuit simulator from duplicated EM‐evaluated data blocks of a single cell and is well correlated with the corresponding high‐fidelity model. The key advantage of this work is the utilization of a reconfigurable, cheap, and well‐aligned low‐fidelity model. The proposed approach is demonstrated through design of a minimum‐size two‐section branch‐line coupler with quasi‐periodic dumbbell‐shaped cells and meander lines. Excellent circuit performance as well as its small size showcase the reliability and usefulness of the presented method. Experimental verification is also provided.     

Fault detection in finite frequency domains for multi‐delay uncertain systems with application to ground vehicle

This paper is concerned with the actuator fault detection (FD) problem in finite frequency domains for multi‐delay systems subject to time‐varying affine uncertainties. Because of the existence of time‐varying uncertain parameters, the generalized Kalman–Yakubovic–Popov lemma based finite frequency FD filter design approaches cannot be applied. To tackle this difficulty, a new delay‐dependent bounded real lemma (BRL) is established by using Lyapunov theory and Parseval's theorem to characterize the finite frequency disturbance attenuation and fault sensitivity performances. Moreover, via the obtained BRL, convex FD filter design conditions are then derived by constructing a hyperplane tangent. Finally, the effectiveness and advantages of the proposed FD method are illustrated through a simulation example on a ground vehicle. Copyright © 2014 John Wiley & Sons, Ltd.     

A 2D design and modeling of micro strip structures on inhomogeneous substrate

The wave concept iterative process method (WCIP) is presented for full wave investigation of a microwave structure on inhomogeneous substrate. These structures can be exploited to achieve a high performance due to their versatility, which can reduce manufacturing cost, size, and weight. Using the spectral operator in the WCIP method permits to resolve this inhomogeneous problem. The use of the spectral operator keeps the advantages of the 2D mesh of the circuit plan. Two different examples are studied; the dispersion characteristics of a microstrip transmission line on inhomogeneous substrate and a microstrip T‐resonator on a low temperature Co‐fired ceramic (LTCC) with embedded vertical air cavities. Compared with measurement and the finite different method, the present technique is found to be efficient with 2D analysis and a required low memory. © 2008 Wiley Periodicals, Inc. Int J RF and Microwave CAE, 2009.     

Optimal precoding for full-duplex base stations under strongly correlated self-interference channels

We study the optimal precoding for a full-duplex (FD) system, where one FD multi-antenna base station (BS) respectively transmits to and receives from two half-duplex single-antenna mobile users (MUs) on the same time slot and frequency band. At the FD BS, the received signal from the desired MU is severely affected by the extremely strong self-interference (SI) from its transmit antennas to the receive antennas. In the presence of residual SI after imperfect SI cancellation, the downlink transmission rate maximization problem subject to a targeted uplink rate is formulated as a non-convex optimization problem to characterize the achievable rate region for the considered system. Considering the case in which the SI channel is strongly correlated, the above problem is transformed into a convex problem by exploiting the rank-one property of the SI channel, which can be solved efficiently. Finally, numerical results validate the effectiveness of the proposed scheme.     

Surrogate‐assisted EM‐driven miniaturization of wideband microwave couplers by means of co‐simulation low‐fidelity models

This article proposes a methodology for rapid design optimization of miniaturized wideband couplers. More specifically, a class of circuits is considered, in which conventional transmission lines are replaced by their abbreviated counterparts referred to as slow‐wave compact cells. Our focus is on explicit reduction of the structure size as well as on reducing the CPU cost of the design process. For the sake of computational feasibility, a surrogate‐based optimization paradigm involving a co‐simulation low‐fidelity model is used. The latter is a fundamental component of the proposed technique. The low‐fidelity model represents cascaded slow‐wave cells replacing the low‐impedance lines of the original coupler circuit. It is implemented in a circuit simulator (here, ADS) and consists of duplicated compact cell EM simulation data as well as circuit theory‐based feeding line models. Our primary optimization routine is a trust‐region‐embedded gradient search algorithm. To further reduce the design cost, the system response Jacobian is estimated at the level of the low‐fidelity model, which is sufficient due to good correlation between the low‐ and high‐fidelity models. The coupler is explicitly optimized for size reduction, whereas electrical performance parameters are controlled using a penalty function approach. The presented methodology is demonstrated through the design of a 1‐GHz wideband microstrip branch‐line coupler. Numerical results are supported by experimental validation of the fabricated coupler prototype.     

Modeling and synthesis of the interdigital/stub composite right/left‐handed artificial transmission line

An efficient design procedure, including both analysis and synthesis, is proposed for Composite Right/Left Handed (CRLH) interdigital/stub structures. Improved models are developed for both the interdigital capacitor and the shorted stub inductor including its ground via hole. Subsequent optimal formulas are recommended to model these components with their parasitic effects. The models and formulas are verified by both full‐wave and experimental results. A CAD program with a friendly GUI, available online, is provided and its operation is described in details. This program allows a very fast design of the CRLH structure, and its synthesis parameters are proven very accurate without any full‐wave optimization. © 2009 Wiley Periodicals, Inc. Int J RF and Microwave CAE, 2009.     

Dispersion relations and mode shapes for waves in laminated viscoelastic composites by finite difference methods

Dispersion relations and mode shapes for wave propagation in a laminated, infinite viscoelastic composite are obtained by finite difference methods. A complex modulus formulation is used. Free vibrations are studied for which the frequency is complex and the wave number real. Floquet theory is applied. Discretization of the governing differential equations and quasi periodic boundary conditions leads to a generalized eigenvalue problem for a large, sparse complex matrix. This is solved by a method involving permutation of the matrix into a convenient banded form and by writing recursion relations for the determinant. As an example, numerical results are obtained for a two medium composite with the filament material elastic and the matrix material modelled as elastic in dilatation and a standard linear solid in shear. The method proves to be very efficient and accurate for calculating frequencies as well as mode shapes.     

Ultra‐wideband radar cross‐section reduction using polarization conversion metasurface

In this paper, a novel ultra‐wideband (UWB) chessboard structured metasurface has been designed to realize the monostatic radar cross‐section (RCS) reduction. The unit has a periodic simple multi‐V‐shaped structure can convert linear polarized waves into their orthogonal polarized ones in an UWB ranging from 14.5 to 41 GHz. In addition, the RCS reduction of two kinds of chessboard structures which composed of the proposed unit are compared in this paper, and one of them has been fabricated and tested. Experiments and simulation results show that, under normal incidence of electromagnetic wave, it can achieve RCS reduction over 10 dB within an UWB of 20.5 GHz.     

Via‐hole coupled oversized microstrip line and its band‐pass filter application

A simple via‐hole coupled oversized microstrip line filter is proposed and demonstrated in this article. The via‐hole in this case works as an inductor coupling structure whereas the oversized microstrip line resonator has a higher Q‐factor than its conventional counterpart. Full‐wave‐based circuit models of a series of via‐holes embedded in the oversized microstrip line are extracted by using our proposed numerical calibration technique combined with a commercial method‐of‐moments simulator. A simple 3‐pole via‐hole coupled oversized microstrip line filter is designed and fabricated on the basis of the extracted circuit models of via‐holes. Measured results show that the demonstrated filter has a center frequency of 1.853 GHz, a bandwidth of 6.98% and an insertion loss of 1.36 dB. Measured results of the fabricated filter sample are in agreement with its simulated results, showing a good performance of the proposed scheme. © 2008 Wiley Periodicals, Inc. Int J RF and Microwave CAE, 2008.     

Electrical performances of self‐shielded uniplanar guided‐wave structures

This paper presents electrical performance and propagation characteristics of self‐shielded uniplanar guided‐wave structures including circular, elliptic, diamond, and trapezoidal enclosures. Enhanced spectral domain approach is extended to characterize guided‐wave property of these self‐shielded lines. Particular attention is focused on fundamental mode propagation constant and characteristic impedance of coupled uniplanar line having a practical shield profile. Influence of various parameters of the structure on cutoff frequency of the slotline mode is investigated. Interesting features of the self‐shielded uniplanar structures are discussed for design consideration. Results are verified with some data available and presented in support of the technical discussion. ©1999 John Wiley & Sons, Inc. Int J RF and Microwave CAE 9: 22–31, 1999.     

Propagation characteristic analysis of ridged circular waveguide using 2D finite‐difference frequency‐domain method

The propagation characteristics of ridged circular waveguides are analyzed by using 2D finite‐difference frequency‐domain (2D FDFD). Based on the 2D FDFD method in a cylindrical coordinate system, general difference formulas for the ridged circular waveguide are deduced, and modified difference formulas are built at some special points of the ridged circular waveguides. To verify the proposed method, three ridged circular waveguide structures are investigated and the numerical results are compared with available ones obtained by other research methods. © 2005 Wiley Periodicals, Inc. Int J RF and Microwave CAE 15, 2005.     

CAD modeling of coplanar waveguide interdigital capacitor

A lumped‐element circuit is proposed to model a coplanar waveguide (CPW) interdigital capacitor (IDC). Closed‐form expressions suitable for CAD purposes are given for each element in the circuit. The obtained results for the series capacitance are in good agreement with those available in the literature. In addition, the scattering parameters obtained from the circuit model are compared with those obtained using the full‐wave method of moments (MoM) and good agreement is obtained. Moreover, a multilayer feed‐forward artificial neural network (ANN) is developed to model the capacitance of the CPW IDC. It is shown that the developed ANN has successfully learned the required task of evaluating the capacitance of the IDC. © 2005 Wiley Periodicals, Inc. Int J RF and Microwave CAE, 2005.     

A homotopy method for exact output tracking of some non‐minimum phase nonlinear control systems

This paper presents a method for non‐causal exact dynamic inversion for a class of non‐minimum phase nonlinear systems, which seems to be an alternative to those existing in the literature. This method is based on a homotopy procedure that allows to find a ‘small’ periodic solution of a desired equation by a continuous deformation of a known periodic solution of a simpler auxiliary system. This method allows to face the exact output tracking problem for some non‐minimum phase systems that are well known in the literature, such as the inverted pendulum, the motorcycle and the CTOL aircraft. Copyright © 2008 John Wiley & Sons, Ltd.     

Design of nonlinear interconnections guaranteeing the absence of periodic solutions

In this paper, a novel methodology for scheming interconnection structure for a class of nonlinearly interconnected systems is proposed to guarantee the absence of a specific kind of periodic solutions. Those types of systems can be viewed as an interconnection of single-input single-output isolated subsystems with the interconnection structure specified by a square matrix. Frequency-domain inequalities conditions as well as linear matrix inequalities (LMIs) conditions for nonexistence of limit cycles of the second kind in the entire interconnected system are established. The main results also define the frequency range on which cycles of the second kind of the system cannot exist. Based on this LMI approach, an estimate of the largest bound of the frequency of cycles of the second kind can be explicitly computed by solving a generalized eigenvalue minimization problem. Numerical results show the effect of the nonlinear interaction on system behavior and the applicability and validity of the proposed method.     

Composite corporate traveling‐wave power dividers for slot array waveguide antennas

A composite corporate traveling‐wave power divider is presented. The single‐layer structure is composed of three parts: two interdigital traveling‐wave subsections combined with a power splitter. An iterative design technique is described in which the divider is split into a number of basic blocks. Large‐scaled networks are then easily designed because the whole structure does not need to be simulated. A method to take into account the insertion losses is also proposed and bandwidth enhancement is discussed, which is done by increasing the number of corporate layers. Experimental results are also shown for a 1:4 subsection. It provides equal output power with 0.5 dB of insertion loss. The phase‐shift between output ports is close to the specifications of ?150° at 30 GHz, with an error of less than 2°. It is also shown that this topology is well suited for frequency scanning antenna. © 2008 Wiley Periodicals, Inc. Int J RF and Microwave CAE, 2009.     

A finite-difference eigenvalue algorithm for calculating the band structure of a photonic crystal

A new method based on a finite difference of the governing differential equation for the eigenvalue problem is introduced to calculate the band structure of a two-dimensional photonic crystal. The effective medium technique is also used in the method. The problem is reduced to a standard matrix eigenvalue problem. Compared to the conventional plane wave expansion method, the present method improves the convergence of the solution and thus is a fast and accurate algorithm for calculating the band structure of a photonic crystal.     

Leaky‐wave antenna with high gain and wide beam‐scanning angle range based on novel SIW‐CRLH transmission line

A leaky‐wave antenna (LWA) with high gain and wide beam‐scanning angle is proposed in this article using a novel substrate integrated waveguide (SIW) composite left/right‐handed transmission line (CRLH TL). The novel SIW‐CRLH TL is analyzed and the equivalent circuit model is also provided. Considering the continuous phase constant of the balanced SIW‐CRLH TL from negative to positive values, the proposed LWA can obtain a continuous beam steering property from backward to broadside to forward. For verification, a periodic LWA, which is comprised of 10 unit cells of the balanced SIW‐CRLH TL, is fabricated and measured. The measured and simulated results agree well, showing that the proposed periodic LWA operates from has continuous beam‐scanning capabilities of about 90° from backward to forward (including the broadside) with gains of better than 10 dB within the operating band. © 2015 Wiley Periodicals, Inc. Int J RF and Microwave CAE 26:36–41, 2016.     

Design of defective electromagnetic band‐gap structures for use in dual‐band patch antennas

Generally, the surface wave of an antenna can be suppressed by integrating the electromagnetic band‐gap (EBG) structures. However, to achieve this effect, the EBG cells must be reasonably designed, otherwise it may lead to performance degradation instead. In this article, a dual‐band pinwheel‐shaped slot EBG structure is proposed. When applied to a patch antenna, defects are introduced into 3 rows of the EBG unit cells. The proposed antenna, incorporating EBGs designed with structural defects, to radiate at 4.9 and 5.4 GHz is simulated and tested. The measured results show that the ?10‐dB bandwidth of the proposed EBG antenna is extended by 41% and 25.4% at low frequency and high frequency, respectively. In addition, the peak gain of the proposed EBG antenna is increased by 2.44 dB at 4.9 GHz and 2.86 dB at 5.4 GHz with >40% efficiency. When compared with the periodic EBG antenna, this antenna is more effective. Thus, these experimental results show that the performance of the EBG antenna can be improved by interrupting the periodicity of the EBGs structures.