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This study is based on the application requirements of fully polarimetric radar for aircraft detection and target recognition. We focus on fully polarimetric scattering characteristics of air targets, particularly the cross-polarimetric scattering mechanism and its availability. In the ultra-high frequency band, we conduct numerical modeling and simulation of stealth and non-stealth aircraft targets. The spatial distribution characteristics of polarimetric scattering for different aircraft targets are studied and compared, and the structures that could cause strong cross polarization are analyzed. The results of this study suggest that nonstealth aircraft have more polarimetric characteristics; this will help people use polarimetric radar for detecting aircraft.

The time modulated (TM) linear arrays with variable aperture sizes (VAS) ate applied to the airborne pulsed Doppler (PD) radar system in this paper. The VAS arrays which have great flexibility in the control of the aperture excitations are employed to synthesize low/ultra-low sidelobe patterns in the PD radar system, and the optimized time sequences obtained via the differential evolution (DE) algorithm is used to minimize the average power loss due to the time modulation. Numerical simulation results of the signals transmitted and received by PD radar based on the VAS arrays show that the time modulation changed the waveforms of the signals in time domain but the received signals also contain the correct range and velocity information of the moving target. Index Terms--Antenna arrays, differential evolution, pulsed Doppler radar, time modulation.

Two modified junction basis functions were presented based on the principles of current continuity and zero charge. A new type of wires-to-surface basis was used to analyze wire conical/pyramidal monopoles mounted on conducting plates more efficiently than with the traditional wire-to-surface junction element: To obtain an uniform current distribution along all the wire elements of the antenna, the mesh size and the iterative tolerance can be set as 0.08-0.10[lambda] and le-3 to le-2 with this new basis function or its multiple version, while these two parameters need to be about 0.04[lambda] and le-5, respectively, as for the old one. A single-point surface-to-surface junction basis function was introduced to resolve thin sheet pyramidal or conical monopole antennas or any other single-point surface-to-surface junction problems which role the traditional junction basis is not yet suited to. Index Terms--Antennas, basis functions, method of moments (MoM), surfaces, wires.

In this paper, a local multilevel fast multipole algorithm (LMLFMA) based on an improved electric field integral equation (IEFIE) is developed to achieve fast and efficient solution of electromagnetic scattering from 3-D conducting structures. The IEFIE is used to reduce iteration number, and LMLFMA is applied to further accelerate the computation of matrix-vector multiplications in iteration, in which only the local interactions between subscatterers are taken into account. Numerical results show that the present method attains faster iterative convergence than traditional EFIE and less computational cost than MLFMA. The speedup can achieve at least 4-5 times while keeping an rms error of less than 2 dB. Index Terms--Electric field integral equation (EFIE), electromagnetic scattering, fast solvers, radar cross section (RCS).

The synthesis of a nonuniform antenna array with as few elements as possible has considerable practical applications. This paper introduces a new non-iterative method for linear array synthesis based on the matrix pencil method (MPM). The method can synthesize a nonuniform linear array with a reduced number of elements, and can be also used to reduce the number of elements for linear arrays designed by other synthesis techniques. In the proposed method, the desired radiation pattern is first sampled to form a discrete pattern data set. Then we organize the discrete data set in a form of Hankel matrix and perform the singular value decomposition (SVD) of the matrix. By discarding the non-principal singular values, we obtain an optimal lower-rank approximation of the Hankel matrix. The lower-rank matrix actually corresponds to fewer antenna elements. The matrix pencil method is then utilized to reconstruct the excitation and location distributions from the approximated matrix. Numerical examples show the effectiveness and advantages of the proposed synthesis method. Index Terms--Array synthesis, low rank approximation, matrix pencil method (MPM), nonuniform antenna arrays, singular value decomposition (SVD), sparse antenna array.

The graphics processing unit (GPU) has been used to speed up the conventional method of moments (MoM) calculations for electromagnetic scattering from arbitrary three-dimensional conducting objects. The acceleration ratio of filling impedance matrix has reached 30, while the total acceleration ratio (including iteration) is about 20. Moreover, a matrix splitting algorithm is developed to break up the texture limit. It splits the huge matrix into multiple, tiny matrixes, each of which can be fit into one texture. Then, the system memory can be used to store all the elements of the impedance matrix, making it possible to deal with electrically large problems, since the capacity of video card's memory is no longer a limit. Index Terms--Graphics processing unit (GPU), hardware speed-up, matrix splitting algorithm, method of moments (MoM), numerical solution.

A high convergence rate and a robust exploration ability constitute a contradiction in modern heuristic optimization techniques. This paper describes a modified differential evolution strategy (MDES) that builds up a balanced relationship between the two contradictive elements by introducing several modifications into the conventional differential evolution strategy (DES). The novel MDES opens up an effective and robust approach for global optimization problems. Several representative mathematical functions are minimized using various optimization methods and the convergence rates are compared to evaluate the performance of MDES. Moreover, array synthesis examples which can be formulated as non-convex problems are presented, including the optimal synthesis of sum and difference patterns and the synthesis of unequally spaced arrays. Numerical results demonstrate that the proposed approach improves the performance of the algorithm significantly, in terms of both the convergence rate and the exploration ability. Index Terms--Antenna arrays, differential evolution (DE), pattern synthesis.

The higher order vector basis functions defined in large patches have been utilized in the numerical solution of integral equations in this paper to sparsify the impedance matrix and relieve the memory pressure. The physical explanation for the sparsification of the impedance matrix is also elucidated. Furthermore, the maximally orthogonalized bases have been applied to improve the condition number of the impedance matrix. The scaling factor was reformed to speed up the iteration convergence in the numerical solution. Finally, the iterative method for sparse matrix equations is applied to improve the solution efficiency. Some numerical results are provided to illustrate the excellent performance both in the sparsification of the impedance matrix and solution efficiency for numerical analysis of the scattering problem. Index Terms--Higher order basis functions, matrix sparsification, maximum orthogonalization, scaling factor.

An effective frequency domain full-wave approach is proposed for the simulation of time modulated (TM) linear arrays with variable aperture size (VAS) time schemes. The full-wave simulation of the TM linear arrays is fulfilled by combination of the frequency domain full-wave simulation results of the corresponding regular linear arrays at multiples of the modulation frequency. Numerical simulation results of two TM linear arrays with VAS time schemes are presented and are compared with reported measurement results, and close agreement is obtained through comparison. Index Terms--Antenna arrays, frequency-domain, time modulation.

In this paper, a novel improved electric field integral equation (IEFIE) is developed to achieve fast and accurate solution of electromagnetic scattering from 3-D conducting structures. By adding the principle value term of the magnetic field integral equation (MFIE) operator into the EFIE operator, a well-conditioned improved EFIE operator is constructed. To achieve a reasonable accuracy, several update steps for the current vector are required. A multilevel fast multipole algorithm (MLFMA) is also applied to accelerate the computation of matrix-vector multiplications in the iteration. The present method attains much faster convergence of iterations than traditional EFIE and much better accuracy of the solution than the traditional combined field integral equation, particularly for 3-D structures with open or sharp surfaces. Numerical results show the validity and efficiency of the present method. Index Terms--Combined field integral equation, conjugate gradient iteration, electric field integral equation, magnetic field integral equation, multilevel fast multipole algorithm (MLMFA).

Both the interchannel coupling (CC) among multiple compact circuitry channels and the antenna polarization coupling (APC) due to angular deviation of antennas (ADA) in diversity systems cannot be neglected in practice. The effects of these two problems are considered in this paper. The investigation of the CC in a two-branch orthogonal polarization antenna diversity system is extended to the case of a general multibranch polarization antenna diversity system, and the CC is investigated together with APC by applying a network theory framework. Their effects on both the correlation and the mean power ratio among received signals are obtained. It is found that the effects of both kinds of couplings as well as that of the combining signal processing on the received signals can be considered as a series of weightings. Two equivalent relations among APC, CC, and ADA are developed. Such equivalences reveal insights into CC from the point of view of antennas and provide additional degree-of-freedom for system optimization to improve the diversity performance. Furthermore, two kinds of compensation techniques are also developed to mitigate the APC and CC, as well as to enhance the diversity performance. Finally, some numerical results are presented to validate the analyses. Index Terms--Angular deviation of antenna (ADA), antenna polarization coupling (APC), diversity, equivalent relations, interchannel coupling (CC).

An active learning framework is introduced to reduce the number of frequencies and angles one must consider for wideband monostatic scattering computations or measurements. This method is used to optimally select those frequencies and angles that would be most informative, resulting in nonuniform sampling and often a reduced number of points (vis-a-vis uniform sampling). In this paper we focus on jointly two-dimensional optimal sampling in frequency and incident angle [theta] for monostatic scattering. The method consists of two basic steps. One step involves estimation of model parameters using a least-square (LS) algorithm. The next step is to optimally choose the next point (frequency and [theta]) for analysis by the computational model or experiment. This new point is selected with the goal of reducing uncertainty in the parametric model (quantified via the Fisher information matrix). Iterating these two steps, a sequence of numerical computations or measurements are performed, each at the most informative point far learning the parameters of the associated simpler parametric model. This idea is demonstrated here in the context of reducing the number of points (frequencies and orientations) at which a computational model must be employed. And in order to avoid repeatedly gathering samplings at the edge of the input space, an alternative objective function is applied which makes the actively selected points closer to a region of interest. Index Terms--Active learning, Fisher information, multilevel fast multipole method (MLFMM), nonuniform sampling.

The diagonally perturbed incomplete factorization preconditioning scheme is applied to the conjugate gradient (CG) method fur solving a large system of linear equations resulting from the use of edge-based finite-element method (FEM). This scheme contains more global information about the coefficient matrix when compared with banded-matrix schemes. The efficient implementation of this preconditioned CG (PCG) algorithm is described in detail for complex coefficient matrix equation. On several electromagnetic problems the PCG approach converges in CPU time, which is 8.6-19.5 times shorter with respect to the CG approach. By comparison with other preconditioned techniques, the results demonstrate that incomplete factorization preconditioning strategy is especially effective for CG iterative method when edge-FEM is applied to solve large-scale time-harmonic electromagnetic field problems. Index Terms--Conjugate gradient (CG) method, finite-element method (FEM), incomplete factorization, preconditioning technique.