Shunlian Chai

Transmit–Receive Isolation Improvement of Antenna Arrays by Using EBG Structures

The mushroom-like electromagnetic band-gap (EBG) structure combined with a choke structure is applied to improve the transmit-receive (T/R) isolation of two X-band large antenna arrays. The basic idea presented is to suppress the surface wave propagation between the T/R arrays by utilizing EBG walls formed through a vertical configuration of the EBG cells, so as to improve the T/R isolation. The isolation level of the arrays with the EBG walls and a metal choke is measured and compared to other isolation methods applied. The experimental results show that by using this combined structure, the isolation level can be improved by at least 30 dB in the radars operation band, which shows its high efficiency.

Solving the Axial Line Problem for Fast Computation of Mixed Potential Green's Functions for Cylindrically Stratified Media

An improved approach is proposed for solving the axial line problem (|ρ^→-ρ^→| = 0) of mixed potential Greens functions (MPGFs) in spatial domain for cylindrically stratified media. Different from the previous work, to alleviate the convergence problem of the summation of cylindrical eigenmodes in the spectral domain, two asymptotic terms with respect to <i>n</i> are extracted analytically. In this way, the higher order nonphysical singularities with respect to |ρ^→ - ρ^→| are proven to vanish, and the lower order physical singularities can be transformed into the spatial domain completely using the Sommerfeld identity. Therefore, the remaining parts of the summation are free of singularities related to |ρ^→ - ρ^→| and fast convergent, which can be easily transformed into closed-form spatial-domain expressions by applying the generalized pencil of function method. The present approach can be used to accurately calculate the spatial domain MPGFs over the entire cylindrical surface. Several numerical results are presented to verify the accuracy and efficiency of this approach.

A Robust Method for the Computation of Green's Functions in Cylindrically Stratified Media

Closed-form Greens functions for cylindrically stratified media are derived in terms of spherical waves and surface-wave contributions. The methodology is based on a two-level approximation of the spectral-domain Greens functions. In the first step, the large argument behavior of the zeroth-order Hankel functions is used for the extraction of the quasistatic images from the spectral-domain Greens functions with the use of the Sommerfeld identity. In the second step, the remaining part of the Greens functions are approximated in terms of pole-residue expressions via the rational function fitting method. This robust, efficient, and fully numerical approach does not call for an analytical cumbersome extraction of the surface wave poles, prior to the spectrum fitting. Moreover, it can be applied in both the near- and far-field. Numerical results for the Greens functions of one-layer and two-layer structures are presented to verify the accuracy and efficiency of this approach.

COUPLING MATRIX DECOMPOSITION IN DESIGNS AND APPLICATIONS OF MICROWAVE FILTERS

The relationship between the immittance inverter co- e-cients and the coupling coe-cients is obtained under the non- resonating coupling condition. With the relationship and the deter- minant properties of the transformation matrix, the coupling matrix could be decomposed to many sub-matrixes for fllter designs. The physical signiflcance of the decomposition is discussed. Using this idea, a fllter can be decomposed to a number of sub-fllters, which could be connected by sections of transmission lines with the same characteris- tics kept.

Design and Manufacture of the Wide-Band Aperture-Coupled Stacked Microstrip Antenna

The X-band wide band aperture-coupled stacked mi- crostrip antenna is studied and manufactured. Based on the finite- difference time-domain method, a parametric study of the input impedance of the antenna is presented, and the effects of some pa- rameters which are not easy to control in manufacture of the antenna impedance are illustrated. The structure of screw-plane-support is used for the manufacture of this kind of antenna. And several notes for manufacture are considered based on the parameter analysis. The measured bandwidth in which VSWR ≤ 2 is greater than 50%. The measured results are basically accordant to numerical simulated re- sults. It testifies the capability of the model in expanding bandwidth and validity of this structure. The radiation patterns within operation bandwidth are presented and discussed.

AN EFFICIENT METHOD FOR THE COMPUTATION OF MIXED POTENTIAL GREEN'S FUNCTIONS IN CYLINDRICALLY STRATIFIED MEDIA

Closed-form mixed potential Greens functions (MPGFs) for cylindrically stratifled media are derived in terms of quasistatic- wave and surface-wave contributions. In order to avoid possible over∞ow/under∞ow problems in the numerical calculations of special cylindrical functions such as Bessel and Hankel functions, a novel form of the spectral-domain MPGFs is developed. Then, a two- level methodology is used for the approximation of the spectral- domain MPGFs. In the flrst step, the qusistatic components are extracted from the spectral-domain MPGFs, and then transformed into the space domain with the use of the Sommerfeld identity and its derivatives. In the second step, the remaining parts of the spectral-domain MPGFs are approximated in terms of pole-residue expressions via the rational function fltting method (RFFM). The proposed method is e-cient and fully automatic, which avoids an analytical cumbersome extraction of the surface wave poles (SWPs), prior to the spectrum fltting. In addition, this method can evaluate the spatial-domain MPGFs accurately and e-ciently for both the near- and far-flelds. Finally, numerical results for the spatial-domain MPGFs of a two-layer structure are presented and discussed.

Scattering Analysis of Periodic Arrays Using Combined Cbf/P-FFT Method

In this paper, an improved CBFM/p-FFT algorithm is presented, which can be applied to solve electromagnetic scattering problems of large-scale periodic composite metallic/dielectric arrays, even when the array has electrically small periodicity or separating distance. Using characteristic basis function method (CBFM), scattering characteristics of any inhomogeneous targets can be represented by special responses derived from a set of incident plane waves (PWs). In order to reserve the dominant scattering characteristics of the targets and remove the redundancy of the overfull responses, a singular value decomposition (SVD) procedure is applied, then, new series of basis functions are built based on the left singular vectors after SVD whose corresponding singular values beyond a predeflned threshold. However, the algorithm of CBFM combined with method of moments (MoM) still requires a lot of memory and CPU resources to some large scale problems, so the precorrected- fast Fourier transform (p-FFT) method is applied based on the novel built basis functions, with which, the required memory and solve time for solution can be reduced in an extraordinary extent. For a near correction technique is applied to process the interactions between cells placed within a distance less than a predeflned near-far fleld threshold, arrays with electrically small periodicity can be analyzed accurately. Moreover, the incomplete LU factorization with thresholding (ILUT) preconditioner is applied to improve the condition number of the combined algorithm, which improves the convergence speed greatly.

Wide band e-shape wearable antenna for wireless body area network

In this paper, we present a wideband planar e-shape textile antenna and study its performance numerically and experimentally. The textile material and the radiation patch shape provide the antenna a wide working band (2.25ghz-2.75ghz). A digital human model is established to numerically analyze the affect of human body on antenna. Experiments are carried out to study the performance of antenna on human body, The simulated and measured results in free space and on human chest show good agreement with each other. the wideband planar e-shape textile antenna is suitable for body-centric wireless communications with high quality.

Overview and classification of a priori information used in microwave tomography

Different kinds of a priori information used to enhance microwave tomography are introduced and classified into two main categories. The first main category of a priori information is about features of target, these can introduce some information of the target to the tomography algorithm. This type of a priori information can lead to a better initial guess and an appropriate regularization as well. The second category is about the imaging system, these can enhance the imaging though loading system configuration information, such as the boundary condition, background and so on. This type of a priori information changes the field which inquires the target, thus it can enhance the systems ability of acquiring the information of target. Both the categories are aimed to improve the quality and quantity of available information, and help the algorithm seeking an accuracy solution stably.

Adaptive dynamic meta particle swarm optimization algorithm and application in 3D power pattern synthesis for conformal arrays

The adaptive dynamic Meta particle swarm optimization (ADMPSO) algorithm is proposed to apply in power patterns synthesis for conformal arrays. To begin, the dominated subgroup and nondominated subgroup are defined on the basis of traditional Meta particle swarm. Meanwhile, the adaptive dynamic modulating for multiple-subgroup is realized by introducing the downsizing of nondominated subgroup, and the expansion of dominated subgroup, which improves both the convergence speed and exploration ability significantly. Finally, based on the target fitness function built for power pattern synthesis in arbitrary arrays, ADMPSO algorithm has been used in synthesizing three dimensional (3D) power patterns for conical array, with all polarization fields considered.