L.-W. Li

National University of Singapore, Singapore, Singapore

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Publications (176)85.54 Total impact

  • [show abstract] [hide abstract]
    ABSTRACT: In this paper, a full-wave volume-surface integral equation approach is presented for the analysis of conformal microstrip antenna arrays with finite curved frequency selective surface (FSS) radomes. The volume integral equation is applied to the dielectric region of the composite structure, while the surface integral equation is used on the conductive surface. The integral equations are solved using the method of moments (MoM), with the precorrected-FFT (P-FFT) method used to reduce the memory requirement and accelerate the matrix-vector products in the iterative solution of the equation. With this method, FSSs and antennas of arbitrary shape and finite size can be modelled and the effects of the FSS on the characteristics of the antenna can be accurately investigated.
    Journal of Electromagnetic Waves and Applications 04/2012; 21(13):1745-1760.
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    ABSTRACT: When the Fast Hankel Transform filter technique is used to calculate the dyadic multilayered Green's functions, it can be difficult to obtain accurate numerical results because of the branch-cut singularity and the surface wave poles singularity. The Modified Fast Hankel Transform filter algorithm is proposed to overcome this problem by expressing the Bessel function with a complex argument as a sum of terms of product of Bessel function with the real part of the argument and Bessel function with the imaginary part of the argument. Then the Fast Han-kel Transform filter technique is applied to each expansion term. Numerical results confirm that the proposed approach has high accuracy and efficiency and successfully extends the applicability of the conventional Fast Hankel Transform method to general multilayered geometries.
    01/2010;
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    N.V. Venkatarayalu, L.-W. Li
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    ABSTRACT: The stable hybrid finite element time domain - finite difference time domain (FETD-FDTD) method is extended by incorporating higher order hierarchical basis functions in the finite element region. The use of unstructured tetrahedral elements in the modeling of antenna structure enables the application of the hybrid method to accurately model geometrically complex radiators. Pyramidal elements are used in the transition from unstructured tetrahedral elements to structured hexahedral elements of the FDTD grid. The finite element formulation incorporates the excitation of antennas using coaxial line or stripline feed with transverse electromagnetic mode (TEM). Traditional FDTD method with anisotropic perfectly matched layer (PML) is used to simulate unbounded media. The technique is extended for scattering problems, enabling the modeling and simulation of reception by antennas. Application of this method in the modeling of typical wideband antennas along with the numerical results is presented.
    Applied Electromagnetics Conference, 2007. AEMC 2007. IEEE; 01/2008
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    ABSTRACT: The finite-element time-domain (FETD) method based on the use of hanging variables to generate nested grids is used as an interface between the coarse and fine grids in the FDTD subgridding method. Since the formulation for treating hanging variables is based on a Galerkin-type intergrid boundary operator, the resulting FDTD subgridding algorithm is guaranteed to be stable. Numerical examples such as the computation of resonant modes of a 3-D rectangular resonant cavity and 2-D TE<sub>z</sub> scattering by a PEC cylinder and a NACA airfoil are presented to verify the stability and accuracy of the proposed subgridding algorithm
    IEEE Transactions on Antennas and Propagation 04/2007; · 2.33 Impact Factor
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    Jin Sun, L.-W. Li
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    ABSTRACT: Dispersion of waves guided along a perfectly electric conducting cylinder coated with two-layer lossy dielectric materials is studied in this paper. With a slight dielectric loss considered in both the substrate and the superstrate, the high-attenuation mode is much easier to be observed and anisotropic effects of the wave propagation are enhanced, as compared to that of the lossless dielectric coating. The azimuthal propagation constants, which are parameterized in terms of the layer radius, dielectric constant and coating thickness, are obtained in this paper by solving the dispersion equation
    IEEE Transactions on Antennas and Propagation 04/2007; · 2.33 Impact Factor
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    ABSTRACT: A novel on-chip vertical tapered solenoid inductor has been designed fabricated and experimentally characterised. Owing to its vertical tapered solenoid structure, it is observed that the frequency corresponding to the peak quality factor (f<sub>Qmax</sub>) increases by 160%, i.e. from 4.05 to 10.55 GHz, and the self-resonance frequency (f<sub>sr</sub>) increases from 21.3 to more than 25 GHz, when compared to a traditional planar inductor of similar inductance and on-chip area. Further, this inductor is found to exhibit a self-shielding characteristic which has a strong impact on the requirements of floating shields underneath the inductor. The performance of the inductor is also characterised at different temperatures.
    Electronics Letters 02/2007; · 1.04 Impact Factor
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    ABSTRACT: Extensive studies on the performance of on-chip CMOS transformers with and without patterned ground shields (PGSs) at different temperatures are carried out in this paper. These transformers are fabricated using 0.18-mum RF CMOS processes and are designed to have either interleaved or center-tapped interleaved geometries, respectively, but with the same inner dimensions, metal track widths, track spacings, and silicon substrate. Based on the two-port S-parameters measured at different temperatures, all performance parameters of these transformers, such as frequency- and temperature-dependent maximum available gain (G<sub>max</sub>), minimum noise figure (NF<sub>min</sub>), quality factor (Q<sub>1</sub>) of the primary or secondary coil, and power loss (P<sub>loss</sub>) are characterized and compared. It is found that: 1) the values of the G<sub>max</sub> and Q<sub>1</sub> factor usually decrease with the temperature; however, there may be reverse temperature effects on both G <sub>max</sub> and Q<sub>1</sub> factor beyond certain frequency; 2) with the same geometric parameters, interleaved transformers exhibit better low-frequency performance than center-tapped interleaved transformers, whereas the center-tapped configurations possess lower values of NF<sub>min</sub> at higher frequencies; and 3) with temperature rising, the degradation in performance of the interleaved transformers can be effectively compensated by the implementation of a PGS, while for center-tapped geometry, the shielding effectiveness of PGS on the performance improvement is ineffective
    IEEE Transactions on Microwave Theory and Techniques 02/2007; · 2.23 Impact Factor
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    ABSTRACT: When deriving dyadic Green's functions for the spherical structures with gyrotropic or bianisotropic materials, an integral whose integrand function consists of two spherical Bessel functions and a power function needs to be evaluated. Therefore, this paper revisits thoroughly the evaluation of the integral of I<sub>l,l</sub>'(kappa,kappa'). Starting from pointing out an error, it provides the correct solution to the integral in spherical coordinates in terms of distribution, in particular, step functions and delta functions. The formulation is further extended to a more generalized integral H<sub>l,l</sub>'<sup>lambda</sup>(kappa,kappa'); and it is newly found that the solution to the generalized integral varies differently in the cases of even and odd values of l-l'. The mistakes that we found in the previous literature can also be proved easily by some of our intermediate solutions
    IEEE Transactions on Antennas and Propagation 02/2007; · 2.33 Impact Factor
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    ABSTRACT: Alternative expressions of the dyadic Green's function for an infinite rectangular waveguide with and without a load are presented for defining the electromagnetic fields due to a microstrip patch, a slot, and an equivalent magnetic surface source placed along the broad and narrow walls. The alternative formulations of the dyadic Green's functions are given in terms of rectangular vector eigenfunctions defined by using the piloting vector y and z instead of the conventional z. The sign change in the z direction involved in the double numerical integration influences the accuracy and stability of the numerical computation. The alternative forms of the dyadic Green's functions can be used to overcome this problem in the practical applications.
    Microwave and Optical Technology Letters 01/2007; 8(2):98 - 102. · 0.59 Impact Factor
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    ABSTRACT: The Pruppacher-and-Pitter (PP) raindrop model has been well accepted as a realistically distorted raindrop model. To employ this model for the investigation of radio wave attenuation by rain requires tedious numerical calculations. In this Letter a simple mathematical formula is used to simplify and improve the expression of the nonaxisymmetric distorted raindrop shapes, enabling the scattered field by rain to be derived analytically. The coefficients in the formula are determined for different raindrop sizes by translating the polar coordinates vertically and fitting the shapes of the Pruppacher-and-Pitter-form raindrops to the shapes discussed in this Letter. Using the least-squares method, a general expression for the coefficients in terms of the raindrop sizes is obtained. A comparison of the raindrop shapes obtained from the Pruppacher and Fitter expression and the expression in this Letter demonstrates the validity of the simplified formula for realistically distorted raindrops. © 1994 John Wiley & Sons, Inc.
    Microwave and Optical Technology Letters 01/2007; 7(4):201 - 205. · 0.59 Impact Factor
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    ABSTRACT: The use of a gamma distribution of raindrop sizes in the calculation of microwave rain attenuation is gaining acceptance. A method of calculating the general gamma distribution of raindrop sizes based on the rain attenuation relation aRb has been developed in this article using the measured microwave attenuation data with respect to the rain rates. The method, which has been tested using the CCIR model, overcomes the common problems of instability and does not suffer from the possibility of multiple solutions. The parameters of the gamma distribution suitable for the moderate climatic region are computed from the calculated attenuation data provided by CCIR model. Comparison of the calculated results with the data from the CCIR model shows that the method provides the ideal modeling of raindrop size distribution (DSD). As an application, the two-year experimental data recorded in Singapore have been used to obtain another set of parameters of the gamma DSD function for the tropical region. © 1994 John Wiley & Sons, Inc.
    Microwave and Optical Technology Letters 01/2007; 7(5):253 - 257. · 0.59 Impact Factor
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    ABSTRACT: This paper presents an accurate and efficient method of moments (MoM) analysis for characterizing a large-scaled phased dipole array. In this method, the mixed potential integral equation (MPIE) is utilized and solved iteratively using the generalized conjugate residual method. The precorrected fast Fourier transform (pFFT) method is employed to accelerate the entire computational process so as to reduce significantly both the memory requirement and the computational time for designing large arrays. Numerical results of an example, which had to be solved using approximate approach due to the large number of elements, are presented to demonstrate the efficiency and accuracy of the present method.
    Journal of Electromagnetic Waves and Applications - J ELECTROMAGNET WAVE APPLICAT. 01/2007; 21(6):737-754.
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    ABSTRACT: We have performed an experimental characterization of hybrid temperature and frequency effects on the performance of on-chip square transformers. Using measured two-port S-parameters at different temperatures, we extracted and compared the maximum available gain $G_max$ and fractional power loss $P_ loss$ in each of three transformers (with turn numbers of $N = 2, 3$ , and $4$ of the primary and secondary spirals, respectively). We found that, as temperature increases, the transformer performance degrades significantly. This is caused by the increase in the conductive loss of metal tracks and the dielectric loss of silicon substrate. However, beyond a certain temperature, such as at 418 K in the case of $ N=4$ , further increase in temperature has little effect on performance, mainly because of the constitutive characteristics of silicon substrate. In addition, the decrease in $G_max$ and increase in $P_ loss$ with temperature depend on the number of turns.
    IEEE Transactions on Magnetics 09/2006; · 1.42 Impact Factor
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    ABSTRACT: A fast solution to the combined field volume integral equation (CFVIE) for electromagnetic scattering by large three-dimensional dielectric bodies of arbitrary permittivity and permeability is presented. The CFVIE is formulated in the region of the scatterers by expressing the total fields as the sum of the incident wave and the radiated wave due to both the electric and magnetic polarization currents. The resultant integral equation is solved using the method of moments (MoM). Then the precorrected fast Fourier transform (P-FFT) method is applied to reduce the memory requirement and accelerate the matrix-vector multiplication in the MoM solution. In the implementation of the P-FFT method, two sets of projection operators are constructed respectively for the projections of the electric sources and magnetic sources. In addition, two sets of interpolation operators are also applied respectively for the computation of the vector/scalar potentials and the curl of the vector potentials in the support of the testing functions. The resultant method has a memory requirement of O(N) and a computational complexity of O(NlogN) respectively, where N denotes the number of unknowns
    IEEE Transactions on Antennas and Propagation 04/2006; · 2.33 Impact Factor
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    ABSTRACT: A method based on the volume-surface-wire integral equation and precorrected-fast Fourier transform (FFT) method is presented for accurate analysis of probe-fed microstrip antennas on arbitrarily-shaped, finite-sized ground plane and substrate. The method of moments (MoM) is used to solve the integral equation. Three triangular-type basis functions are used to represent the unknown currents in the substrates, on the conducting surfaces, and probes respectively. The connection of a vertical probe feed to the patch is rigorously modeled by an attachment mode at the junction. The precorrected-FFT method is applied to reduce the memory requirement and computational cost of the traditional MoM to facilitate analysis of large antenna arrays.
    IEEE Transactions on Antennas and Propagation 03/2006; · 2.33 Impact Factor
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    ABSTRACT: In this paper, the transmission line theory is utilized to characterize the metamaterials comprising of microscopic elements of a periodic array, specifically, the traditional split-ring resonators (SRRs), as an example. The bianisotropic property of the metamaterials is characterized in a new way different from the existing wave methods. As evident from both simulations and experiments, the SRR array structure is found to be quite lossy even though it is made of very good conductor or even perfect conductor as in simulation. With the present characterization, we are able to explain physically very well on how or why the energy gets lost in this structure. Finally, the theoretical result is compared with numerical simulation data obtained based on quasi-static Lorentz theory to further verify our analysis.
    Journal of Electromagnetic Waves and Applications - J ELECTROMAGNET WAVE APPLICAT. 01/2006; 20(1):13-25.
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    ABSTRACT: Abstract—A hybrid method that combines the integral equation (IE) method,and Physical Optics (PO) is proposed for efficient analysis of slot arrays enclosed in an electrically large radome. The integral equations are applied over the aperture of the slot antenna and the tip region of the radome by enforcing continuity of the tangential magnetic or electric field. Equivalent PO currents are assumed on the relatively smooth region of the radome walls, induced by the radiated fields from the antenna. The radiated fields due to the PO currents on the radome are coupled back to the integral equations to account for interactions between the radome and the antenna, as well as the interactions between the tip and other parts of the radome. This method,leads to considerable reduction in memory and computational time, since PO is applied to a large part of the electrically large radome, with unknown,currents defined only on the aperture of the slot antenna and the small tip region of the radome. Better efficiency can be attained if the integral equation is applied only to the slot aperture and the entire radome wall taken as the PO region, with acceptable degradation in accuracy. 250,Nie et al.
    Journal of Electromagnetic Waves and Applications - J ELECTROMAGNET WAVE APPLICAT. 01/2006; 20(2):249-264.
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    ABSTRACT: Based on the distributed parameters extracted and the thermal model proposed to calculate the rise in temperature, the average power handling capabilities (APHCs) of finite-ground thin-film microstrip lines (TFMLs) on benzocyclobutene (BCB) and polyimide films over ultra-wide frequency ranges are investigated. Numerical results are given to show the enhancement in APHCs achieved by choosing appropriate metallization conductivity and thickness, BCB thermal conductivity, and polyimide thickness etc. It is believed that the proposed approach is useful in the design of practical TFML circuits.
    IEEE Microwave and Wireless Components Letters 11/2005; · 1.78 Impact Factor
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    ABSTRACT: This correspondence presents an accurate and efficient method of moments (MoM) for analyzing electromagnetic scattering by a large finite array of waveguide slots with finite thickness. In this method, the mixed potential integral equation (MPIE) is utilized onto both upper and lower surfaces of the slots, and the subdomain MoM procedure is implemented to obtain the equivalent magnetic current distributions. The precorrected fast Fourier transform (P-FFT) method is employed to accelerate the entire computational process to reduce significantly the memory requirements for large arrays. During the MoM procedure, the Rao-Wilton-Glisson (RWG) functions are used as the basis and testing functions, with both z- and x-directional magnetic currents considered. This approach extends applicability of the present method to solve the MPIE for characterizing waveguide slots of arbitrary shape and distribution and the capability of the integral equation based fast algorithm. Numerical results are obtained and compared with the experimental results, to verify the accuracy and efficiency of this technique.
    IEEE Transactions on Antennas and Propagation 10/2005; · 2.33 Impact Factor
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    ABSTRACT: A V-shaped wire-loop antenna with a butterfly-like structure is proposed in this paper. The antenna consists of two identical half-loops inclined at an angle to each other and joined together at the feed point and the point at which they meet the ground plane. The performance of the structure was evaluated analytically for included angles 90°, 120° and 150° between the half-loops. At a frequency at which the lengths of the half-loops making up the antenna are equal to a wavelength, the radiation characteristics were superior to that of a conventional half-loop. The results were verified through measurements for the case where the included angle was 120°. The dominant radiation was in the direction perpendicular to the ground plane and the gain was better by 4.5 dB. The measured 3-dB beamwidth was also narrower, at 60°, compared to 135° in the case of the conventional half-loop.
    IEEE Transactions on Antennas and Propagation 07/2005; · 2.33 Impact Factor

Publication Stats

669 Citations
109 Downloads
85.54 Total Impact Points

Institutions

  • 1994–2007
    • National University of Singapore
      • Department of Electrical & Computer Engineering
      Singapore, Singapore
  • 2005
    • Shanghai Jiao Tong University
      Shanghai, Shanghai Shi, China
  • 2003
    • Northumbria University
      Newcastle-on-Tyne, England, United Kingdom
  • 2002
    • The University of Arizona
      • Department of Electrical and Computer Engineering
      Tucson, AZ, United States
  • 2001
    • University of Mississippi
      • Department of Electrical Engineering
      Oxford, MS, United States