Transmission line matrix modeling of disperse wide-band absorbingboundaries with time-domain diakoptics for S-parameterextraction

A numerical modeling procedure based on Johns time-domain diakoptics approach with space interpolation techniques for efficient transmission-line matrix (TLM) analysis of two-dimensional microwave circuits is discussed. Frequency-dispersive boundaries are represented in the time domain by their characteristic impulse response or numerical Green's function (Johns matrix). Almost perfect wide-band absorbing boundary conditions have been obtained with this technique, permitting accurate characterization of waveguide discontinuities and compounds. The application of these techniques saves considerable computer run time and memory when compared with conventional TLM analysis     

Modeling of general constitutive relationships in SCN TLM

The modeling of general constitutive relationships in SCN (symmetrical condensed node) TLM is presented. The technique consists of decoupling the impulse scattering at the nodes from equations describing the medium by using equivalent node sources with state-variable formulation of the constitutive relationships. The procedure requires few modifications of TLM. Numerical examples are presented     

Full-wave analysis of guiding structures using a 2-D array of 3-DTLM nodes

A TLM approach to the full-wave analysis of guided wave structures is introduced. Instead of real pulses as in the conventional TLM method, complex pulses are used. Therefore a nonreciprocal phase shift in the z-direction can be introduced and used to connect the z arms in a 3-D node directly. As a result, the 3-D array of 3-D nodes, normally required in the TLM method to calculate the propagation and attenuation constant, is reduced to only one mesh unit in the z direction (a 2-D array of 3-D nodes). The propagation constant is determined by choosing a value and then calculating the frequency at which this value is valid from the Fourier transform of the impulse response. Losses are found by computing the exponential decay of time harmonic solutions at the eigenfrequencies of the structure     

A new procedure for interfacing the transmission line matrix (TLM)method with frequency-domain solutions

The authors present a new procedure that interfaces the transmission-line matrix (TLM) method with frequency-domain solutions of electromagnetic fields. Frequency-domain solutions are transformed into appropriate time-domain sequences using the discrete Fourier transform (DFT). Hence, the corresponding boundary Johns matrix can be determined with minimum computational effort. The subsequent treatment consists in convolving the streams of TLM impulses incident on the boundary with a Johns matrix generated with the new approach. The method is used to obtain the time-domain reflection sequence of wideband absorbing terminations in a rectangular waveguide in the dominant mode operation. In addition, the time-domain analysis of pulse penetration through a sheet with high, but finite, conductivity is presented. Good results demonstrate the efficiency of the proposed procedure     

Scattering of EM waves from a slightly rough surface of a generallyanisotropic plane-layered halfspace

The problem of plane wave scattering by a slightly rough boundary of an arbitrarily layered medium with general anisotropy of electric and magnetic properties is considered using boundary conditions transference method and a small perturbation approximation combined with the Green's functions technique. It is shown that in the Born approximation, the second statistical moments of the fluctuation field scattered from an inhomogeneous anisotropic halfspace can be expressed exclusively in terms of the external parameters referring to a flat boundary and the spectral density of roughness. In the present circumstance, the set of the external parameters includes the four plane-wave reflection coefficients from a flat boundary and the (limiting) values of constitutive parameters of the adjacent media at said boundary, Within the framework of the outlined approach, the covariance dyads, Poynting's vector, scattering coefficients and Stokes parameters of the fluctuation field, and the Mueller matrix for a rough boundary are calculated     

Coupling phenomena in concentric multi-applicator phased arrayhyperthermia systems

The coupling between the waveguide applicators of a four-element phased array hyperthermia system irradiating a three-layered cylindrical tissue model of circular cross section is analyzed theoretically. The fields inside the tissue layers are expressed in terms of cylindrical vector wave functions satisfying the corresponding wave equations, while the fields inside each waveguide are expanded in terms of guided and evanescent normal modes. Then, by implementing the appropriate boundary conditions, a system of four coupled integral equations is derived in terms of the unknown electric field distributions on the open waveguide apertures. This system is solved by expanding the unknown electric field on each aperture into waveguide normal modes and by applying a Galerkin's procedure. The self reflection coefficient and the mutual coupling coefficients are then determined and numerical results for a four-element phased array hyperthermia system are computed and presented for different waveguide applicator sizes and settings     

Scattering and radiation from a slitted parallel-plate withrectangular grooves: TE-wave

TE-wave scattering and radiation from a slitted parallel-plate waveguide with rectangular grooves is considered. The Fourier transform and the mode matching are used to represent the scattered field in terms of the continuous and discrete modes. The simultaneous equations for the discrete modal coefficients are obtained by matching the boundary conditions. The fast-converging series solutions are presented to evaluate the far-zone radiation, reflection, and transmission coefficients. The numerical computations illustrate the angular behavior of far-zone radiation in terms of the slit size, groove size, and operating frequency. The antenna radiation pattern of the slitted parallel-plate is measured and compared with theory     

Second-Order Absorbing Boundary Conditions for TLM Analysis of Open Region Scattering Problems

A set of second-order absorbing boundary conditions is derived for the three-dimensional finite-difference TLM analysis of open region scattering problems. These boundary conditions are implemented into a computer code using a two-level discretization scheme. A simulation experiment using a 4-term Blackman-Harris pulse excitation with very low sidelobes is used to examine the efficiency of the boundary conditions. It is observed that the absorbing boundaries are perfectly transparent to waves impinging upon them at angles between 0 and 45 degrees, and the normal incidence reflection coefficient is less than -35 dB.     

TLM coefficients with distributed losses

In TLM modelling of lossy systems, instead of concentrating losses in lumped resistors or stubs, an attempt is made to model the losses as distributed, in the `connect' or `propagate' stage, while leaving the scattering lossless. TLM transmission and reflection coefficients are derived using a novel, distributed, quasi-black box approach. The coefficients agree with those from lumped resistor analysis. The implications are discussed in the context of lossy TLM modelling in general     

Absorbing and connecting boundary conditions for the TLM method

Absorbing and connecting boundary conditions are implemented for the transmission-line matrix (TLM) method. The approach is based on an equivalence previously established between the finite-difference-time-domain (FD-TD) method and the TLM method. Boundary conditions presently used for the FD-TD algorithm can be transformed into conditions that can be interfaced with two-and three-dimensional (2D and 3D) TLM schemes. Additional conditions are introduced for 3D-TLM symmetrical condensed node simulations to suppress instabilities caused by spurious modes, inherent to the model, and which are amplified by absorbing boundaries. Numerical results and the comparison with other methods show the good performance of the proposed algorithms     

Maximum likelihood estimation with side information of a 1-Ddiscrete layered medium from its noisy impulse reflection response

We consider the problem of computing the maximum likelihood estimates of the reflection coefficients of a discrete 1-D layered medium from noisy observations of its impulse reflection response. We have side information in that a known subset of the reflection coefficients are known to be zero; this knowledge could come from either a priori knowledge of a homogeneous subregion inside the scattering medium or from a thresholding operation in which noisy reconstructed reflection coefficients with absolute values below a threshold are known to be zero. Our procedure converges in one or two iterations, each of which requires only setting up and solving a small system of linear equations and running the Levinson algorithm. Numerical examples are provided that demonstrate not only the operation of the algorithm but also that the side information improves the reconstruction of unconstrained reflection coefficients as well as constrained ones due to the nonlinearity of the inverse scattering problem     

Research on the Radiation Characteristics of Patched Leaky Coaxial Cable by FDTD Method and Mode Expansion Method

Patched leaky coaxial cable (PLCX) is proposed as an alternative to the conventional leaky cable for wireless links in a complex environment. It is expected to have the capability of adjusting the coupling between the cable and the environment and give smoother electric field coverage. In this paper, the radiation characteristics of the PLCX with general inclined patches are studied by a hybrid method that involves the finite-difference time-domain (FDTD) method for the near-field computation and the mode expansion method for the transformation of near field to far field. In the method, the space around the patched leaky cable is divided into two regions by an artificial closed cylindrical surface that is incorporated with the FDTD lattice surface when implementing the FDTD iteration. The field distribution on the artificial surface is obtained after the implementation of the FDTD method. Meanwhile, the field outside the artificial boundary is expanded in terms of the Floquet modes with coefficients to be determined. By matching the field expressed by modes and the field obtained from the FDTD method at the artificial boundary, a matrix equation with unknown coefficients is obtained. Solving this matrix equation, the expansion coefficients are known, and the field outside the artificial boundary is ready to be obtained.     

Reconstruction of conductivity profiles from pulse response

Two methods for reconstructing the conductivity profiles of layered media from reflection data in time domain are presented. First, we derive simple recurrence formulas, starting with the reflected field at the boundary. The conductivity profile can then be reconstructed step by step. Secondly, we find that the impulse response relates approximately but directly to the conductivity profile. Examples of the two techniques are shown by both smooth profiles and ones with discontinuities.     

Mutual Coupling Between Parallel-Plate Waveguides

The radiation field and mutual coupling between two identical parallel-plate waveguides having the same axis of symmetry are investigated. Jones' method of formulation is applied and a modified Wiener-Hopf equation is obtained. Expressions for the radiated field in free space, reflected field in the exciting waveguide, and transmitted field in the coupled waveguide are obtained and the reflected and transmitted fields are expressed in terms of waveguide modes. The reflection coefficient for each mode is represented by three terms, two of which are due to reflections at the open end of the exciting waveguide and are constant along the waveguide. The third term is the contribution from the field scattered by the open end of the coupled waveguide and decays along the waveguide according to the radiation condition. Similarly, the transmission coefficient of each mode is represented by three terms, two of which decay along the coupled waveguide and the third one is constant. The radiation field is also divided into three terms. One of them is due to the radiation from the open end of the exciting waveguide and the other two are the contribution of multiple interactions between the two waveguides. Computed results for the reflection and transmission coefficients and the radiation field are shown for TE/sub 0,1/ excitation and various separation distance of the waveguides. The results for the reflection and transmission coefficients are oscillating functions of period /spl pi/, and approach gradually the well-known final values of a single excited wavegnide.     

Absorbing boundary conditions for the TLM method

The numerical behavior of different absorbing boundary conditions when applied to the transmission-line modeling (TLM) method is presented. These conditions may be classified into three different groups according to the way they are derived. The first group is obtained by discretizing one-way analytical conditions derived for the analytical wave equation. The second group is a set of discrete conditions directly obtained for the discrete wave equation. The last group is based on appropriate reflection coefficients derived purely from transmission-line theory. Because of their different behaviors, the numerical study is explicitly carried out for both two- and three-dimensional free-space scattering problems     

Reflection and transmission of electromagnetic waves normally incident on a plasma slab moving uniformly along a magnetostatic field

Power reflection and transmission coefficients are found for linearly and circularly polarized plane electromagnetic waves, normally incident on a plasma slab, moving uniformly along a magnetostatic field, normal to the slab boundaries. The solution is found by applying the boundary conditions in the rest frame, and then using relativistic transformations for the fields and the plasma parameters to find the reflection and transmission coefficients observed in the laboratory frame. The results for the circularly polarized incident waves are found in closed form. Numerical results are presented for linearly polarized incident waves. It is found that with an increase in the magnetostatic field, the absolute maximum of the reflection coefficient increases at different velocities. An increase in the magnetostatic field makes the slab more transparent at velocities for which the transmission coefficient with no magnetostatic field is very small. A dielectric-like behavior is observed for large magnetostatic fields. The sum of the power reflection and power transmission coefficients is found to be no longer equal to unity for velocity different from zero.     

不同边界条件下生物组织吸收系数的确定

时间分辨反射的 Monte Carlo法计算的数据作为模拟实验数据 ,运用 R.K.Wang提出的两点测量法 ,在不同的边界条件下 ,对半无限生物组织的吸收系数进行模拟测量。结果表明选择外延边界条件(EBC、EBCF)、部分流边界条件 (PCBC)的吸收系数测定较为准确。同时 ,讨论了有效时间和测量位置的选择对测量结果的影响     

The frequency-domain transmission line matrix method-a new concept

A frequency-domain transmission line matrix (TLM) method for the frequency-selective S-matrix computation of 3D waveguide discontinuities is presented. It combines the flexibility of the conventional TLM method with the computational efficiency of frequency-domain methods. The basis for this technique is the excitation of an impulse train of sinusoidally modulated magnitude, which retains the form of an impulse while its envelope contains the information of the structure at the modulation frequency. Utilizing the diakoptics technique in conjunction with the concept of the intrinsic scattering matrix, the original electromagnetic field problem is simplified to a matrix algebra problem. A variety of structures have been analyzed in order to check the accuracy of the approach, and excellent agreement has been observed in all cases. S-parameters for CPW air-bridges including finite thickness and conductivity of the metallizations are computed. The effect of superconducting air-bridges is analyzed