Hsi-Tseng Chou

A novel acceleration algorithm for the computation of scattering from rough surfaces with the forward‐backward method

The forward-backward method has been shown to be an effective iterative technique for the computation of scattering from one-dimensional rough surfaces, often converging rapidly even for very large surface heights. However, previous studies with this method have computed interactions between widely separated points on the surface exactly, resulting in anO(N2) computational algorithm that becomes intractable for large rough surface sizes, as are required when low grazing incidence angles are approached. An acceleration algorithm for more rapidly computing interactions between widely separated points in the forward-backward method is proposed in this paper and results in an O(N) algorithm with increasing surface size. The approach is based on a spectral domain representation of source currents and the Greens function and is developed for both perfectly conducting and impedance boundary surfaces. The method is applied in a Monte Carlo study of low grazing incidence backscattering from very rough (up to 10 m/s wind speed) ocean-like surfaces at 14 GHz and is found to require only a small fraction of the CPU time required by other competing methods; such as the banded matrix iterative approach/canonical grid and fast multipole methods.

A Focused Planar Microstrip Array for 2.4 GHz RFID Readers

The specific problems encountered in the design of near-field focused planar microstrip arrays for RFID (Radio Frequency IDentification) readers are described. In particular, the paper analyzes the case of a prototype operating at 2.4 GHz, which has been designed and characterized. Improvements with respect to conventional far-field focused arrays (equal phase arrays) are discussed and quantified.

Novel Gaussian beam method for the rapid analysis of large reflector antennas

A relatively fast and simple method utilizing Gaussian beams (GBs) is developed which requires only a few seconds on a workstation to compute the near/far fields of electrically large reflector antennas when they are illuminated by a feed with a known radiation pattern. This GB technique is fast, because it completely avoids any numerical integration on the large reflector surface which is required in the conventional physical optics (PO) analysis of such antennas and which could take several hours on a workstation. Specifically, the known feed radiation field is represented by a set of relatively few, rotationally symmetric GBs that are launched radially out from the feed plane and with almost identical interbeam angular spacing. These GBs strike the reflector surface from where they are reflected, and also diffracted by the reflector edge; the expressions for the fields reflected and diffracted by the reflector illuminated with a general astigmatic incident GB from an arbitrary direction (but not close to grazing on the reflector) have been developed in Chou and Pathak (1997) and utilized in this work. Numerical results are presented to illustrate the versatility, accuracy, and efficiency of this GB method when it is used for analyzing general offset parabolic reflectors with a single feed or an array feed, as well as for analyzing nonparabolic reflectors such as those described by ellipsoidal and even general shaped surfaces.

Formulation of forward-backward method using novel spectral acceleration for the modeling of scattering from impedance rough surfaces

F-BM/NSA, H.T. Chou et al., Radio Sci., vol.33, p.1277-87, with computational complexity of O(N) is very efficient in method of moment (MoM) modeling of large-scale scattering problems from rough surfaces. The previous formulation for PEC surfaces is extended to treat impedance surfaces. Similarly, numerical experiment shows F-BM/NSA is far more efficient than the competitive BMIA/CAG in the order of magnitude.

A hybrid uniform geometrical theory of diffraction–moment method for efficient analysis of electromagnetic radiation/scattering from large finite planar arrays

A hybrid uniform geometrical theory of diffraction (UTD)-moment method (MOM) approach is introduced to provide an efficient analysis of the electromagnetic radiation/scattering from electrically large, finite, planar periodic arrays. This study is motivated by the fact that conventional numerical methods become rapidly inefficient and even intractable for the analysis of electrically large arrays containing many antenna or frequency-selective surface (FSS) elements. In the present hybrid UTD-MOM approach, the number of unknowns to be solved is drastically reduced as compared to that which is required in the conventional MOM approach. This substantial reduction in the MOM unknowns is essentially made possible by introducing relatively few, special ray-type (or UTD) basis functions to efficiently describe the unknown array currents. The utility of the present hybrid approach is demonstrated here for the simple case of a large rectangular phased array of short and thin metallic dipoles in air, which are excited with a uniform amplitude and linear phase distribution. Some numerical results are presented to illustrate the efficiency and accuracy of this hybrid method.

Uniform asymptotic solution for electromagnetic reflection and diffraction of an arbitrary Gaussian beam by a smooth surface with an edge

A closed form solution is obtained to describe, in a physically appealing manner, the reflection and diffraction of a general astigmatic Gaussian beam which is incident on an arbitrary smooth, electrically large, slowly varying curved, perfectly conducting screen (or reflector). This closed form solution is obtained via an asymptotic evaluation of the radiation integral for the fields scattered from the reflector, to within the physical optics approximation that remains valid for the present situation. The analysis developed here is particularly well suited for the fast analysis of electrically large reflector antennas by representing the feed illumination by a relatively small set of Gaussian beams launched from the feed plane. Each of these Gaussian beams after being launched undergoes reflection and diffraction at the reflector; the expressions for the reflected and diffracted fields are developed in this paper and utilized by Chou [1996] to compute the radiation pattern of large reflector antennas in a matter of a few seconds as compared to the conventional numerical physical optics integral method which takes hours on the same computer.

Design of a Near-Field Focused Reflectarray Antenna for 2.4 GHz RFID Reader Applications

The design of a reflectarray antenna is presented when the radiated field is focused in the near-zone of the array aperture. In particular, the reflectarray antenna is implemented for RFID reader applications at 2.4 GHz. Numerical investigations on the radiation characteristics of this reflectarray, as well as an experimental validation, are presented to demonstrate its feasibility.

On the Poisson sum formula for the analysis of wave radiation and scattering from large finite arrays

Poisson sum formulas have been previously presented and utilized in the literature for converting a finite element-by-element array field summation into an alternative representation that exhibits improved convergence properties with a view toward more efficiently analyzing wave radiation/scattering from electrically large finite periodic arrays. However, different authors appear to use two different versions of the Poisson sum formula; one of these explicitly shows the end-point discontinuity effects due to array truncation, whereas the other contains such effects only implicitly. It is shown, via the sifting property of the Dirac delta function, that first of all, these two versions of the Poisson sum formula are equivalent. Second, the version containing implicit end point contributions has often been applied in an incomplete fashion in the literature to solve finite-array problems; it is also demonstrated that the latter can lead to some errors in finite-array field computations.

A novel acceleration algorithm for the computation of scattering from two-dimensional large-scale perfectly conducting random rough surfaces with the forward-backward method

The forward-backward method with a novel spectral acceleration algorithm (FB/NSA) has been shown to be an extremely efficient iterative method of moments (MoM) for the computation of scattering from one-dimensional (1D) perfect electric conducting (PEC) and impedance rough surfaces. The NSA algorithm is employed to rapidly compute interactions between widely separated points in the conventional FB method and is based on a spectral domain representation of source currents and the associated Greens function. For fixed surface roughness statistics, the computational cost and memory storage of the FB/NSA method are /spl Oscr/(N/sub tot/) as the surface size increases, where N/sub tot/ is the total number of unknowns to be solved. This makes studies of scattering from large surfaces, required in low grazing-angle scattering problems, tractable. In this paper, the FB/NSA method is extended to analyze scattering from two-dimensional (2D) rough surfaces. The NSA algorithm for this case involves a double spectral integral representation of source currents and the 3D free-space scalar Greens function. The coupling between two spectral variables makes the problem more challenging, and the efficiency improvements obtained for 2D surfaces are appreciable but not as dramatic as those for 1D surfaces. However, the computational efficiency of the FB/NSA method for 2D rough surfaces remains /spl Oscr/(N/sub tot/) as one of the surface dimensions increases. Comparisons of numerical results between the conventional FB method and the FB/NSA method for large-scale PEC rough surfaces show that the latter yields identical results to the former with a reduction of CPU time and only a slight increase in memory storage.

A Time Domain Formulation of the Uniform Geometrical Theory of Diffraction for Scattering From a Smooth Convex Surface

A time-domain version of the uniform geometrical theory of diffraction (TD-UTD) is developed to describe, in closed form, the transient electromagnetic scattering from a smooth convex surface excited by a general time impulsive astigmatic ray field. This TD-UTD impulse response is obtained by employing an analytic time transform (ATT) for the inversion in time of an available and accurate corresponding frequency domain UTD (FD-UTD) solution. The ATT is employed because it overcomes the difficulties that occur when inverting FD-UTD fields associated with rays that traverse line or smooth caustics. Furthermore, the TD-UTD response to a general pulsed astigmatic wave excitation may be found by a convolution of the general excitation with the TD-UTD impulse response which can be performed in closed form. Some numerical examples illustrating the utility of this TD-UTD are presented.