In this paper, we present a novel frequency-selective surface (FSS) design aimed at enhancing the performance of broad-band reconfigurable antenna apertures. In particular, reconfigurable printed dipole arrays are examined in the presence of a multilayer FSS. Of particular interest is the design of FSS structures whose reflection coefficient has prespecified phase response over a broad set of frequencies. Previous FSSs primarily considered designs on the basis of the reflection-coefficient amplitude and were intended for radome applications rather than substrates. Designing FSSs subject to phase requirements will be seen to require some compromise in the magnitude. Broad-band requirements also present us with a need for noncommensurate FSS designs.

An extrapolation technique is adapted to predict the monostatic radar cross-section (RCS) pattern from a few pattern value calculations. This approach eliminates a need to resolve the system when an iterative solver is employed. A three-dimensional application is considered to demonstrate the accuracy of the technique.Index Terms-Angular extrapolation, electromagnetic scattering, Padè approximation. I. FORMULATIONThe asymptotic waveform evaluation (AWE) method provides a reduced-order model and has already been successfully used in various electromagnetic and circuit applications [1]- [6]. These applications dealt with frequency extrapolation implementations. In this letter, we present a new implementation of AWE for rapid monostatic scattering pattern fill calculations when an iterative solver is employed.On the basis of AWE, a Taylor series expansion is generated about a specific value of the system parameter (frequency, angle, etc.). The Taylor coefficients or moments are then used to extract poles of the system yielding a rational function (Padè approximation) of the system parameter. Padè representations have a larger circle of convergence and can therefore provide a broader extrapolation when compared to Taylor series representations. Below we describe the implementation of AWE in connection with method of moment (MoM) systems for computing monostatic patterns using only a few angular points.A usual form of the MoM system iswhere V m (k; ; ) = T m 1 E inc (k; ; ) ds (1b) and I n refer to the summation coefficients of the current density expansion weighted by the subsectional basis function T n (r). Among the other parameters, Zmn(k) represents the weighted integral used for generating the matrix, fV m (k; ; )g is the excitation vector, and E inc (k; ; ) denotes the external incident field excitation. Also, k is the free-space wave number and (; ) is the incidence angle. For monostatic pattern evaluations using a direct solver, an order of O(N 3 ) computational complexity is required to factorize the dense matrix [Zmn]. Additional O(N 2 ) computations are then needed for each right-hand side term. For iterative solvers, the entire solution must be repeated for each excitation. The proposed pattern fill procedure eliminates repetition of the iterative solutions altogether Manuscript received . Publisher Item Identifier S 0018-926X(98)08910-8. except for a few pattern points used as the expansion points of the extrapolation.To show how AWE can be applied, we assume that fI n (; )g has already been computed at a given direction ( 0 ; 0 ). A Taylor series expansion on (with constant) is then given by fI n ()g = fI n ( 0 )g + 1 q=1 M qwhere V (q) m (0) is the qth derivative of Vm() with respect to evaluated at 0 and we have dropped implied dependencies on and k. For plane wave excitations, the moments M q n can be trivially calculated and one could therefore increase the order of the expansion as needed to extend the validity of the approximation. This is better achieved by casting (2) into a Padè rational functi...

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Hybrid finite element (FE) -boundary integral (BI) analysis of infinite periodic arrays is extended to include planar multilayered Green's functions. In this manner, a portion of the volumetric dielectric region ci~n be modeled via the finite element method whereas uniform multilayered regions can be modeled using a multilayered Green's function. As such, thick uniform substrates can be modeled without loss of efficiency and accuracy.The multilayered Green's function is analytically computed in the spectral domain and the resulting 3 1 matrix-vector products are evaluated via the fast spectral domain algorithm (FSDA). As a result, the computational cost of the matrix-vector products is kept at