S. Maci

Dispersion properties of periodic grounded structures via equivalent network synthesis

The increasing interest on metamaterials and frequency band-gap structures has been motivated by the large number of recently discovered engineering applications in the field of microwaves and antennas. A method is presented for the efficient dispersion analysis (associated to both surface-waves and leaky-waves) for printed periodic FSS-type surfaces. The method has been tested for a gangbuster FSS backed by a ground plane. Preprocessing is performed on the broadband reflection-coefficient data obtained from a full-wave analysis. After that, the FSS is characterized via its resonances for a few values of the incident angle. This determines, in a straightforward way, poles and zeros of an equivalent impedance, simply synthesized by an L-C dispersive circuit. Due to the weak dispersivity of poles and zeros parameters, the L-C circuit can be identified in a simple analytical form which, after analytical continuation, is applied to formulate the transverse resonance equation. The resulting process is much simpler and faster than a full-wave solution for dispersion and provides an accurate initial guess for the iterative identification of the leaky- and surface-wave solutions.

Dual band circularly polarized patch antenna

The need for frequency reuse has stimulated interest in multi-frequency antennas. Although multi-frequency operations can be obtained by using wide band antennas and suitable electronic circuits, this solution has several drawbacks in terms of efficiency and noise performances. Most of all, wide bandwidths are difficult to be achieved with planar structures. An inherently multi-resonant planar antenna allows to benefit from the well known advantages in size/weight/cost reduction while retaining rather good noise temperature and efficiency. Typically, a dual frequency operating mode is attained by using multi-layer stacked patches. Single layer structures for linear polarization have been proposed in Maci et al. (1987) and Yazidi et al. (1993). Both structures are based on a rectangular patch loaded by two slots. As demonstrated in Maci et al., when these slots are etched close to the radiating edges, they do not change significantly the first resonant frequency and the radiation pattern of the patch. Furthermore they introduce another resonance with a radiation pattern similar to the former. This latter resonance is strongly dominated by the slot lengths. In the present paper the use of slot loaded patches is extended to circular polarization (CP). This is obtained by properly shaping the patch and by introducing two other loading slots. Experiments are shown and discussed.<<ETX>>

Double diffraction coefficients for source and observation at finite distance for a pair of wedges

A closed form, high-frequency solution is presented for describing the double diffraction mechanism at a pair of parallel wedges, when they are illuminated by a spherical wave. The solution is obtained by using a spherical waves spectral representation of the first order diffracted field from each wedge. For the sake of simplicity, only the scalar case is considered when either hard or soft boundary conditions may be imposed on the faces of the two wedges. This provides a basic step for constructing the solution in the more general electromagnetic case. Although the procedure is applicable to any couple of parallel wedges, the case of two wedges sharing a common face is explicitly considered in the solution presented. The asymptotic evaluation of the double spectral integral leads to transition functions involving generalized Fresnel integrals. Numerical calculations show that the high frequency formulation fails so gracefully that it gradually blends into the solution of a single wedge when the distance between the two edges vanishes. This is a desirable property for analysing the shadowing effects of a thick screen.

Hybrid spatial-spectral analysis of periodic structures

For the MoM analysis of complex printed periodic structures, the use of sub-domain basis functions is essential. In this work, triangular mesh and Rao-Wilton-Glisson (RWG) functions are assumed. The aim of this work is to propose a hybridization of the standard spectral- and spatial-domain MoM approaches, through a convenient manipulation of the periodic Greens function (PGF)

Dispersion analysis of printed periodic structures by using a pole-zero network synthesis

A new method is presented, for the efficient derivation of the dispersion equation associated to periodic planar structures. In order to illustrate the basic concepts, the method is presented here for the case of a periodic dipole structure above a ground plane, but it is potential applicable to any kind of any kind of frequency selective surfaces (FSS) printed on or embedded in stratified dielectric media. This method appears powerful for determining bandgaps in artificial magnetic surfaces composed by FSS backed by ground planes, as well as for the dispersion analysis of leaky wave structures. The method is based on a pole-zero large bandwidth synthesis of the periodic structure, obtained starting from the reflection coefficient response of the FSS on a large frequency band. This synthesis provide an analytical expression of the equivalent network associated to the printed surface, and allows a simple determination of the dispersion equation. Preliminary results confirm the accuracy of the process and its efficiency with respect to conventional methodology.

Full-wave analysis of a large rectangular array of slots

Large, finite arrays are often studied in hypothesis of infinite structure, thus allowing the reduction of the numerical effort to that of a single periodic cell. Sometimes this approximation leads to reasonable results in predicting the input impedance of elements far out from the edges. However, for near edge elements it is visibly wrong. Furthermore, when wide beam angle scanning occurs, the effects of truncation can be relevant also for elements very far from the edges. A truncated Floquet waves (TFW) method has been presented, for predicting the distributions of the radiating currents, including those belonging to the edge elements of the array, while retaining a number of unknowns which is comparable with that occurring for the infinite array problem. This approach is based on the method of moments (MoM) solution of an integral equation in which the unknown function can be interpreted as due to the edge diffracted field excited by the Floquet waves of the infinite structure. In this paper, the formulation of the TFW method is applied to the 3D case of rectangular array of slots in an infinite ground-plane.

Generalization to complex source excitation of the incremental theory of diffraction

This paper analyzes this complex source point (CSP) extension of discrete and incremental ray techniques by comparing the uniform theory of diffraction (UTD) and incremental theory of diffraction (ITD) to some reference cases

Double diffraction at a pair of coplanar skew wedges

The present work is a generalization of that dealing with a pair of knife-edges. In particular, a high-frequency solution is obtained for the scattering in the near zone of a pair of coplanar skew wedges with arbitrary exterior angle, illuminated by a source at finite distance. The solution is achieved by using a spherical wave spectral representation of the incident, singly diffracted field by the first wedge. The electromagnetic solution is constructed from the relevant scalar formulation by choosing appropriate ray-fixed reference systems. The final dyadic closed form expression is cast in the uniform theory of diffraction (UTD) framework, and includes terms up to the second order. These latter become of the same order as the leading terms in overlapping transition regions of the two wedges. This solution is also directly applicable when the two wedges share a common face for both polarizations; in this case, the second order terms provide an accurate description of those contributions that are usually neglected for soft polarization.

Edge fringe approach for the full-wave solution of large finite arrays

The electromagnetic modeling of large finite arrays has been the object of a number of investigations. A method of moment (MoM) formulation is suggested, for predicting the distributions of the radiating currents (including those belonging to the edge elements of the array) but retaining a number of unknowns which is comparable with those occurring in the infinite array approach. This formulation is based on an integral equation in which the unknown function is the difference between the exact current distribution on the truncated array and the current distribution pertinent to an infinite array. This unknown function can be associated to the field diffracted at the edge of the array by the Floquet modes of the infinite array. Following this physical interpretation the unknown of the integral equation are efficiently represented by a few entire domain basis functions which are properly shaped.

Pattern distortion for corrugated horns open-ended on a finite ground plane

A method is proposed for predicting the radiation pattern of rectangular apertures on a finite rectangular ground plane. This method includes an accurate description of single, double and vertex diffraction mechanisms. The first step of the analysis is the full-wave estimation of the aperture field. To this end, the external and the internal regions are separated by replacing each aperture with a metallic plug with equivalent magnetic currents on its external and internal sides; these currents are of equal amplitude and of opposite sign to ensure the continuity of the electric tangential field through the aperture. Then, an integral equation is formulated, that represents the continuity of the tangential H-field through the apertures. To solve the internal-external coupling, the external flange is initially treated as an infinite ground plane. After the determination of the magnetic currents, these currents are considered as radiating on the finite ground plane. For the sake of simplicity, but without loss of generality, a single horn is considered. The internal region of the corrugated horn is analyzed by using a generalized admittance matrix method.