Fernando J. S. Moreira

Generalized classical axially symmetric dual-reflector antennas

This work presents a generalized study of classical axially symmetric dual-reflector antennas. The antenna dishes are simply described by conic sections, arranged to reduce the main-reflector radiation toward the subreflector surface. The dual-reflector configuration provides a uniform-phase field distribution over the illuminated portion of the aperture, starting from a spherical-wave feed source at the antenna primary focus. All possible configurations are characterized into a total of four distinct groups. Simple closed-form design equations and the aperture field distribution are derived, in a unified way, for all these kinds of generalized antennas using the principles of geometrical optics. The formulation is applied in a parametric study to establish the configurations yielding maximum radiation efficiency (not including diffraction effects). The design procedure is exemplified in the synthesis of a novel configuration, which is further analyzed by the moment method.

Efficient Ray Tracing for Radio Channel Characterization of Urban Scenarios

This paper presents an efficient ray tracing algorithm for the use of the uniform theory of diffraction (UTD) in radio channel characterizations of urban environments. The algorithm is based on a 2-D image theory, which is properly modified to yield 3-D ray trajectories. Multiple reflections and diffractions are considered through the classification of multipath components into four different categories. The adoption of such ray classes eliminates redundant calculations throughout the ray-tracing process, improving the analysis of the electromagnetic propagation over the coverage area

A Meshless Local Petrov–Galerkin Method for Three-Dimensional Scalar Problems

In this paper, we apply a meshless method based on local boundary integral equations (LBIEs) to solve electromagnetic problems. The discretization process is carried out through the use of special basis functions that, unlike the Finite Element Method, are not confined to an element and do not require the support of an underlying mesh. The approach herein developed can be applied to general three-dimensional scalar boundary value problems arising in electromagnetism.

Classical axis-displaced dual-reflector antennas for omnidirectional coverage

The aim of this work is to discuss the synthesis and performance of classical dual-reflector antennas suited for an omnidirectional coverage. The reflector arrangements are axially symmetric with surfaces of revolution generated by axis-displaced conic sections, established from geometrical-optics (GO) standpoints to achieve omnidirectional radiation characteristics. Closed-form equations are derived for the design of all possible reflector configurations. The vector GO aperture field is also obtained, yielding an approximate analysis by the aperture method. Some pertinent geometrical characteristics and efficiency curves are then presented and discussed for several antenna configurations fed by transverse electromagnetic coaxial horns (for vertical polarization). A practical antenna design is conducted and analyzed by the method-of-moments technique, demonstrating the accuracy of the efficiency analysis yield by the aperture method for moderately large antenna apertures.

The Meshless Local Petrov–Galerkin Method in Two-Dimensional Electromagnetic Wave Analysis

This paper deals with one member of the class of meshless methods, namely the Meshless Local Petrov-Galerkin (MLPG) method, and explores its application to boundary-value problems arising in the analysis of two-dimensional electromagnetic wave propagation and scattering. This method shows some similitude with the widespread finite element method (FEM), like the discretization of weak forms and sparse global matrices. MLPG and FEM differ in what regards the construction of an unstructured mesh. In MLPG, there is no mesh, just a cloud of nodes without connection to each other spread throughout the domain. The suppression of the mesh is counterbalanced by the use of special shape functions, constructed numerically. This paper illustrates how to apply MLPG to wave scattering problems through a number of cases, in which the results are compared either to analytical solutions or to those provided by other numerical methods.

Shaping Axis-Symmetric Dual-Reflector Antennas by Combining Conic Sections

A simple procedure for the shaping of axis-symmetric dual-reflector antennas is described. The shaping procedure is based on the consecutive concatenation of local conic sections suited to provide, under geometrical optics (GO) principles, an aperture field with uniform phase, together with a prescribed amplitude distribution. The procedure has fast numerical convergence and is valid for any circularly symmetric dual-reflector configuration. To illustrate the procedure two representative configurations are investigated. The GO shaping results are validated using accurate method-of-moments analysis.

A self-checking predictor-corrector algorithm for efficient evaluation of reflector antenna radiation integrals

An efficient algorithm for numerically evaluating diffraction integrals is presented. The algorithm employs a predictor-corrector scheme combined with Ludwigs (1968) integration procedure. The predictor-corrector eliminates the amplitude and phase ambiguities present in the real+imaginary algebra used in machine calculations and provides accuracy self-checking capabilities. The end result is a reliable and efficient integration method that does not require independent integrand phase information, can handle arbitrarily shaped integration domains, and is capable of monitoring its own accuracy as the integration proceeds. The performance of the algorithm is investigated by computing, using the physical optics technique, the electromagnetic field scattered by representative reflector antenna geometries. These tests demonstrate that the proposed algorithm is particularly efficient in the analysis of multi-reflector systems. >

Novel Heuristic UTD Coefficients for the Characterization of Radio Channels

This work presents a novel heuristic coefficient for the uniform theory of diffraction, suited to characterize radio channels. The new coefficient combines features from two previously proposed heuristic coefficients. The diffraction by a conducting wedge is investigated in order to demonstrate the usefulness of the proposed coefficient

Generalized classical axially-symmetric dual-reflector antennas

Classical axially-symmetric Cassegrain and Gregorian reflectors are widely used in high-gain antenna applications. The main disadvantage of these configurations is the subreflector blockage, which causes a number of deleterious effects. However, this problem can be reduced by decreasing the main-reflector radiation toward the subreflector. This may be accomplished either by shaping both reflectors or by using alternative classical configurations. This work considers the second option by presenting, in an unified way, generalized classical axially-symmetric configurations that prevent, from a geometrical optics (GO) stand point, the main-reflector scattered energy from striking the subreflector surface. Starting from initial design variables, closed-form expressions are derived for the relevant surface parameters, as well as for the corresponding aperture field distributions. These expressions can be used as effective design tools to determine the final antenna geometry or even to establish an initial configuration for a shaping procedure.

A Local Boundary Integral Equation (LBIE) Method in 2D electromagnetic wave scattering, and a meshless discretization approach

In this work, we apply the Local Boundary Integral Equation (LBIE) Method to the scattering problem of a plane wave by a dielectric cylinder. This method together with a meshless discretization approach, known as meshless, provides very accurate results, when compared to analytical solutions. The method is an efficient tool in electromagnetic scattering analysis because among its principal characteristics, it is based on local interactions, thus leading to a sparse matrix.