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INTRODUCTION
Horn antennas have been extensively investigated for a long time: due to their radiating properties they are used both as direct microwave radiators as well as feeders for large reflectors. They are also high gain elements in phased arrays and standard gain devices for the calibration of other antennas. Corrugated and stepped rectangular horns may provides identical principal plane patterns, low sidelobe levels and high polarization purity. If employed as feeders, they are able to illuminate a reflector antennas with low loss of energy and an high efficiency. Moreover array of horn antennas are used as feeder in spacecraft antennas to generate either multiple spot beams or contoured beams. One of most critical issues in the design of phased array matching is the active impedance of the radiating element. A good design is achieved if the mismatch and the variations of the impedance with respect to the frequency and scan angle is minimized. This is generally accomplished by properly selecting the element spacing, lattice structure, dimensions of the element.
Moreover, in array analysis the coupling effects prediction is a challenging task generating pattern degradation and increases the overall cross-polarization of the array feed.
The development of accurate and efficient electromagnetic analysis methods for array antennas is of increasing interest to the electromagnetic designer. Accurate and, at the same time, efficient numerical simulation tools are a key component of the design and fabrication chain of this type of antenna. The capability of electromagnetic simulations technique in characterizing the scattering behavior of the overall radiating structure represents a challenging task for both large as well as complex geometries allowing a fine and efficient tuning of all the design parameters.
In this work, novel hybrid numerical techniques for the full wave analysis of single element and array of aperture antenna will be proposed. All the approaches are based on the joint application of different numerical techniques dedicated to the analysis of the different electromagnetic portions of the antenna. All the proposed solutions provide an accurate and efficient simulation tool which allows the analysis of a large class of waveguide aperture antennas.
In Chapter 1 the features and benefits of an hybrid approach will be motivated. In Chapter 2 an hybrid full wave mode matching (MM) / finite element method (FEM) for the
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computation of the generalized scattering matrix (GSM) of the inner waveguide portion of the antenna will be proposed. In Chapter 3 the previous mentioned technique will be hybridized with the method of moments (MoM) to compute the scattering of the discontinuity between the inner waveguide section and the outer aperture free space problem. In Chapter 4 the MoM formulation will be extended to the analysis of arrays. In Chapter 5 the characteristic basis functions method (CBFM) will be applied in conjunction with a fast matrix generation (FMG) technique to overcome the limitations of the MoM analysis dealing with large arrays. In Chapter 6 a spectral decomposition (SD) approach will be discussed for the scattering characterization of the outer free space coupling problem when large periodic arrays are accounted. All the proposed solution will be validated in comparison with other simulation tools or reference in the open literature to prove their effectiveness.
