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E - Bibliothèque et Archives Canada

E - Bibliothèque et Archives Canada

The analysis of

The analysis of scattering problem using the uniform aieory of diffraction (UTD) is used usually for abjects with dimensions of many wavdengths or Iarger. This stems from the fact that UTD solutions are generated by high frequemy asyrnptotic techniques. The pu- of this work is to evaluate the behavior of UTD at low frequency. The UT0 Edge diffraction solution is one of the most used tods [l] to solve problerns dealing with modeling of antennas on cornplex platfoms. Proving the accuracy of UTD at low frequencies muid be very advantageous. A typical application would be to predict the radiation pattern and henœ the performanœ of an antenna mounted on the fuselage of an airplane Men the frequency used is low or the airplane strudure h elecbically srnail. This will niinimize the usage of rneasurernents requiring expensive scale rnodeling to design and locate antennas. To achieve this evaluation. a wbfl proven independent methoci is needed to be established and used for cornparison wïth UTD. It is of great importance also to study rnany scattering cases to be able to establish the major reasons that cause UTD to feil. At a first glance, going low in frequency by itself t supposed to be a sufficient cause for UTD solutions to degrade. It will be shown however that UTD solutions for some electncally small scatterers are adequate and major parameters affecong the sucœss of UT0 are the scattering problem gemwûy, the incidence angle and the field polarization.

60th TEz and TMz plane wave illumination with diflsrent angles of incidence are considerd here. Different scatterer shapes also are used. Thase shapes are of simple geomtry and they indude the infinite eâge, the strip and the redangular cylinder. The choiœ of aiese shapes is not arbitrary sinœ they are considered building blocks for mon, cornplex geometries and they can be considered as a series of testo. asœnding in diniculty, for UT0 to pass. The edge is a canonical problem. The strip is a combination of two back to back half planes where difiradion is the rnogt severe and multiple dinraction is existent but for which UT0 is an exact solution to first order diffraction (special case). The rectangular cylinder is the combination of four 90° edges for which diffraction is in a sense halfway betwwn aie highest diffraction (ha# plane ) and no diffraction at al1 (full plane) but for which UT0 is not an exact solution and where multiple diffraction is present. Radiation pattems are usually the results thought of in a scattering problem. Sinœ a low frequency is useâ however, the radiation patterns for such small electrical scatterers hold few surprises. The surface current density distributions are the most sensitive variables to changes on an electrically small scatterer. In the next chapters, the surlace current densities on edges, sûips and rectangular cylindeis for different illumination incidenœ angles and polarization are plotted and a condusion is made regarding the results obtained. Through out this work the da' tim variation is unâerstood and suppressed.

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