Scattering From a Circular Waveguide Coated With Lossy Material (Radar Cross-Section, Jet Intake, Gtd, Magnetic, Equivalent Current)

127 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1986.Scattering from a circular waveguide coated with a lossy material and terminated by a plate of perfect electric conductor (PEC) is theoretically analyzed and compared with available experimental data. Particular attention is given to the reduction scheme of the radar cross section (RCS) from such a structure.First, the normal modes in an overmoded waveguide coated with a lossy material are analyzed, particularly for their attenuation properties as a function of material parameters, layer thickness, and frequency. When the coating material is not too lossy, the low-order modes are highy attenuated even with a thin layer of coating. This coated guide serves as a mode suppressor of the low-order modes, which can be particularly useful for reducing the radar cross section (RCS) of a cavity structure such as a jet engine inlet. When the coating material is very lossy, low-order modes fall into two distinct groups: highly and lowly attenuated modes. However, as a/(lamda) (a = radius of the cylinder; (lamda) = the free-space wavelength) increases, the separation between these two groups becomes less distinctive. The attenuation constants of most of the low-order modes become small, and decrease as a function of (lamda)/a.Second, the RCS of an open-ended, semi-infinite circular waveguide is calculated by GTD and the equivalent-current method. We include both single- and double-diffraction terms. It is found that the double diffraction-term is stronger than the single-diffraction term for the horizontal polarization at wide-angle incidence. Its inclusion is necessary in order to check with experimental data and the asymptotic expansion of the exact Wiener-Hopf solution.Third, the RCS from a circular waveguide terminated by a perfect electric conductor is evaluated and it is found that the RCS is reduced significantly by coating the interior waveguide wall. At low frequency (a/(lamda) = 1), the significant RCS reduction with a thin layer of coating is theoretically predicted and experimentally observed over a broad range of incidence angle if the coating material is magnetic and lossy. However at high frequency (a/(lamda) > 2.5), the thin layer of coating is usually effective only near axial incidence. A thicker layer of coating is required for the RCS reduction for a wider incidence angle.U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD