Incremental Diffraction Coefficients for Planar Surfaces. Part 3. Pattern Effects of Narrow Cracks in the Surface of a Paraboloid Antenna
Interim rept. Oct 1987-Mar 1988
ROME AIR DEVELOPMENT CENTER GRIFFISS AFB NY
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In this report, incremental diffraction coefficients for a narrow slit in an infinite, perfectly conducting plane are used to investigate the effects of cracks in the surface of a focal-fed circular paraboloidal reflector antenna. Such cracks can result from the imperfect fitting together of panels to form a large reflector. The imperfections in fitting together are modeled by narrow slits in a paraboloidal surface. The far fields scattered by the cracks are computed by integrating the slit incremental diffraction coefficients multiplied by the illuminating field along the cracks. Two forms of cracks are modeled azimuthal cracks, the projections of which on the aperture plane are concentric circles, and radial cracks, the projections of which on the aperture plane are radii of the projection of the paraboloid. The feed is assumed to be a Huygens source. It is found that narrow azimuthal cracks hardly change the H-plane pattern, moderately change the further out sidelobes of the E-plane pattern, and moderately change the entire cross-polarization pattern. Depending on their orientation the radial cracks can strongly change the E-plane and cross-polarization patterns. However, like the azimuthal cracks, the radial cracks hardly change the H-plane pattern.
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