APPLICATION OF INCREMENTAL LENGTH DIFFRACTION COEFFICIENTS TO CALCULATE THE PATTERN EFFECTS OF THE RIM AND SURFACE CRACKS OF A REFLECTOR ANTENNA

Incremental length diffraction coefficients (ILDC's) are applied to the calculation of reflector antenna patterns. ILDC's for the half-plane are integrated around the rim of a paraboloid reflector antenna to obtain well-behaved far fields of the nonuniform current for all angles of observation. These computed nonuniform current far fields, when added to the physical optics far field, produce a more accurate total far field of the reflector. Excellent agreement with the far fields obtained from a method-of-moments solution to the electric field integral equation applied to a 20-wavelength-diameter reflector shows that the cross polarization, further-out sidelobes, and fields near nulls of reflector antennas can be appreciably modified by the fields of the nonuniform currents. ILDC's are also used to investigate the effect of cracks on the surface of reflectors that can result from the imperfect fitting together of panels to form large reflectors. Three models of cracks are studied: 1) a slit in a perfectly electrically conducting (PEC) screen; 2) a channel with a semicircular cross section in a PEC screen; and 3) a boss with a semicircular cross section in a PEC screen. Significant pattern effects of cracks are found depending on the model and orientation of the cracks.