Axial-Conductances Angular Filter Investigation

This report describes the concept, analysis, design, construction, and tests of an angular filter using an axial-conductance medium. The filter provides rejection that increases with incidence angle in the E plane. It is essentially invisible at broadside incidence, does not have critical tolerances on dimensions or materials, and operates over a wide frequency band. Analysis of an ideal homogeneous axial-conductance medium shows that the optimum value for the axial loss tangent is unity. With this value, the homogeneous medium provides approximately 8 dB of absorptive rejection per wavelength of filter thickness at a 45 E-plane incidence angle. Analysis of a practical inhomogeneous axial-conductance medium shows that some loss is introduced at broadside incidence, and that two types of waves can exist in the medium when only one wave is incident at an oblique angle. When the practical medium has dimensions that are properly chosen, its broadside loss can be negligible, and its rejection versus incidence angle can approximate that of the ideal medium. Tests of inhomogeneous samples in simulator wave guide confirm these analytical results. A screen printing method for depositing thick-film resistive ink on thin dielectric sheets has been investigated. With this method a 5x5 foot angular filter, designed for operation at 10 GHz, has been constructed containing over 70,000 axial-conductance elements.