Approximation techniques for the mutual admittance of slot antennas on metallic cones

The effect of scattering from a sharp tip on mutual admittance is investigated for pairs of circumferential and radial slots on a semi-infinite metallic cone. Mutual coupling measurements as a function frequency and angular separation are presented and compared with numerical calculations. The base of the conical model used in the experimental study was terminated in a spherical cap to minimize scattering from the finite length of the apparatus. The circumferential slot results illustrate interference effects between the direct coupling from slot-to-slot via the geodesic path over the conical surface and the tip back scattering. For the radial slot configuration, the results exhibit negligible tip scattering effects. The analysis utilizes a computer calculation of mutual coupling between circumferential and axial slots on an equivalent cylinder to predict the direct coupling contribution and utilizes the tip diffraction coefficient for calculating the component scattered by the apex. The tip diffraction coefficient is derived from an asymptotic expression for the tip-scattered component of the surface magnetic field excited on a semi-infinite conical surface.