Dielectric Sphere Diffracted Fields of a Dielectric Coated Conical Horn

Abstract

Analytical and experimental investigations into the radiation characteristics of a high gain antenna feed system with a variable beamwidth facility are reported. The antenna consists of a homogeneous dielectric sphere placed in front of, but displaced from, the aperture of a dielectric coated conical horn. First the analysis of the fields in the dielectric coated conical horn is presented briefly using spherical hybrid modes, and the characteristic equation for the separation constant is derived. The condition for the support of balanced hybrid modes that contribute to low cross polarization is also derived. Then the theoretical expressions for the radiated far field components of the feed system are derived by treating the homogenous dielectric sphere as a scatterer of electromagnetic waves based on scattering theory approach, and also by treating the displaced dielectric sphere as a microwave lens illuminated by an aperture source. There is good overall agreement between the theoretical and experimental results. It is concluded that the test feed system has improved radiation characteristics with increased gain and directivity at reduced sidelobe levels. The low cross polarization nature of this horn makes it an attractive feed for satellite communications and radio astronomy applications.