Loop Excitation of a Conical Horn

A Green's function technique is developed for the full-wave analysis of a loop excited conical horn, considering all excited modes. The technique avoids the need for discretization of the horn antenna structure into several cascaded sections or the imposition of unknown currents over the entire horn surface, thus reducing the computational burden significantly. The aperture fields of the conical horn are investigated for varying horn lengths. Reflection effects at the aperture is found to be minimal for horn lengths over 5λ, corresponding to a horn aperture diameter of more than 2λ. Analytical expressions used to represent the aperture fields over the conical cap at the aperture are provided from which the radiation fields are derived to characterize the aperture radiation. It is observed that the loop excitation can be appropriately located relative to the horn apex to couple to appropriate modes to generate broadside or monopole-like pattern characteristics. Also, in addition to the simplicity of the feed configuration, the loop position can be properly selected to couple to multiple modes that has a significant impact on the pattern directivity and sidelobe levels. The developed technique is also used to rigorously evaluate and characterize the contribution of each mode at resonance.