Feed gap aperture field and input admittance of an infinitely long insulated antenna

Expressions are derived for the input admittance and aperture electromagnetic fields in the feed gap of a solid insulated antenna of infinite extent. This is done by solving the boundary value problem where the fields are represented by Fourier series built up by superposition of basic sets of φ-independent cylindrical waves. Such waves are obtained from the solution of the Helmholtz equation governing the z-directed electric Hertzian potential. The axial aperture electric field in the gap is in the form of a Fourier-Bessel series and is used to find the input admittance. Results are plotted for the aperture fields and tabulated for the input admittances at different radii, insulating shell dielectric constants, and gap widths