Input admittance and complete near-field distribution of an annular aperture antenna driven by a coaxial line

An annular aperture antenna driven by a coaxial transmission line is studied. An analytical and closed-form expression for input admittance is obtained. The effect of all higher order TMonto the admittance is found to be quite significant. Aperture field distribution is expressed in terms of the TEM and the first five TMonmodes. Higher order modes at the aperture can be estimated from the edge behavior. Near-field distribution based upon these modes is then cast into a form suitable for numerical evaluation.