Rigorous evaluation of the input impedance of a sleeve monopole by modal-expansion method

A rigorous modal-expansion analysis is developed for obtaining the current distribution and input impedance of a sleeve-monopole antenna fed through an infinite ground plane by a coaxial line. A “perfectly matched boundary,” which is simply the combination of an electric wall and a magnetic wall, is initially proposed to facilitate the modal-expansion analysis. The walls are parallel to the ground plane at a variable height above it. Expansion coefficients of the electromagnetic fields are then determined by enforcing the boundary conditions at conducting surfaces and regional interfaces. Numerical results for the current distribution and input impedance of the sleeve monopole are presented and compared with available data in the literature. Very good agreement is observed. Also examined are the radiation pattern for different sleeve lengths and the effects of the length and finite thickness of the sleeve conductor on the antenna´s input impedance