This article presents a numerical technique based on the modal-expansion method for modeling a multi-sleeve monopole antenna fed through an infinite ground-plane by a coaxial transmission line. The modal-expansion analysis is facilitated by introducing a perfectly conducting boundary at a variable height over the ground-plane of the monopole. The resulting structure is then divided into a number of regions and the electromagnetic field components in each region are expanded into the summation of its modal functions. The current distribution over the monopole and sleeve surfaces as well as the antenna's input impedance are computed by finding the expansion coefficients through matching the tangential field components across the regional interfaces. Numerical results for the surface current distribution and input impedance are presented and discussed with emphasis laid on single- and double-sleeve monopole antennas.
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