A broad-band double-sleeve monopole antenna

This paper presents a modal-expansion analysis of a double-sleeve monopole antenna fed by a coaxial cable. 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 input impedance of double-sleeve monopoles are presented and discussed with emphasis laid on broadening the bandwidth