The dielectric-filled tubular monopole

The integral equation for the current in a dielectric-filled tubular monopole when driven by a coaxial line with an assumed pure TEM mode is formulated and solved by a numerical method. It is found that when the dielectric-filled tube can support a TM
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circular waveguide mode, the characteristics of the antenna are drastically different. In this case a very sharp resonance behavior is observed. The resonant length depends primarily on the properties of the internal waveguide mode rather than on the free-space behavior of the antenna.