Understanding the bandwidth limitations of segmented capped monopole antennas using eigenmode analysis

The antenna discussed here belongs to a general class of segmented and/or folded capped monopoles, similar to that described by Goubau and subsequently studied by others. These antennas achieve a low radiation Q-factor for their size, and exhibit a multi-resonant impedance response, leading to excellent bandwidth behavior in a vertically polarized electrically short structure. Because of the multi-resonant impedance, it is difficult to directly relate bandwidth to Q in these antennas. For this reason, the bandwidth limitations of these antennas relative to fundamental lower bounds on Q have not been quantitatively derived in the literature. In this paper, we illustrate how to determine these bandwidth limitations using an eigenmode analysis of the antenna.