This communication finds that it is the combined characteristic modes (CMs), rather than the purely magnetic ones, that are always associated with low-resistance resonances of slot antenna. Thus, combined CMs could help control its bandwidth. A ring slot antenna with loaded stubs is proposed and analyzed to show how to use combined CMs to explain and achieve wideband characteristic. It is shown that by loading stubs, a new combined CM is introduced. A prototype antenna is then fabricated to experimentally validate the analysis and designing approach. The fractional bandwidth of the proposed ring slot antenna could be effectively increased from 12.5% to 18.2%.
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Lin et al. (2018) studied this question.