Abstract
The radiation pattern of a quarter-wavelength monopole antenna placed off-center above a circular ground plane is seriously distorted due to the noncircular symmetric discontinuity encountered at the edge of the ground plane. An annular ring structure surrounding the monopole antenna is proposed to localize most currents for omnidirectional radiation with an enforced circular symmetric discontinuity. The characteristic mode analysis (CMA) shows that two modes with desired radiation patterns contribute most to the radiation of the proposed ring-loaded monopole. With the information of the calculated modal significances (MSs) and characteristic currents in CMA, the size of the loading ring is optimized. The simulated and experimental study shows that after introducing the ring structure, the maximum unroundness of the off-center monopole is reduced by 66%, which is improved from 3.5 to 1.2 dB.
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