Analysis of Ultra-Wideband Printed Planar Quasi-Monopole Antennas Using the Theory of Characteristic Modes

This paper presents an analysis of ultra-wideband (UWB) printed planar quasi-monopole antennas using the theory of characteristic modes. It is shown that the modal voltage-standing-wave ratio, mode current distribution, and modal significance provide deeper physical insight into the operating mechanisms of these popular types of antennas. Using the printed planar bevel-shaped quasi-monopole antenna as an example, we characterize the antenna's performance. It is found that there are only a few significant modes excited. The dominant mode controls the antenna's behavior over the lower band, while the higher-order modes control the behavior over the upper band. The ground-plane size affects the mode properties. The ground-plane height should be taller than a quarter and shorter or equal to a half-guided-wavelength. The width should be equal to a half-guided-wavelength at the lower-frequency edge. We also examine the notched-frequency characteristics of the antenna by embedding a slot in the monopole. We point out the potential self-interference for the first time. Furthermore, we investigate the miniaturization of the antenna by simply chopping off half of the antenna's structure. We explain why the miniaturized antenna exhibits a wider impedance bandwidth and higher cross-polar radiation at higher frequencies.