Metasurface-Loaded Off-Center Monopoles With Wideband Radiation Performance Using Characteristic Mode Analysis

A metasurface (MTS)-based reactive impedance surface (RIS) is proposed to load an eccentrically placed monopole above a ground plane for controllable wideband omnidirectional radiation. With the off-center placement, the radiation pattern of the monopole is not omnidirectional anymore, with ripples in the H-plane measured by un-roundness. The RIS is introduced above and in parallel with the finite ground plane and the monopole is placed in the center of the RIS. Characteristic mode analysis (CMA) is used to analyze the RIS-loaded monopole and to reveal the mechanism. It is found that two desired modes determine the bandwidth of the omnidirectional radiation together. The resonant frequency of the RIS mode can be easily tuned by changing the sheet impedance of the RIS. A metal-dielectric-metal constructed MTS is proposed to realize the RIS. The grid impedance of the MTS can be tuned easily by changing the size of the patch in the unit cell. For a single monopole placed off-center above a rectangular ground plane (150 mm
$\times300$
mm), the impedance bandwidth is increased from 19.6% to 32.4% with
$\vert \text{S}_{11}\vert < -10$
dB at
$f_{0} =2.5$
GHz. And the maximum un-roundness is reduced by 66.7% from 4.8 to 1.6 dB over the frequency band of 2.2–2.8 GHz. For a two-monopole antenna system with the same ground plane mentioned above, the loading MTS increases the impedance bandwidth of the monopole from 19.2% to 32.4% with
$\vert \text{S}_{11}\vert < -10$
dB at
$f_{0} =2.5$
GHz. When only one monopole is loaded with MTS in the system, the maximum un-roundness is reduced by 65.5% from 5.5 to 1.9 dB in the frequency band of 2.2–2.8 GHz. When both monopoles are loaded with MTS in the system, the maximum un-roundness is reduced by 40% from 5.5 to 3.3 dB in the frequency band of 2.2–2.8 GHz. The antenna prototypes are fabricated and measured.