Improving Radiation Pattern Roundness of Henge-Like Metaring-Loaded Monopoles Above a Finite Ground for MIMO Systems

A henge-like metaring (HMR) is proposed for improving the radiation pattern roundness of monopole antennas off-center mounted on a finite ground by localizing the radiation from the monopole and suppressing the scattering by the ground. The improved patterns enhance uniform coverage of multiple-input and multiple-output (MIMO) systems. The study reveals that the radiation pattern of an off-center monopole is distorted by the asymmetric ground currents excited by both the feed and the radiation of the monopole. The distorted radiation patterns severely degrade wireless communication link quality. The HMR, composed of an annular array of mushroom unit cells, simultaneously functions as an electromagnetic bandgap (EBG) and a radiator, and encircles the monopole to form a henge monopole antenna (HMA). The HMR as an EBG is used to suppress the ground currents outside the HMR analyzed by an equivalent circuit model. The HMR as a radiator is designed to decouple the monopole from the ground with its elevated radiation pattern using characteristic mode analysis. As examples, two prototypes of single and four off-center MIMO HMAs are designed and investigated in the 2.45-GHz band. Simulated and measured results show that the single HMA and each of the four HMAs achieve the un-roundness of the radiation pattern at θ = 65° plane lower than 2 dB and 3 dB in the 2.45-GHz band. As a result, near the radiation nulls, the SNR is improved by 6 dB. The compact construction and efficient current suppression facilitate the application of HMAs in multi-antenna systems above a finite ground with uniform coverage and reliable connections.