Compact MIMO Antenna With Frequency Reconfigurability and Adaptive Radiation Patterns

A frequency-reconfigurable antenna for a multiple-input–multiple-output (MIMO) system with electronically steerable radiation patterns in azimuth plane is proposed in this letter. For MIMO systems, the steerable radiation patterns are able to reduce the correlation between signals and increase the channel capacity. There are two folded-monopole electronically steerable parasitic array radiator (FM-ESPAR) antennas in the proposed MIMO antenna system in this letter. The radiation pattern of each FM-ESPAR antenna is steerable in azimuth plane from 0
$^{\circ}$
to 360
$^{\circ}$
. Its frequency is reconfigurable between 2.3 and 2.6 GHz. A top-disk loaded monopole and folded monopoles are employed to reduce the height of each FM-ESPAR antenna to 12 mm (
$\lambda /10$
at 2.4 GHz). The FM-ESPAR antennas are separated from each other by using a 30-mm-long sleeve ground plane. The measured results confirm that the MIMO antenna comprising two FM-ESPAR antennas achieves good isolation at any angle between the main lobes of the FM-ESPAR antennas. The proposed MIMO antenna has compact size, electronically steerable beams, and frequency-agile capability.