A Compact Planar Omnidirectional MIMO Array Antenna With Pattern Phase Diversity Using Folded Dipole Element

This paper presents a planar omnidirectional multiple-input multiple-output (MIMO) antenna with pattern phase diversity. The profiles of conventional dual-polarized omnidirectional MIMO antennas are too high to be planar integrated due to the limited bandwidth of vertical-polarized element. Therefore, a dual-channel horizontal-polarized MIMO antenna is proposed for the planar integration of omnidirectional MIMO antenna. The antenna is formed with four-element folded dipole array, and a compact feed network with two sets of 90 degrees progressive phases is employed to feed the two channels with shared elements, thus the antenna can be integrated into a sheet of substrate with an ultralow profile of 0.8 mm. Although the polarization and radiation pattern (amplitude) of the two colocated channels are the same with each other, a high isolation is still achieved as a result of the orthogonal pattern phase. A prototype was fabricated to validate the performance. The measured highest isolation between two channels is 31.7 dB, and -10 dB isolated bandwidth is 2.29-2.57 GHz (11.7%) with S-11 < -14 dB and envelope correlation coefficient less than 0.05, which shows a good diversity performance. Therefore, the proposed dual-channel antenna gives a feasible solution for the planar integration of the omnidirectional MIMO system.