Isolation Enhancement of Closely Packed Dual Circularly Polarized MIMO Antenna Using Hybrid Technique

Mutual coupling always seriously degrades the antenna performance in multiple-input multiple-output (MIMO) systems but this issue has been rarely investigated for antennas with circular polarization. In this paper, a planar and compact circularly polarized MIMO patch antenna with the polarization diversity is presented. Three grounded stubs and a mirrored F-shaped defected ground structure are used to achieve simultaneous matching and isolation between the two patches with offset feeding for circular polarization. The antenna elements are closely packed with the edge-to-edge distance of <inline-formula> <tex-math notation="LaTeX">$0.06\lambda _{0}$ </tex-math></inline-formula> at the desired frequency of 2.5 GHz. The measured results indicate that the proposed antenna achieves the impedance matching (<inline-formula> <tex-math notation="LaTeX">$\text{S}_{11} < -10$ </tex-math></inline-formula> dB), high isolation (<inline-formula> <tex-math notation="LaTeX">$\text{S}_{12} < -20$ </tex-math></inline-formula> dB), and circular polarization (axial ratio< 3 dB) within the frequency band of 2.5-2.55 GHz. The radiation patterns and realized gains are measured, showing good agreement between the simulation and measurement.