Extremely Compact Interconnected Half-Mode Cavity Antennas With Enhanced Isolation for MIMO System

We propose the interconnected half-mode cavity antennas (HMCAs) with enhanced isolation that have a very close center-to-center distance of
$0.13 \lambda _{0} $
. First, by both tuning the differential mode (DM)-first impedance through a pair of inductors and creating an extra coupling path through the feeding branches, the half-mode of the cavity could be formed, and the isolation between the dual feeding ports in a single cavity could be greatly enhanced from 2 to 25–50 dB over the working band from 4.9 to 5.1 GHz. Then, we extend the dual-port to four-port HMCAs with a total length of
$0.54 \lambda _{0} $
through the connection of dual cavities, and the isolation between the adjacent cavities could be enhanced from 7 to better than 23 dB by employing the tuning lines to tune the common mode (CM)-second impedance. Furthermore, the extremely compact interconnected eight-port HMCAs are presented with a total length of
$1.08 \lambda _{0} $
. Finally, the experimental results also validate the effectiveness of our proposed HMCAs with excellent decoupling performance. Our proposed design should provide promising application values for the extremely compact multiple-input multiple-output (MIMO) systems.