Compact Radiation Pattern Decoupling Design of 2-D MIMO Microstrip Antennas

For the first time, this article presents a radiation pattern decoupling (RPD) method for compact 2-D multiple-input-multiple-output (MIMO) microstrip antenna (MA). With two shorting walls loaded at its nonradiating edges, an MA element is analyzed under its fundamental ${\text {TE}}_{110}^{x}$ mode with a cavity model. With extra current paths introduced by U-shaped strips and slots, H- and E-plane RPD characteristics can be in turn obtained based on the superposition principle. For each MA element, four shorting walls in total can make the element performance insensitive to the surroundings. Therefore, dummy elements are no longer needed in this design, which makes it compact. A prototype with $4\times 4$ antenna elements is designed, fabricated, and measured for the 5.8-GHz band. The measured results show good agreement with the simulated ones. A measured overlapping impedance bandwidth of 5.9% can be obtained, over which the isolation is higher than 18.7 dB between any two ports. It should be highlighted that this compact MIMO array features RPD characteristics even without dummy elements, thus being promising for practical applications.