Compact and Low-Coupled Tripolarized Microstrip MIMO Antenna Based on Parasitic Patch Loading

A miniaturized and low-coupled tripolarized microstrip multiple-input multiple-output (MIMO) antenna working at 3.5 GHz is proposed. This antenna includes a main circular radiating patch, which is shorted to the ground plane by an outer layer of vias, and a parasitic patch inside the cavity, which is connected to the main patch by an inner layer of vias. Through the selection of suitable number of the outer layer vias and the loading of the parasitic patch, the resonant frequencies of modes TM
₁₁
and TM
₀₂
can be reduced, individually. The design procedure of this antenna is demonstrated and a transmission-line model for TM
₀₂
mode is presented, which can give a good explanation for the frequency reducing by the capacitance introduced by parasitic patch loading. This design can not only downsize the diameter of the unloaded antenna from 0.7λ
₀
to 0.485λ
₀
, with the same height of 0.035λ
₀
, but also can reduce the interport coupling from -13 to -18 dB. The measured and simulated S-parameters and radiation patterns are in good agreements. Fine MIMO performance of the proposed tripolarized antenna in terms of large capacity and low correlation coefficients has been verified by some propagation measurements in a real-world office.