Compact Quad-Element Vertically-Polarized High-Isolation Wideband MIMO Antenna for Vehicular Base Station

A novel compact multiple-input-multiple-output (MIMO) antenna with a wide impedance bandwidth and high port isolation is proposed for vehicular base stations. The design consists of four identical vertically-polarized monopole-based antenna elements, which are symmetrical about the central axis and are placed 90 degrees apart from each other. The wideband and decoupling features are theoretically analyzed and experimentally demonstrated. Four resonant frequencies are created by the F-shaped radiator, independent reflector and cross-π-shaped structure, which increase the MIMO antenna bandwidth. Parasitic stubs further improve the impedance matching in the high-frequency band. The cross-π-shaped structure acts as a decoupling function and reduces the mutual coupling, which is realized by using two metal strips shorted to the ground plane, perpendicular to each other. The etched slots further improve port isolation. The effects of imitated perfect vehicle metallic surface are also analyzed. The proposed MIMO antenna prototype is fabricated and measured to verify the concept of design. The operating frequency band is from 1.95 GHz to 6.25 GHz with a reflection coefficient of better than −10 dB, where the realized gain is higher than 3.3 dBi, and the maximum value is 6.16 dBi at 2.6 GHz. High isolation of >16.5 dB and low envelop correlation coefficient of <0.028 among designed MIMO antenna elements are also achieved.