Wideband Gain Enhancement of High-Isolation and Quasi-Omnidirectional Metamaterial MIMO Antenna for Vehicular Radar

In this article, a gain-enhanced wideband multiple-input multiple-output (MIMO) antenna with high isolation and quasi-omnidirectional radiation is proposed. It is a hexagonal structure and includes six identical endfire log-periodic dipole array (LPDA) elements. These antenna elements are distributed in the same plane and uniformly placed in a rotating arrangement with an angle difference of 60°, and then are suspended over a large metallic plate (LMP). To enhance the wideband gain, a different metasurface (MS) lens with a high refractive index is deployed around the endfire log-periodic dipole. The uniform MS lens makes electromagnetic waves deflect toward the array axis of the LPDA, whereas the gradient MS lens could form focused plane electromagnetic waves in the endfire direction. Furthermore, these antenna elements are isolated with a complementary split ring resonator (CSRR)-based electromagnetic band-gap (EBG) structure owing to its wide bandstop frequency and small-size characteristics. To validate the design concept, the prototype is fabricated and measured. The measured −10-dB impedance bandwidth covers 2.85–10.9 GHz (FBW 117.1%) and the gain varies from 10.37 to 14.82 dBi. Moreover, high isolation of better than 22.4 dB and low envelope correlation coefficient (ECC) of less than −62.3 dB is achieved between antenna elements. The proposed wideband MIMO antenna could be suitable for vehicular radar.