Wideband MIMO Antennas With Enhanced Isolation Using Coupled CPW Transmission Lines

This article presents a wideband printed-monopole antenna pair with enhanced isolation. A clear two-step method based on even-mode (EM) and odd-mode (OM) analyses is developed for antenna decoupling and matching: first, the OM input impedance of the antenna pair is matched by adjusting the element spacing; next, the EM input impedance is matched by adding a coupled TEM transmission line. Those two steps are independent and easy to implement. Notably, the use of a nondispersive coupled transmission line facilitates wideband decoupling and intrinsically allows an extremely small feed-point spacing. The proposed antenna pair is experimentally tested: it offers isolation higher than 20 dB and total efficiency higher than 87% from 3.7 to 7.3 GHz (a fractional bandwidth of 65.5%). The separation between the two feed points is only 5.6 mm ( $0.069~\lambda $ at 3.7 GHz). Furthermore, we construct a four-element multiple-input–multiple-output (MIMO) array by intersecting orthogonally two of the proposed antenna pairs at their central line, which offer an isolation level better than 20 dB from 3.8 to 7.4 GHz for every two ports. With a straightforward design process, compact feeding structures, and wideband performances, the proposed dual- and four-element MIMO antennas are promising for the fifth-generation wireless communication.