Compact QMSIW Self-Diplexing MIMO Antenna

In this letter, a dual-band, four-port multiple-input multiple-output (MIMO) self-diplexing antenna is designed based on quarter-mode substrate integrated waveguide (QMSIW) resonators. The MIMO antenna configuration includes identical self-diplexing resonators arranged in a mirrored manner. The proposed antenna operates at 3.5 GHz (when Port 1 and Port 2 are active) and 4.9 GHz (when Port 3 and Port 4 are active), occupying a compact area of 0.25$\lambda _{g}^{2}$ ($\lambda _{g}$ is calculated at 3.5 GHz). A full-mode substrate integrated waveguide (SIW) structure is initially designed and then modified into an asymmetric QMSIW configuration. Capacitive slots with different effective capacitances are added to enable operation at two distinct frequencies. Compactness is achieved through the use of QMSIW resonators instead of full-mode SIW resonators and the integration of capacitive slots within each QMSIW resonator. The antenna exhibits low mutual coupling ($-$22.89 dB) due to sufficient spacing between the resonators. The antenna gains are found to be 4.38 dBi at 3.5 GHz and 5.98 dBi at 4.9 GHz. The MIMO channel parameters, such as envelope correlation coefficient (ECC) and diversity gain, are also evaluated and found to be satisfactory. The antenna's attractive features; compact size, low mutual coupling, independent frequency control, and excellent MIMO channel performance, make it well-suited for compact RF devices. To the authors knowledge, this is the most compact MIMO self-diplexing antenna reported to date.