Compact Self-Diplexing MIMO Antenna With Improved Isolation for IoT Connectivity

This letter proposes a cavity-backed substrate-integrated-waveguide dual-feed self-diplexing multiple-input–multiple-output (MIMO) antenna for Internet of Things applications. To implement a self-diplexing operation, the half-mode substrate-integrated waveguide (SIW) cavity is loaded with a rectangular slot carved on the top metal cladding. This causes the cavity to be divided into two unequal apertures, resulting in the development of a quarter-mode SIW. The patch has orthogonally positioned feed lines that provide two distinct resonant frequencies, 5.18 GHz and 5.8 GHz, with respect to the fundamental mode $\text{TE}_{110}$. Employing metallic vias on the cavity improves the self-isolation between the orthogonal ports. Furthermore, a square aperture between orthogonal patches can achieve maximum isolation without additional units. The proposed compact MIMO antenna with a self-diplexing feature using an aperture slot achieves maximum isolation of 46.84 dB at 5.17 GHz and 42.76 dB at 5.8 GHz between patches, interport isolation of 28.6 dB at $f_{L}$ and 27.6 dB at $f_{H}$ among ports, an impedance bandwidth of 130 MHz (5.236 GHz to 5.106 GHz) and 174 MHz (5.88 GHz to 5.72 GHz), and high gain of 4.5 dBi, and 5.46 dBi was achieved at 5.17 GHz and 5.8 GHz, respectively. In addition, the MIMO antenna's performance metrics are envelope correlation coefficient (ECC) $< $0.01 and diversity gain $>$9.9 dB was assessed. The proposed MIMO antenna's performance is evaluated through full-wave electromagnetic (EM) analysis and physical measurements of the fabricated prototype.