Optimized Compact MIMO Antenna Design: HMSIW-Based and Cavity-Backed for Enhanced Bandwidth

This paper introduces a compact, rhombus-shaped antenna based on a Half-Mode Substrate Integrated Waveguide (HMSIW) with a cavity-backed design, tailored to operate at 5.8 GHz within Industrial, Scientific, and Medical (ISM) band. The HMSIW cavity is designed to maintain the half-dominant TE110mode while reducing the cavity size by 50%. Initially, the antenna's bandwidth is insufficient for the entire ISM band. To address this, a rectangular slot is etched into the center of the cavity, which divides the dominant mode into TE110-odd and TE110-even modes resonating at 5.74 GHz and 5.8 GHz, respectively. This modification results in around 63% increase in bandwidth. For a Multiple Input Multiple Output (MIMO) application, the design extends from a single element to a four-element configuration, arranged orthogonally to achieve a high isolation level of approximately -23 dB in a compact footprint. The antenna has been prototyped and experimentally validated. The measured -10 dB impedance bandwidth spans 240 MHz (from 5.63-5.87 GHz), covering the full ISM band. The peak gain of the antenna is measured of 5.95 dBi, with an efficiency exceeding 90% across the operating frequency range. The 4-ports MIMO antenna diversity metrics parameters have been evaluated which are found in satisfactory limit.