Abstract
In this paper a compact four-port crown-shaped MIMO antenna is proposed for sub-6 GHz wireless applications and fabricated on FR4 substrate. The peak gain is 4.57 dBi, and the radiation efficiency is above 75% at wide impedance bandwidth of 4.0–4.8 GHz, with operation against about 4.5 GHz as expected. The crown geometry allows for improved impedance matching, better structural symmetry and lower mutual coupling when compared to conventional patch antennas. In order to increase practical performance, Nd:YAG laser surface treatment is used as a post-fabrication approach that decreases the RMS surface roughness from 385 nm to 78 nm in order to reach maximum electrical conductivity and minimum conductor losses. As a result, the reflection coefficient below −30 dB is measured, the VSWR reaches values close to unity and provides 50 Ω input impedance. The MIMO system in this case also shows an optimal diversity with ECC near idle (0.01) and a diversity gain of approximately 10 dB. Closer agreement between simulated and measured results, especially after laser treatment, has validated the design and demonstrated the ability of laser post-processing to decrease fabrication tolerances. The antenna is structurally unique in integrating a crown-shaped fractal patch geometry with the complement split-ring resonator, which breaks the surface current symmetry to excite orthogonal modes to realize a significantly wide impedance bandwidth across sub-6 GHz spectrum. This achieves natural decoupling through its inherent spatial diversity and orthogonal port orientations without complex but bulky decoupling networks crucial for the compact form factor needed in dense 5G terminals.