Simulation-based geometry evolution and performance assessment of a compact ultra-wideband 2 × 2 MIMO antenna from mmWave to Sub-THz frequencies

This study presents a simulation-based geometry evolution and performance assessment of a compact four-port 2 × 2 MIMO antenna for frequencies extending from the mmWave to the sub-THz region. The design evolves from an initial circular patch radiator to a modified petal-like radiator formed by five overlapping and rotated elliptical sections, followed by integration into a four-port MIMO configuration. The radiator, slots, feed and matching sections, partial ground plane, substrate dimensions, and element arrangement were parametrically investigated to improve the simulated impedance-matching and inter-port coupling responses without employing an additional defected ground structure, electromagnetic band-gap structure, metasurface, or dedicated decoupling element. The single-element footprint was reduced from 10 × 10 mm 2 to 10 × 9 mm 2 , while the complete four-port footprint was reduced from 24 × 24 mm 2 to 22 × 22 mm 2 . Under the investigated CST Studio Suite software, the conventional reflection coefficients remained below − 10 dB from 13.6 to 151.91 GHz, corresponding to a simulated impedance bandwidth of 138.31 GHz and a fractional bandwidth of approximately 167.1%. The transmission coefficients remained mostly below − 20 dB across this range. The peak realized gain reached 11.87 dBi, while the simulated radiation efficiency remained mostly above 90% up to approximately 120 GHz before decreasing in the upper-frequency region. Surface-current and radiation-pattern analyses showed a transition from comparatively broad lower-frequency current and radiation distributions to increasingly localised, directional, and multi-lobed behaviour at higher frequencies. Active reflection coefficients demonstrated that simultaneous four-port matching depends on the applied excitation amplitudes and relative phases and does not fully coincide with the conventional individual-port bandwidth. The full-band S -parameter-based envelope correlation coefficient (ECC) remained below approximately 0.005, while far-field ECC calculated at 16, 58, 100, and 148 GHz ranged from 0.000042 to 0.010246. The corresponding diversity-gain (DG) values remained close to 10 dB. The total active reflection coefficient (TARC) showed that simultaneous four-port matching is excitation-dependent, while channel capacity loss (CCL) was below 0.4 bit / s / Hz across the operating band. These results support the numerical feasibility of the proposed geometry-evolution approach under the stated simulation assumptions. Future work will focus on prototype fabrication and calibrated four-port mmWave-to-sub-THz measurements to evaluate practical antenna and communication-system performance.