Multiple Input Multiple Output Antenna at 28 GHz Using Two Planar Inverted F Antennas

This study investigates a multiple-input-multiple-output (MIMO) antenna system comprising two planar inverted F antennas (PIFAs) designed for operation at 28 GHz. Each PIFA element has a compact size of 0.0106λ03, where λ0 represents the free space wavelength at 28 GHz. The overall size of the two-element MIMO antenna system measures 0.0212 λ03. Three different design approaches are explored for this system. In the first design, the two PIFAs are positioned side by side with their vertical metal walls aligned parallel. This configuration achieves an impedance bandwidth of 2.25 GHz, ranging from 26.92 GHz to 29.17 GHz, with a directivity of 6.6 dBi at 28 GHz. The isolation exceeds 16 dB, and the envelope correlation coefficient (ECC) remains below 0.01, with a diversity gain over 9.951 across the bandwidth. The radiation efficiency and total efficiency for this design are 72.15% and 70.25%, respectively, at 28 GHz. In the second design, the metal walls of the PIFAs are oriented at 90 degrees to each other to enhance isolation. This adjustment results in improved performance parameters, including an isolation increase of approximately 10 dB and a directivity of 8.27 dBi at 28 GHz. The radiation and total efficiencies rise to 75.72% and 75.68%, respectively, indicating a slight improvement over the first design. The ECC remains below 0.00024, and the diversity gain consistently exceeds 9.9988 throughout the entire 2.2 GHz bandwidth, which is slightly narrower than that of Design 1. The third design introduces a slot in the metal patch of each antenna element, further miniaturizing the MIMO system to 0.0195 λ03. This configuration achieves a minimum isolation of 26.27 dB and a directivity of 8.06 dBi at 28 GHz. Although the bandwidth reduces to 1.78 GHz, the ECC stays below 0.00033, and the diversity gain remains above 9.9983. The efficiencies slightly decrease to 73.02% and 71.88%, respectively. The Channel Capacity Loss values at 28 GHz are 0.048 bits/s/Hz, 0.017 bits/s/Hz, and 0.007 bits/s/Hz for Designs 1, 2, and 3, respectively, all within the acceptable limits. The performance of the MIMO antenna system is evaluated through computer simulation and compared with similar antennas documented in recent literature.