Realization of a Flexible Dual-Band Circularly Polarized mmWave MIMO Antenna with Aligned CP Bands for Wearable Applications

This paper presents a fully flexible dual-band circularly polarized (CP) millimeter-Wave (mmWave) multiple-input multiple-output (MIMO) antenna for wearable applications. The proposed two-port antenna is implemented using a PF4-foam substrate and Shieldex Nora-RS conductive fabric to achieve a lightweight and mechanically compliant structure suitable for conformal operation. The design is based on slot-loaded circular radiators and a defected ground plane, while two tuning stubs are introduced in each feed line to bring the axial-ratio minima into close alignment with the desired resonant frequencies. The antenna operates at 32.76 and 40.4 GHz, exhibiting strong impedance matching with S11 below −30 dB, high isolation with S21 of −29 and −41 dB, and excellent CP performance with axial ratios of 0.37 and 0.54 dB at the two bands, respectively. The corresponding lower- and upper-band impedance bandwidths are 4.08 and 1.824 GHz. In addition, the antenna maintains high total efficiency of 92% and 89%, respectively, together with stable radiation characteristics. Bending analysis confirms only minor resonance shifts without significant degradation of the impedance or CP response. The proposed antenna also demonstrates excellent MIMO characteristics, including very low envelope correlation coefficient, low channel capacity loss, and favorable total active reflection coefficient under different excitation phases. These results confirm the suitability of the proposed design for future flexible wearable mmWave communication systems.