This paper introduces the design of a circularly polarized (CP) single input single output (SISO) antenna, along with its 2 \(\times \) 2 MIMO version, functioning within the 0.64-0.66 THz frequency range. The antenna is designed on low-loss polyimide dielectric substrates, with various slot shapes etched into radiating elements to transform linear polarization into circular polarization. A defected ground structure (DGS) is incorporated to tune the antenna’s resonance frequency at 0.65 THz. The design features low return loss, reduced mutual coupling, and a low envelope correlation coefficient (ECC). The specific absorption rate (SAR) of the antenna is also analyzed for human-body interaction. The MIMO antenna offers a -10 dB impedance bandwidth of 0.64-0.66 THz and axial ratio bandwidth (ARBW) between 0.647 and 0.666 THz. It achieves a gain of 5.76 dBi and a radiation efficiency of 88% at 0.65 THz. The isolation is more than 25 dB and the ECC value observed is \(10^{-3}\) in the operating band. The channel capacity loss ranges from 0.07 to 0.32 bits/s/Hz within the operating band, and the mean effective gain (MEG) ranges from -2.2 to -5.7 dB in isotropic and Gaussian environments.
