-0_10 S. K. Goudos and M. A. Matin Moreover, recently the Institute of Electrical and Electronics Engineers (IEEE) introduced the first standard for low THz bands, namely, the IEEE 802.15.3d [1]. This defines global wireless personal area network (WPAN) application at frequen- cies from 252 to 325 GHz. Thus, this is the first global wireless communications standard for the 250-350 GHz frequency band. This will have channel bandwidths ranging from 2 to 70 GHz and a nominal PHY data throughput of 100 Gbps. Taking into account all the above, the research in 6G wireless systems is a highly challenging topic. Antennas are one of the most important parts of any communication system. Moreover, antennas perform the task of receiving and trans- mitting electromagnetic waves. Because of the high transmission loss of low THz electromagnetic waves in free space, it becomes necessary to design low THz (more specifically in the frequency band of 252-325 GHz) antennas having very high gain to make up for the high path loss values. As a result, the THz communication system's range is extended, and the power limitation of the THz source generator is overcome. Antennas of this type can be used in a variety of ways, for example, high- speed short distance communication [24]. One of the key characteristics of antenna design for such communication systems is circular polarization (CP). CP has the benefit of freeing the receiver from the direction of arrival dependence in an input electromagnetic wave. Moreover, circular polarization has the potential to enhance the antenna's efficiency and eliminate the mismatch between the transmitter and the receiver caused by misalignment. In a multipath environment, CP plays an important role because it can reduce the delay spread [19]. To this end, new antennas must be designed and evaluated for 6G wireless systems. These antennas should meet all the forthcoming requirements of 6G networks, including the operation in the terahertz band and circular polarization. Considering the complexity of the