# Radiator Enablers for Wireless Communication Evolution
## 3 Radiators' Novel Technologies, Design, and Implementation Issues
### Terahertz Antennas for 6G Applications
#### 3.7.4 Example Designs  
According to ref. [134], the application of graphene on fixed beam reflectarray antennas to operate at the THz frequency is a highly promising idea, and here is why: special electronic properties of graphene, like complex surface conductivity, support the propagation of the slow plasmonic modes. A reflect array designed at 1.3 THz, consisting of more than 25,000 elements, each approximately \(\lambda_{0}/16\), was analyzed using a full vectorial approach accounting for incidence angle variation and local periodicity. The array shows excellent bandwidth, cross-polarization, and grating lobe suppression performance, confirming the feasibility of graphene-based reflect arrays. This work also opens the route toward reconfigurable THz reflect arrays based on the electric field effect in graphene. Photoconductive antennas (PCAs) are used for the generation and detection of the THz wave (ref. [135]). A PCA design is proposed in ref. [136] based on the use of spatially dispersive graphene strips parallel to each other in such a way that enhances their tunability without compromising their single-strip properties. Working in a high-index medium, the reduced group velocity requires a nonlocal model of conductivity, realized using a per unit length circuit including quantum capacitance. In the deep subwavelength regime, the GS couples are effectively replaced by one strip with doubled conductivity. With wideband photocurrent driving, graphene-based PCA is promising for effective terahertz generation and detection. In a high-speed THz communication system, the horn antenna can be used as a standalone antenna or as a feed source for a lens antenna or a transmitting antenna. In ref. [137], a novel, compact, highly efficient circularly polarized conical horn antenna is presented operating at a 300 GHz frequency for 6G wireless communication. The antenna configuration consists of three main elements: a WR-03 rectangular waveguide feed, a circular polarizer disk with crossed slots of unequal length for the generation of the circular polarization, and the conical horn mounted onto the polarizer disk. An economical technique called Wire EDM is employed for fabricating the antenna operating over the WM-860 frequency band. The experimental results show that an impedance bandwidth of 20% (270-330 GHz) with a reflection coefficient \(\leq-15\) dB and a 3 dB axial ratio bandwidth of 7 GHz from 309 to 316 GHz is achieved. The normalized radiation patterns exhibit excellent symmetry and strong agreement with simulations, confirming the antenna's performance and suitability for next-generation wireless applications. In ref. [138], the authors improved the performance of microstrip antennas by applying the metamaterials concept. Though microstrip antennas are favorable because of their simple design, low cost, and conformability, traditional techniques for achieving high gain and narrow beams significantly raise complexity, size, and coupling effects. The emergence of metamaterials with their subwavelength structures and their unique properties has drawn a considerable amount of attention in this regard due to the property of negative refraction. In this paper, an inductive-capacitive \"I\"-shaped element-based metamaterial structure is proposed, which ensures the realization of negative permeability from 31.18 GHz to 38.80 GHz. Loading these metamaterial structures periodically around the patch antenna improved the antenna gain by 1.62 dB at 35 GHz. Further research on metamaterials for antenna design may provide another option for compact and high-performance antennas. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Antenna Type** & **Advantages** & **Characteristics** & **Application Examples** \\ \hline \multirow{4}{*}{ \begin{tabular}{c} Graphene-Based Antennas (ref. [134]) \\ \end{tabular}  } & \multirow{4}{*}{ \begin{tabular}{c} Graphene is a 2D material with \\ exceptional electrical and tunable \\ properties at THz frequencies. \\ \end{tabular}  } & Dynamic frequency tuning & \multirow{4}{*}{