# Radiator Enablers for Wireless Communication Evolution
## 3 Radiators' Novel Technologies, Design, and Implementation Issues
### Orbital Angular Momentum Antennas
#### 3.4.6 OAM Example Designs  
There are a lot of examples of OAM configurations that can be implemented in the new era of 6G and beyond communications. To begin with, Substrate Integrated Waveguides (SIW) formed as an antenna array of two circular concentric parts with four and eight slots accordingly can propagate OAM (ref. [87]). For mmWave applications, we have two main sectors of antennas that can be used: phase-modulating (using metasurfaces, holographic plates, etc.) and directly OAM-producing antennas, like uniform circular arrays (UCAs), patch antennas with a variety of dielectric materials (ref. [88]). One noticeable way to implement OAM waves is Conical Conformal Array (CCA) patch antennas. By changing the cone's angle, beam steering can be easily achieved, thus providing the desired purity levels (ref. [85]). The usage of metasurfaces in OAM production and more precisely with hexagonal patches loaded with rectangular slots allows the construction of a relative transmit and reflectarray (ref. [90]).  
OAM can find some applications in radar technology with the usage of time division multiplexing (TDM) by utilizing UCA in multiple rings, propagating a variety of modes (ref. [89]). For increased channel capacity, TDM and phased arrays are necessary, but due to the complexity of the feeding mechanism, one concludes the use of primary uniform circular arrays (PUCAs), which can be assembled from four circularly polarized patch antennas loaded with \(\Upsilon\)- and \(\Delta\)-etched stripes (ref. [86]). Again, SIW can be used with eight etched L-shaped slots in a circular pattern, thus creating a circular resonant cavity that provides a higher quality factor (ref. [83]). On the other hand, except for TDM, Orthogonal Frequency Division Multiplexing (OFDM) in MIMO configurations with two UCAs can beused (ref. [84]). In addition, OAM can be used on the THz band and utilized by reflectarray Antennas (RAs), such as a conical horn antenna, but with caution due to low gain and efficiency (ref. [91]). All the examples are summarized in Table 7.  
In general, the most commonly used methods in OAM waves are spiral phase plates (SPPs), metasurfaces, UCA, dielectric resonator antennas (DRAs), and parabolic reflector antennas (PRAs). All of them could propagate those waves, but each one has distinct characteristics, which are listed in Table 8 and Figure 1 (ref. [82]).  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Ref. No** & **Frequency (GHz)** & **Method** & **Mode Numbers (I)** & \multicolumn{2}{c}{**Total Number**} & \multicolumn{1}{c}{**Study Type**} \\ \hline
[82] & Variable & Variable & Variable & Variable & Theory \\
[83] & 24 & SIW & \(-1\) & N/2 \(-1\) & Measurement \\
[84] & 10 & UCA/MIMO & Variable & Variable & Theory \\
[85] & 10 & CCA & +1, +2 & 2 & Measurement \\
[86] & 2.5 & PUCA & \(-1,0,+1\) & 3 & -//- \\
[87] & 10.190 & SIW & 0, +1 & 2 & -/- \\
[88] & 10 & UCA & \(-1,+1\) & 2 & Simulation \\
[89] & Variable & UCA & \(-8\) to +7 & 16 & -/- \\
[90] & 21 (K-Band) & Meta-surfaces & +1 & 1 & Measurement \\
[91] & 185–188 & RA & +2, +3 & 2 & Simulation \\ \hline \hline \end{tabular}
\end{table}
Table 7: OAM design examples.  
Figure 1: Diagram of OAM methods.