## 4 Millimeter-Wave CP DRA
\begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline
**Ref.** & **Geometry** & \(f_{r}\) **(GHz)** & **CP BW** & **CP BW** & \(\varepsilon_{r}\) & **Feed** & **Techniques** & **Gain** & **Mode** & **Isolation** \\ \hline
[65] & RDRA & 3.45 & 3.15-3.93 & 24.76 & 9.8 &
\begin{tabular}{c} Microstrip \\ line \\ \end{tabular} & EBG & 4.83 & TE\({}_{111}\) & \(-\)26 \\ \hline
[66] & CDRA & 3.22–3.72 & 3.34-3.54 & 5.99 & 9.8 & \begin{tabular}{c} Microstrip \\ line \\ \end{tabular} & \
\begin{tabular}{c} Polarization \\ and \\ pattern \\ diversity \\ \end{tabular} & \textgreater{}4.2 &
\begin{tabular}{c} HEM\({}_{115}\) \\ \end{tabular} & \textgreater{}15 \\ \hline
[67] & SDRA & 2.4 & 2.39–2.54 & 4.9 & 9.5 & \
\begin{tabular}{c} Microstrip \\ line \\ \end{tabular} &
\begin{tabular}{c} Decoupling. \\ Metal \\ strips on \\ the DRAs \\ \end{tabular} & 5.2–5.6 & TE\({}_{111}\) & 31 \\ \hline
[68] & CDRA & \begin{tabular}{c} 2.9–3.2 \\ 3.44–3.64 \\ 4.75–5.52 \\ \end{tabular} & \begin{tabular}{c} 3.32–3.58 \\ 5–5.32 \\ \end{tabular} & \begin{tabular}{c} 7.8 \\ 5–5.32 \\ \end{tabular} & \begin{tabular}{c} \\ 6.4 \\ 6.4 \\ \end{tabular} & NM & \
\begin{tabular}{c} Aperture \\ coupled \\ feeding \\ antenna \\ \end{tabular} & \textgreater{}2 &
\begin{tabular}{c} Orthogonal \\ orientation of \\ antenna \\ \end{tabular} & \textgreater{}15 \\ \hline
[69] & \begin{tabular}{c} V-shaped \\ DR \\ \end{tabular} & 4.89–5.42 & 5.16–5.38 & 4.17 & 9.8 & \
\begin{tabular}{c} Microstrip \\ line \\ \end{tabular} &
\begin{tabular}{c} Orthogonal \\ feeding \\ network \\ \end{tabular} & 5.04 & TE modes & \(\leq\)\(-\)14 \\ \hline
[70] & CDRA & 3.38–3.8 & 3.4–3.57 & 5 & 9.8 & \begin{tabular}{c} Microstrip \\ line \\ \end{tabular} & \
\begin{tabular}{c} Decoupling \\ network \\ \end{tabular} & 4.91 &
\begin{tabular}{c} HEM\({}_{115}\) \\ \end{tabular} & 21 \\ \hline
[71] & RDRA & 3.5–4.95 & 3.58–4.4 & 23 & 10 & \
\begin{tabular}{c} Conformal \\ metal \\ strip \\ \end{tabular} &
\begin{tabular}{c} Parasitic \\ patch and \\ diagonally \\ position \\ \end{tabular} & 6.5 & TE\({}_{315}^{3}\)/TE\({}_{183}^{3}\) & \textgreater{}28 \\ \hline
[72] & \begin{tabular}{c} Pyramid \\ DRA \\ \end{tabular} & \begin{tabular}{c} 7.74– \\ 12.39 \\ \end{tabular} & 8.86–9.69 & 9.37 & 10.2 & \begin{tabular}{c} Microstrip \\ line \\ \end{tabular} & \
\begin{tabular}{c} Symmetry \\ antenna \\ elements \\ \end{tabular} & 7 &
\begin{tabular}{c} Multiple \\ modes \\ \end{tabular} & \textgreater{}21.8 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Comparison between various MIMO circularly polarized DRAs. frequency. However, the proposed antenna will be thick as it is implemented by four layers of substrates and needs to be tightly bonded with metal screws. In [77], a CP substrate-integrated CDRA has been proposed for 60 GHz applications. The DRA element has been isolated from the substrate using two circular arrays of holes and vias, as shown in Figure 18. A slotted circular patch is loaded on the DRA element at the top of the substrate to generate a CP field. The measured overlapping bandwidth between the reflection coefficient and AR has been increased by 15.9% from 55 to 64.5 GHz, with a peak measured gain of 11.43 dBic when the sequential rotation (SR) feeding method has been used for the 2 \(\times\) 2 array. However, substrate-integrated antenna often leads to large leakage losses which depends on the separation of vias and holes. One of the popular methods to enhance impedance bandwidth, AR bandwidth, and gain of the proposed antenna is by integrating a SP feeding network into the antenna design [78]. The DRA using a SP feeding network is proposed in for the mm-wave application at 30 GHz. As depicted in Figure 19, the proposed antenna consists of a 2 \(\times\) 2 array of a flower-shaped DR placed on a ground plane. The cross-slots and SP feeding network are etched on the top and bottom sides of the Rogers RO3006 substrate, respectively.