## 2 Dielectric Resonator Antenna
### Dual- or Multiple-Feed CP DRA
\begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Ref.**} & \multirow{2}{*}{
\begin{tabular}{c} **Basic Ge-** \\ **ometry** \\ \end{tabular} } & \multirow{2}{*}{\(f_{r}\) (GHz)} & \multicolumn{2}{c}{**CP BW**} & \multirow{2}{*}{\(\mathbf{c_{r}}\)} & \multirow{2}{*}{\(\mathbf{c_{r}}\)} & \multirow{2}{*}{**Feed**} & \multirow{2}{*}{**Techniques**} & \multicolumn{2}{c}{**Gain**} & \multirow{2}{*}{**Mode**} & \multirow{2}{*}{**Eff. (\%)**} \\  & & & & & & & & & & **Type** & & **(dBi)** \\ \hline
[42] & RDRA & 3.67–4.73 & 3.67–4.4 & 19.9 & 10 & \
\begin{tabular}{c} Metal \\ feeding \\ strip \\ \end{tabular} & 10 &
\begin{tabular}{c} H-shaped \\ conformal \\ metal strip \\ \end{tabular} & 6.8 & \(\text{TE}^{\text{x}}_{\text{133}}/\text{TE}^{\text{y}}_{\text{183}}\) & NM \\
[43] & CDRA & 2.97–4.52 & 3.07–4.15 & 29.91 & 9.8 & \begin{tabular}{c} Microstrip \\ line \\ \end{tabular} & 2.84 & \
\begin{tabular}{c} ground \\ plane \\ \end{tabular} & 2.84 &
\begin{tabular}{c} HEM\({}_{\text{115}}\) \\ 94.69 \\ \end{tabular} \\
[44] & L-shaped & 2.9–3.8 & 2.97–3.35 & 42 & 30 &
\begin{tabular}{c} Microstrip \\ line \\ \end{tabular} & 3.4 & NM & 93 \\
[45] & Truncated & 2.08–2.98 & 2.31–2.72 & 17.75 & 6.6 &
\begin{tabular}{c} Coaxial \\ feed \\ \end{tabular} & 5.5 & \(\text{TE}_{\text{811}}\) & 70 \\
[46] & CDRA & 4.3 & 4.2–4.55 & 8.3 & 9.8 & \
\begin{tabular}{c} Coaxial \\ feed \\ \end{tabular} & 9.8 &
\begin{tabular}{c} Stacked \\ DRA \\ \end{tabular} & 5 & NM & NM \\
[47] & CSDRA & 4.45–7.3 & 5.47–6.37 & 16.45 & 9.8 & \
\begin{tabular}{c} Coaxial \\ feed \\ \end{tabular} & 9.8 &
\begin{tabular}{c} Factal \\ geometry \\ \end{tabular} & 5.8 & TM\({}_{\text{415}}\)/TM\({}_{\text{2v15}}\) & 80–94 \\
[48] & HDRA & 4.16 & 4.2–5.3 & 20.9 & 4.3 & \
\begin{tabular}{c} Coaxial \\ feed \\ \end{tabular} &
\begin{tabular}{c} Fractal \\ geometry \\ \end{tabular} & 6.38 & NM & 93 \\
[49] & RDRA & 10–13 & 10.4–11.44 & 10 & 10 & \begin{tabular}{c} Aperture \\ Coupling \\ \end{tabular} & 10.2 & \
\begin{tabular}{c} Two-layer \\ RDRA \\ \end{tabular} & 11.1 &
\begin{tabular}{c} TE\({}_{\text{11,11}}\) \\ \end{tabular} & NM \\
[50] & CDRA & 20.1–28.2 & 20.3–26.45 & 30.3 & 10.2 & \
\begin{tabular}{c} Aperture \\ Coupling \\ \end{tabular} & 10.2 &
\begin{tabular}{c} Cross-slot, \\ substrate \\ integrated \\ \end{tabular} & 8.15 & \(\text{HE}_{\text{115}}\)/\(\text{HEM}_{\text{125+1}}\) & NM \\
[51] & SDRA & 2.22–3.5 & 2.22–3.72 & 67.57 & 10 & \
\begin{tabular}{c} Aperture \\ Coupling \\ \end{tabular} & 10 &
\begin{tabular}{c} Metal plate \\ \end{tabular} & 4.73 & \(\text{TE}_{\text{111}}\) & 97 \\
[52] & Stacked & 19.7–21 & 19.8–20.8 & 5.2 & 2.2 & \begin{tabular}{c} Aperture \\ coupling \\ \end{tabular} & 10.2 & \begin{tabular}{c} Aperture \\ DRA, \\ slot-dipole \\ \end{tabular} & 6.6 & \
\begin{tabular}{c} Shared- \\ aperture \\ \end{tabular} & 6.6 &
\begin{tabular}{c} TE\({}_{\text{111}}\) \\ \end{tabular} & NM \\
[52] & RDRA & 27.5–31.2 & 28.7–29.9 & 4.1 & 10.2 & \begin{tabular}{c} Aperture \\ coupling \\ \end{tabular} & 10.2 & \
\begin{tabular}{c} Aperture \\ DRA, \\ slot-dipole \\ \end{tabular} & 8.2 &
\begin{tabular}{c} Aperture \\ DRA, \\ slot-dipole \\ \end{tabular} & 10 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Comparison between different single-fed circularly polarized DRAs. etry can be employed for bandwidth enhancement. Moreover, the number of the array elements can be increased to enhance the gain. A polarization reconfigurable cylindrical DRA with tunable feed network is presented in [57]. The feed network comprises of Wilkinson power divider, two pairs of shifters and four pairs of PIN diodes, which generates two-way signals of equal amplitude and phase or quadrature phase to achieve LHCP, RHCP, or LP states. However, the measured impedance bandwidths are slightly different than the simulated one due to the influences in the dc biasing network and errors in the fabrication process. In [58], a dual CP DRA with a much wider AR bandwidth is offered for electromagnetic energy harvesting.