## 5 Applications of DRA
### RF Energy Harvesting  
RF energy harvesting refers to the utilization of the ubiquitous RF energy transmitted by different wireless systems to feed electronic devices with a low-power consumption remotely. It is widely adopted to replace conventional batteries. Recent growth in wireless technology has led to an enlargement of the existing RF energy in different frequency bands. Thus, a broadband antenna is required to capture the ambient RF energy over this wide frequency range. Moreover, the proposed antenna should also have a high radiation efficiency and high gain to maximize the received RF power from the ambient RF sources. In previous works, numerous planar wideband antennas have been developed to collect RF energy from several frequency bands [81, 82, 83, 84]. However, most of them are unable to obtain the satisfactory gain. Hence, high input power is required to enhance the performance. Given that the ambient RF radiations are weak, unstable, and span over a wide range of frequencies, antennas with a wide impedance bandwidth, high gain, and high radiation efficiency are essential to collect a huge amount of power from the low-density environment. DRAs are proposed in [58, 85, 86, 87, 88] to overcome these problems. Compared to the planar antennas, DRAs offer a wide bandwidth, high gain, high radiation efficiency, low conductor loss, and compatibility to different feeding mechanisms. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline
**Ref.** & **Geometry** & \(f_{r}\)**(GHz)** & **CP BW** & **CP BW (\%)** & \(\epsilon_{r}\)** & **Feed Type** & **Techniques** & **Gain (dBi)** & **Mode** \\ \hline
[76] &
\begin{tabular}{c} Tooth- \\ shaped \\ DRA \\ \end{tabular} & 29.40–34.66 & 29.75–34.15 & 13.77 & 10.2 & PGW & SR method & 8.33 & \(\text{TE}^{\text{A}}_{113}/\text{TE}^{\text{V}}_{113}/\text{TE}^{\text{F}}_{115}\) \\ \hline
[77] & CDRA & 57–64 & 55–64.5 & 17.27 & NM &
\begin{tabular}{c} Aperture- \\ coupled \\ feeding \\ \end{tabular} & SR method & 11.43 & \(\text{HEM}_{115}\) \\ \hline
[78] & \begin{tabular}{c} Flower- \\ shaped \\ DR \\ \end{tabular} & 27–38 & 29.2–30.7 & 5.14 & 10.2 & \
\begin{tabular}{c} Sequential \\ feeding \\ network \\ \end{tabular} & 10.2 &
\begin{tabular}{c} Cross-slot \\ coupling \\ \end{tabular} & 9.5 & NM \\ \hline
[79] & RDRA & 28.6–27.26 & 24.04–26.88 & 23.54–24.04 & 2.12 & 10 & \
\begin{tabular}{c} Microstrip \\ line \\ \end{tabular} &
\begin{tabular}{c} Aperture- \\ coupled \\ slot \\ \end{tabular} & 6.53 & \(\text{TE}_{1y1}\) \\ \hline
[80] & CDRA & 25.1–30.1 & 27.2–29.2 & 7.35 & 7 and 6 & \
\begin{tabular}{c} Coaxial \\ feed \\ \end{tabular} & 7 and 6 &
\begin{tabular}{c} DRA loaded \\ with parasitic \\ dielectric \\ cylinder \\ \end{tabular} & 6.5 & \(\text{TE}_{113}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Comparison between various mm-wave circularly polarized DRAs. Figure 19: Proposed DRA: (**a**) perspective view; (**b**) bottom feed network [78]. A stacked rectangular DRA for RF energy harvesting is proposed in [85]. The DR is made from soda-lime glass. A slot feeding method is used to feed the antenna as it gives a better impedance matching, low spurious radiation, and better polarization purity owing to the indirect electromagnetic interaction between the DRA and the feed line. The DR is stacked at seven layers and a thin air gap is added to improve the gain to 6.69 dB. In the air gap technique, air is used as the substrate in between the ground and DR. Due to the use of air as a substrate radiation efficiency, the impedance bandwidth is enhanced, and the dielectric loss is reduced. However, the impedance bandwidth is limited to 18.8% only. In [86], a broadband high-gain hybrid DRA is proposed, and a 120% impedance bandwidth is recorded from 1.67 to 6.7 GHz with the maximum gain value of 9.9 dBi. The proposed antenna consists of a rectangular DRA backed by a rectangular slot in the ground plane, as shown in Figure 20. The proposed antenna is excited by using an inverted T-shaped feed line which consists of three arms of different lengths.