## 5 Applications of DRA
### Radar  
Radar is a detection system that uses radio waves to measure distances, scattering parameters, or angles velocities. It is widely used in autonomous driving, security, defense, meteorology, and many more applications. Radar should have a high resolution, high efficiency, low false alarm rate, and small sensor dimensions in order to cope with various conditions depending on its application. \begin{table}
\begin{tabular}{c c c c c c c c} \hline
**Ref.** & **Geometry** & \(f_{r}\) **(GHz)** & **BW (\%)** & \(\epsilon_{r}\) & **Gain (dBi)** & **Feed Type** & **Polarization** \\ \hline
[89] & Cylindrical DR & 0.86–0.92 & 6.74 & 100 & 1.61 & Discrete port & LP \\
[90] & Cylindrical DR & 0.85–0.9 & 5.71 & 506 & 7.5 & Discrete port & LP \\
[91] & Cylindrical DR & 3.5–4.5 & 25 & 35 & NM & Coaxial port & CP \\  & Crossed & & & & & & \\
[92] & split-ring & 0.907–0.914 & 0.77 & NM & 5.51 & Coaxial port & CP \\  & resonator & & & & & & \\
[93] & Disk-type & 4–7 & 54.55 & 35 & 14 & NM & LP \\  & cylindrical DR &
\begin{tabular}{c} Between \\ 100–200 \\ \end{tabular} & NM & 35 & 16.7 & NM & LP \\ \hline \end{tabular}
\end{table} 
Table 8: Comparison between various DRAs for RFID. Figure 22: CP antenna with C-DRRs: (**a**) fabricated prototype; (**b**) reflection coefficient [92]. The authors in [95] proposed a rectangular DRA planar array with a broad bandwidth of 50% and a high gain of 22 dBi for unmanned aerial system radar applications. The \(8\times 8\) array is designed using a stripline feeding network with two different substrates to improve shielding of a multi-stage feeding network. A planar array also being used in [96]. The proposed rectangular DRA obtains an impedance bandwidth of 6.98% centered at 24.09 GHz, with the antenna gains varying from 7 to 9.5 dBi. However, the spacing between the antenna elements should be proper and in exact phase to form a good radiation beam and enhance the directivity of the system. In addition, the feeding network of an array requires a large area to occupy multiple segments of transmission line. This will cause the antenna size to be larger. Moreover, the proposed antennas in [95; 96] radiate in linear polarization. For an antenna with linear polarization, the electromagnetic waves broadcast on a single plane are either vertical or horizontal. If the signal is reflected inaccurately, interference will occur and the signal will lose its strength. For conventional radar systems, it is tough to deal with varied scenes and to accurately identify and classify targets. Therefore, the radar sensor has to transmit and receive multiple polarizations that are affected differently by the observed scene. In [97], a four-element cylindrical DRA array with a simple dielectric fixing slab and glue is proposed for 24 GHz automotive radar, as shown in Figure 23. A conventional one-to-four microstrip power divider feeds the proposed antenna. Unevennes and air gaps need to be avoided by applying necessary and unnecessary glue during fabrication and assembly processes to mitigate the possible problem of glue spreading. In [98], two antennas, element and array antennas, have been fabricated and investigated. The proposed antennas employ a quadrature hybrid structure to excite two perpendicular HEM modes for achieving CP radiation. The hybrid structure is integrated with a cylindrical DR, as shown in Figure 24. Based on the simulated and measured results, these antennas are good candidates for X-band radar applications such as weather monitoring and vehicle speed detection. However, there are certain conditions that need to be satisfied in order to generate the appropriate CP wave. First, the feeding points should be placed perpendicularly to realize the two perpendicular HEM modes. Next, equal amplitude modes must be generated between the two paths. After that, a 90\({}^{\circ}\) phase difference between output ports should be achieved by ensuring a quarter wavelength difference between the two paths. Lastly, a 50 \(\Omega\) input impedance is crucial to get good matching.