## 2 Dielectric Resonator Antenna
### Dual- or Multiple-Feed CP DRA
As shown in Figure 13, a rectangular DR, composed of two microwave dielectric ceramics, is employed on top of a series feeding structure. The feeding structure consists of four intersected slots, a microstrip ring with series feeding lines, and two matching microstrip lines. The measured results indicate that the antenna has a 3 dB AR bandwidth of more than 43.5% over the operating band of 1.83-2.85 GHz and peak radiation efficiencies of about 90%. The DRA generates RHCP fields in the direction of broadside when port 1 is excited, and LHCP fields in the direction of broadside when port 2 is excited due to the symmetrical structure. Thus, this antenna is classified as a dual CP DRA. However, a slight difference between the simulation and measurement results is recorded due to the adhesive and air gap between the DR and the substrate. Moreover, errors during fabrication and measurement need to be avoided to achieve the best results. The summary of previous works on various dual or multiple feed DR and techniques to generate circular polarization is presented in Table 3. It can be concluded that the CP bandwidth increased  
Figure 11: Proposed bowtie DRA: (**a**) 3D view; (**b**) fabricated DRA [28]. Figure 12: Fabricated RDRA array: (**a**) top plane; (**b**) bottom plane with eye slots; (**c**) bottom plane with dielectric resonators mounted over the eye slots [56]. and was almost the same as the impedance bandwidth depending on the properties of the phase shifter used. The feeds will stimulate the dual orthogonal modes for obtaining circular polarization. The advantages and disadvantages of different feeding configurations are summarized in Table 4. Using a metal feeding strip and microstrip feeding is the easiest way and simplest method to feed the antenna. However, the bandwidth of the antenna is frequently affected and dependent on the thickness and dielectric permittivity of the substrate. For a coaxial probe feed, the soldering and drilling process during fabrication is difficult and requires extra care to make sure the conductor tracks on the printed circuit board (PCB) are not damaged. The antenna design will become complex if aperture coupling is used because it requires multilayer structures and the alignment is very important for good input matching [33]. To generate a CP DRA, sequential rotation of DR with a SP feeding network is a good option as it has a simple structural geometry; thus, it is easy to design and fabricate [59; 60; 61; 62]. Through this method, a stable phase difference is produced at the ports of the feedline, resulting in the CP characteristics. Moreover, this technique allows the size of the antenna to be reduced. Figure 13: Proposed broadband dual CP DRA: (**a**) 3D view; (**b**) feeding network [58]. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Ref.**} & \multicolumn{2}{c}{**Basic Geometry**} & \multirow{2}{*}{\(f_{r}\) (GHz)} & \multicolumn{2}{c}{**CP BW**} & \multicolumn{2}{c}{**CP BW**} & \multirow{2}{*}{\(\epsilon_{r}\)} & \multirow{2}{*}{**Feed**} & \multirow{2}{*}{**Techniques**} & \multicolumn{2}{c}{**Gain**} & \multirow{2}{*}{**Mode**} & \multirow{2}{*}{**Eff. (\%)**} \\  & & & **(GHz)** & & **(\%)** & & & **Type** & & **(dBi)** & \\ \hline
[28] & Bowtie- & 1.95–2.56 & 1.95–2.56 & 31.3 & 10 & Microstrip & Sequential & 3.82 & TE\({}_{111}^{\rm x}\)/TE\({}_{111}^{\rm y}\) & 83.9 \\  & DRA & 2.93–3.33 & 2.93–3.33 & 13.65 & & line & rotated & 8.4 & 82 \\
[56] & RDRA & 5.769–5.913 & 5.766–5.911 & 2.48 & & & & & & 82 \\  & 5.788–5.922 & 5.788–5.922 & 2.28 & & & & & & 82 \\
[57] & CDRA & 2.23–3.22 & 2.18–3.07 & 40.83 & 9.9 & Probe & Tunable feed & 6.44 & HEM\({}_{115}\) & NM \\
[58] & RDRA & 1.7–3 & 1.83–2.85 & 55.74 & 9.2 & Series & Orthogonal & 8.4 & NM & 85 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Comparison between different dual- or multi-feed circularly polarized DRAs. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Feeding Type** & **Fabrication** & **Reliability** & \