## 2 Dielectric Resonator Antenna
### Single-Feed CP DRA
The concept of PMC boundary approximation is used to generate a circular polarization wave in a dielectric resonator by changing the air-dielectric interface geometrical profile so that the E-fields change their route by disintegrating their components at dissimilar angles following the applied adjustment. The geometrical shape of a cylindrical dielectric resonator has been deformed periodically with the same angular factor (\(\beta=45^{\circ}\)) along the azimuthal direction, as shown in Figure 7. A coaxial probe is applied to stimulate the dielectric resonator fed through a small hole in the ground plane. The proposed DRA exhibits a 51.2% impedance bandwidth, ranging from 3.37 to 5.69 GHz and an 8.01% AR bandwidth from 4.2 to 4.55 GHz. The main advantage of this probe feed is it can be placed at any preferred location to match with its input impedance. The input impedance of the DRA can be tuned and the resonance frequency can be controlled by optimizing the length and position of the feeding probe. This feed type provides high coupling to the DR which, in turn, results in a high radiation efficiency. However, it has a narrow bandwidth of 2-5% and is hard to fabricate because a hole has to be drilled in the substrate, and the connector should protrude outside the ground plane. The dimensions of the drilled hole need to match the dimensions of the probe otherwise the effective dielectric constant of the resonator will be affected, thus the resonance frequency of the antenna will be shifted. In [48], a new converging fractal geometry is proposed for C-band applications. Fractal structures are created by using recursive procedures which produce large surface areas in limited space. Thus, the geometries and dimensions of fractal structures are essentially the deciding factor for the operation of resonant frequencies [54]. The self-similarity of fractal shapes is achieved by applying the infinite number of iterations. A fractal antenna offers size miniaturization by increasing the effective electrical length in the limited space. The perturbations introduced in the hemispherical DRA (HDRA) through the application of a fractal shape provide an enhanced impedance bandwidth because the removed DR reduces  
Figure 6: Proposed CP reconfigurable LDRA [45]. the Q factor with each step of iteration. However, it is a challenge to apply fractal geometry on HDRA because it offers a zero degree of freedom. Aperture coupling is also one of the most commonly applied feeding techniques for CP DRAs. It is an indirect method, where the input signal is coupled to the radiating elements through the aperture (slot). In [49], a 50 \(\Omega\) microstrip feedline is used and printed on the backside of the Rogers RO4535 substrate, as indicated in Figure 8. The proposed antenna operates in the TE\({}_{11,11}\) higher-order mode from 10 to 13 GHz. The unequal slots are engraved on the copper ground plane with an angle of 45\({}^{\circ}\) concerning the microstrip feedline to excite two near-degenerate orthogonal modes of equal amplitude and a 90\({}^{\circ}\) phase difference for circular polarization generation. By joining a higher-order mode DRA, an outer dielectric layer and a cross-slot excitation, and broader impedance and CP bandwidths of \(\sim\)21% and 9.5%, respectively, together with an improved gain of \(\sim\)11 dBic, are achieved. However, this antenna suffers from a slight drop in the measured gain due to the presence of the uneliminated air-gap spots between the DRA and the dielectric coat. A wideband CP DRA using a single feed can be obtained using parasitic vertical plates as proposed in [51]. By presenting four metallic plates around the dielectric resonator, a very wide AR bandwidth can be generated with the contribution from the cross-slot-fed DRA, the revolved metallic plates, and the interaction among the dielectric resonator and metallic plates. The AR bandwidth of the conventional DRA is successfully enhanced from \(\sim\)10% to \(\sim\)46.9% without increasing either the dimension or height of the antenna.