## 2 Dielectric Resonator Antenna
### Single-Feed CP DRA  
The single-feed CP DRA has a simple configuration because no hybrid coupler is required to yield the 90\({}^{\circ}\) phase difference between the orthogonal degenerate modes [40]. A single-feed DRA achieves circular polarization by exciting two quasi-degenerate modes in the DR which are typically in phase quadrature and spatially orthogonal to each other. Usually, a single-feed CP DRA has a narrow AR bandwidth compared to the dual or multiple feeds type [41]. The AR bandwidth can be enhanced by a slight modification in the DR's shape, by loading a metal strip onto the DR, and adjusting the feed structure. Metal strip [42], microstrip feedline [43; 44], coaxial probe [45; 46; 47; 48], and aperture coupling [49; 50; 51; 52] methods are commonly used in single-feed antennas. The selection of the feed arrangement may depend on the antenna's application. A microstrip feedline is very easy to fabricate because the feed is engraved on a similar substrate. However, excitation of the DRA using a microstrip feedline is very difficult, especially when the dielectric resonator has a low dielectric constant. When the dielectric constant of the DRA is equal to or more than 20, a high degree of coupling is realized because a high dielectric constant material will concentrate the field under the DRA. Thus, a high dielectric constant material is required at the bottom layer when the microstrip line is used to feed the structure [53]. Conversely, increasing substrate thickness will increase the spurious feed radiation and surface waves. However, this resulted in the bandwidth deterioration of the antenna and led to unwanted cross-polarized radiation. A single-feed CP rectangular DRA has been designed in [42]. The proposed antenna is excited by using a H-shaped conformal metal strip, as shown in Figure 5. This proposed configuration does not require any multi-layering or any complex cutting of the DRA to obtain a wide impedance bandwidth of ~27.7% from 3.67 to 4.73 GHz and AR bandwidth of ~20% from 3.67 to 4.4 GHz. With a great gain of ~6.8 dBic, this antenna is suitable for worldwide interoperability for microwave access (WiMAX) and satellite applications. However, it is challenging to obtain a wide bandwidth and a good axial ratio because numerous modifications on a H-shaped feeding strip need to be performed. This will require extra time and work. In addition, a coaxial probe can be used to feed a DRA. The connector's inner conductor extends through the dielectric and is soldered to the radiating elem  
Figure 5: A singly-fed rectangular DRA: (**a**) proposed configuration; (**b**) fabricated [42]. conductor is connected to the ground plane. In [45], two similar truncated corner liquid dielectric resonators (LDRs) are fed by a single coaxial probe for stimulating the essential TE\({}_{S11}\) mode of the LDR antenna, as presented in Figure 6. By changing the dimension of the truncated corner rectangular dielectric resonator and the probe's position, a 90\({}^{\circ}\) phase difference between the orthogonal resonant modes is realized, resulting in CP radiation. The proposed antenna will obtain left-hand circular polarization (LHCP) if the ethyl acetate solution is inserted into the left side zone of the container and right-hand circular polarization (RHCP) if the solution is inserted into the right side zone of the container. A 35.6% operating bandwidth is recorded from 2.08 to 2.98 GHz, covering the broad 3 dB AR bandwidth of 16.3%, ranging from 2.31 to 2.72 GHz. However, the permittivity of the container will affect the impedance bandwidth and AR of the antenna. In addition, the antenna has a lower radiation efficiency due to the presence of the PIN diodes. Thus, a liquid with a suitable dielectric constant and low loss is required to increase the radiation efficiency. In [46], a simple cylindrical-shaped DRA is proposed.