## 6 Conclusions  
This review article is mainly focused on the current development of CP DRAs and its applications. In recent years, tremendous efforts have been made on investigating the LP wideband DRAs and several methods have been proposed. However, there are not many studies about CP DRAs, especially for radar and UAV applications. Different methods in achieving CP DRAs that are emphasized in this article are intended to bring ideas for future research to develop high-performance methods for generating circular polarization waves. This article indicates that CP antennas are the best option for combating the drawbacks of LP systems such as polarization mismatch, fading effects, faraday rotation effects, and inclement weather conditions. However, most of the CP antennas suffer from a narrow bandwidth. A multilayer or stacked DRA with different sizes and dielectric materials is often used to enhance the bandwidth of the antenna. However, this method will increase the antenna size and is not suitable for several applications. Another method is by using special-shaped DRAs, but these DRAs may be difficult to obtain on the market. Several feeding configurations can also be used to obtain a wideband DRA such as by using a SP feeding network with a sequential rotation DR. This method also manages to reduce the size of a conventional antenna. As most of the applications require a compact size, high efficiency, and low loss, various shapes of DRAs are proposed and some of the design approaches are discussed. A dielectric resonator element has a low metallization and can increase the gain and radiation efficiency of the antenna while maintaining a circular polarization response with a wider axial ratio bandwidth. Additionally, a good impedance bandwidth and gain can be realized by introducing a thin air gap between the DR and the ground plane. Moreover, a thin air gap is easier and more practical to apply compared to a wide air gap. However, the impedance bandwidth is limited to 18.8% only. From our point of view, it can be concluded that an array DRA with a cross slot in the ground plane is the best method to generate a CP DRA. This design is simple and easy to obtain orthogonal modes by adjusting the length of the slots. Moreover, an array antenna exhibits a high signal strength and high directivity. Additionally, the feeding network also plays an important role in ensuring good antenna performances. This is because the feeding network will control and generate a phase difference to enable the CP wave and has the benefit of improving impedance and axial ratio performances. Thus, designing a suitable feeding network is important for obtaining a stable phase difference. As mentioned earlier, numerous works on ceramic DRAs have been performed at present. However, the main challenge for implementing ceramic DRAs is the hardness of the materials, which makes it difficult to fabricate. Therefore, new materials which are soft and flexible need to be proposed to replace the traditional ceramic materials such as by using polymer-based materials, water, etc. The information given in this review paper is useful for researchers who are working on CP DRAs. Conceptualization, N.A.A.R. and M.N.M.Y. ; methodology, N.A.A.R. ; writing--original draft preparation, N.A.A.R. ; writing--review and editing, N.A.A.R. ; M.N.M.Y., S.K.N. and M.J.; supervision, M.R.E.M., N.Z. and N.N. ; funding acquisition, M.N.M.Y. and I.M.I. All authors contributed equally to this work. All authors have read and agreed to the published version of the manuscript. This research was funded by publication incentive grant from the Universiti Malaysia Perlis (UniMAP). This research also was funded by Ministry of Higher Education and Universiti Teknikal Malaysia Melaka (UTeM) through Fundamental Research Grant Scheme (FRGS) Grant FRGS/1/2020/TK0/UTEM/02/65 and UTeM Grant JURNAL/2020/FKEKK/ /Q00057. Not applicable. The authors would like to acknowledge the financial support in the form of a publication incentive grant from the UniMAP.