# A Low-Profile Dielectric Resonator Antenna Array for OAM Waves Generation at 5G NR Bands
## 1 Introduction
Moreover, the dielectric plate with high weight generally brings additional losses and reduces the antenna efficiencies. Another popular method to generate OAM waves is through the concept of a parabolic reflector antenna. Parabolic reflector antenna offers high mode purity, gain, and directivity. Moreover, the design is simple. However, there are several disadvantages such as its limitations in generating multiple OAM modes and the feeder blocking the signal. Moreover, the parabolic antenna is difficult to fabricate and integrate with other elements, and its efficiency is low. Metasurface is a promising candidate for generating OAM waves due to its unique advantages such as its artificial material, ease of fabrication, absence of complex feed networks, and capability of designing different scales according to the application. The metasurface controls the wavefront of EM waves by adjusting the phase of the incoming waves to generate OAM beams. However, most metasurface designs have large dimensions, which are not suitable since most current devices are compact. In addition, an additional resource such as a horn antenna is required to transmit the incident waves, and this antenna is placed in front of the metasurface, which can block the reflected OAM waves. The OAM waves can also be generated by using the concept of UCA antenna. UCA is widely used due to its advantages such as great adaptability, capacity to generate multiple OAM modes, compact size, and low cost. A UCA antenna consists of equal antenna elements which are organized uniformly in a circle. The radiating elements are excited with the power of the same amplitude but in different phases. This method can generate multiple OAM modes, and the purity of the OAM mode can be easily increased. However, the spacing among the adjacent radiating elements must be at least 0.7\(\Lambda\) to prevent mutual coupling; thus, the antenna tends to be bigger at lower frequencies. Moreover, the misalignment of radiating elements will distort the spiral phase distribution. Recently, numerous studies have been carried out to generate OAM waves using a dielectric resonator antenna (DRA) [32; 33]. DRAs have several advantages such as compact size, low cost, high radiation efficiency, and flexibility toward various feed lines. Moreover, DRAs have low loss compared to microstrip patch antenna since they do not have any metallic surfaces such as radiating elements. OAM waves can be generated by using different types of DRAs, such as array, cylindrical, and hemispherical. However, not many works present the generation of OAM vortex waves with mode +1. In [32], the authors designed a water spiral DRA to generate multimode OAM waves. The shell of the spiral DRA filled with distilled water is designed as a cylinder with a spiral paraboloid on the top side and fed with two orthogonal signals at the bottom side to excite higher-order modes. By changing the radius, height, and notch height of the spiral DRA, \(l=\pm 1\) OAM modes can be generated at 4.8 and 5.6 GHz, and \(l=\pm 3\) OAM modes can be generated at 5.11, 5.66, and 6.36 GHz, respectively. Although the measurement results proved that OAM waves were generated based on the spiral phase distribution, the purity of the generated OAM mode was not discussed. In [33], an antenna array with 8 slots and coupled cylindrical DRAs fed by microstrip lines was proposed to generate mode +1 and mode +2 at 3.5 GHz. However, the obtained gain value and mode purity were not disclosed in the paper. To the authors' best knowledge, this paper is unprecedented in proposing a rectangular DRA with UCA-based OAM characteristics which is specifically designed for 5G NR applications. Section 2 elucidates the antenna design and its operation mechanism. The effect of array arrangement on compactness is also discussed in this section. The experimental analysis of the proposed antenna in terms of reflection coefficient, radiation pattern, gain, phase distribution, 2D polar form, intensity distribution, and mode purity are discussed in Section 3.