# A Low-Profile Dielectric Resonator Antenna Array for OAM Waves Generation at 5G NR Bands
## 2 Design Methodology  
The geometry of the proposed DRA is illustrated in Figure 1. Four rectangular DRs are placed on top of a feeding structure. The DR is made of microwave dielectric ceramics (\(Al_{2}O_{3}\)) with a dielectric constant \(\varepsilon_{r}\) of 10. The size of the DR is \(W_{dr}\times L_{dr}\times L_{dr}\) = 22 mm \(\times\) 22 mm \(\times\) 32 mm. The proposed DRA is designed and fabricated on an FR-4 substrate with a dielectric constant of 4.3, loss tangent \(\tan\delta\) of 0.025, and thickness \(h\) of 1.6 mm. The overall dimensions of the proposed antenna operating at 3.5 GHz are 185 mm \(\times\) 160 mm (2.16\(\lambda\)\(\times\) 1.87\(\lambda\)). The proposed DRA consists of a copper ground layer with a thickness of 0.035 mm. Four crossed-shaped slots with equal dimensions are inserted in the ground plane for the single-band operation. For this work, a UCA with four elements was designed. For an \(N\)-element UCA, an OAM wave of topological charge \(l\) can be generated by exciting the elements with the power of equal amplitude, with a phase difference of \(\Delta=2\pi l/N\) between adjacent elements. Hence, to generate OAM mode \(\pm 1\) with four dielectric resonators, the phase difference must be 90\({}^{\circ}\) between adjacent antenna elements. The dielectric resonators are placed in a circular arrangement with a radius (\(R\)) of 77 mm. The array radius must be at least 0.7\(\lambda\) to avoid mutual coupling and shift in the resonant frequency. The proposed antenna was designed with a feeding network, as shown in Figure 1b. The feeding network was used to excite the dielectric resonator by supplying similar output energy at output ports with the required phase shift value. Once the antenna was designed and fabricated, parameters such as reflection coefficient, radiation pattern, phase distribution, and intensity distribution were measured with the help of a vector network analyzer and a horn antenna inside an anechoic chamber at Advanced Communication Engineering (ACE) Universiti Malaysia Perlis. After the measured data were collected, Python programming was used for data analysis. The proposed antenna is excited by a single source. Equal power distribution to the elements is realized using a feeding network with T-junction dividers. The desired phase difference between adjacent elements is achieved by optimizing the lengths of the feed lines and the geometry of the feeding network. The feeding network of UCA is shown in Figure 2. Port 1 is the input port, whereas port 2, port 3, port 4, and port 5 are the output ports used to excite DR 1, DR 2, DR 3, and DR 4, respectively. A 90\({}^{\circ}\) phase difference between ports 2 and 3 was obtained by altering the length of the feedline, as illustrated in Figure 2b. Ports 2 and 3 are the mirror image of ports 4 and 5. The mirror image arrangement of the feeding network was adopted for the simplicity of the design. The overall phase difference for port 2, port 3, port 4, and port 5 were \(-97.85^{\circ}\), \(-0.58^{\circ}\), \(-93.36^{\circ}\), and \(-0.22^{\circ}\), respectively, as shown in Figure 3a. The additional \(180^{\circ}\) phase shift due to the mirror image of the feeding network gives port 4 a phase shift of \(86.64^{\circ}\) and port 5 a phase shift of \(179.78^{\circ}\). Thus, the incremental phase shifts for the proposed antenna were \(-97.85^{\circ}\), \(-0.58^{\circ}\), \(86.64^{\circ}\), and \(179.78^{\circ}\). The phase differences were as follows: \(97.27^{\circ}\) between ports 2 and 3; \(87.22^{\circ}\) between ports 3 and 4; \(93.14^{\circ}\) between ports 4 and 5; and \(98.07^{\circ}\) between ports 5 and 1. It has been observed from the simulated result that the obtained values are close to \(90^{\circ}\). Moreover, the amplitudes for all four ports were very similar, as shown in Figure 3b. The amplitudes for port 2, port 3, port 4, and port 5 were \(-9.41\) dB, \(-10.84\) dB, \(-9.23\) dB, and \(-10.46\) dB, respectively.