# Quad-Beam 4 \(\times\) 2 Array Antenna for Millimeter-Wave 5G Applications
## 4 Results and Discussion  
### \(|\)S-Parameter\(|\)  
The design of an array antenna begins with the development of a single-element structure (Ant1) which has compact dimensions of 0.71\(\lambda_{1}\)\(\times\) 0.81\(\lambda_{1}\). Initially, the antenna design resembles a circular patch, but this is modified into a dual-elliptical structure, which leads to an impedance mismatch. To address this, elliptical slots are etched within the structure, reducing the patch area but increasing the path of surface current to \(\lambda_{1}\). This adjustment generates a resonance at 30 GHz. The single-element antenna (Ant1) demonstrates a bandwidth ranging from 30.12 to 30.56 GHz and achieves a gain of 7.8 dBi.  
The antenna's performance is progressively enhanced by expanding Ant1 into a 4 \(\times\) 2 array configuration (Ant4). The proposed array antenna exhibits a quad-beam behavior pattern through which a node device can simultaneously communicate with two other devices in the azimuthal plane and another two in the elevation plane. The distinctive beams forming at an angle of \(\pm\)54\({}^{\circ}\) provide low interference between the communicating devices. However, in the process of achieving quad-beam, the antenna results in a slightly higher side-lobe level at \(\pm\)15\({}^{\circ}\), which could be reduced further by optimally increasing the spacing between the elements or by providing the distinctive phase and amplitude to form the constructive quad-beam with destructive patterns at the center. The spacing larger than optimal may result in undesired lobes, which distort an antenna's performance.  
To obtain a quad-beam, the Ant4 array features non-uniform element spacing along the x-axis, with 0.6\(\lambda_{1}\) spacing between both the first and second rows and the third and fourth rows, while the spacing between the second and third rows is reduced to 0.4\(\lambda_{1}\). This non-uniform spacing separates the elongated beams observed in the previous 2 \(\times\) 2 array (Ant3), and it results in dual beams in the xz-plane. In the y-axis, the spacing is maintained at one wavelength, which preserves the dual beams in the yz-plane like Ant3. The overall dimensions of Ant4 are 2.02\(\lambda_{1}\)\(\times\) 1.82\(\lambda_{1}\). The increase in radiating elements effectively expands the antenna aperture, thereby improving the gain.  
The simulated results are validated through prototype fabrication. The fabricated antenna structure is shown in Figure 17a, with the measurement setup illustrated in Figure 17b. A comparison of the simulated and measured reflection coefficients (\(|\)S111\(\rangle\) indicates that the simulated bandwidth for \(|\)S111\(\rangle\)\(\geq\) 10 dB is 29.81-60.5 GHz, while the measured bandwidth is slightly broader, ranging from 28.5 to 31.1 GHz, as shown in Figure 18. The simulated resonance is at 30.2 GHz, while the measured resonance is at 30.5 GHz. The discrepancies in the simulated and measured results might be due to several factors. One such factor might be the use of an adopter to the interface vector network analyzer (VNA) to the antenna port because the used antenna port is a 2.4 mm  
Figure 16: (**a**) A comparison of the proposed Ant4 reflection coefficient \(|\)S111\(\rangle\) with its previous antenna structures. (**b**) The simulated gain and total efficiency of Ant4.  
SMA connector from Johnson, and its threading dimensions are different than the cable's male connector. Also, other factors like fabrication tolerance and port soldering may have affected the antenna's performance. Despite these differences, the bandwidth of simulated results falls under the measured results, with a shift in resonance of 0.5 GHz.