## 3 Discussion of Experimental Result
### State of the Art Aspects  
A comparison between various characteristics of the proposed design and previously reported results in terms of the number of layers, antenna size, operation frequency, impedance bandwidth, and cross-polarization level are provided in Table 1. Differential feeding methods have been studied in literature to suppress cross-polarized fields in MPAs [34; 58]. Although the use of these methods effectively reduces the cross-polarization radiation, the bandwidth of these structures is still narrow. In addition, the implantation of some of these methods needs an additional number of excitation ports, which leads to an increase in the measurement costs. In [36], a 4 \(\times\) 4 microstrip patch antenna array is reported using the PF method to excite radiating elements. A PF geometry consists of a metal pin and a surrounding ring slot. By changing the diameters of the ring slot and metallic pin, the reactance and conductance \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Ref.**} & **Number of** & **Number of** & **Aperture Size** & **Max Gain** & **Frequency** & **Impedance** & **X-PoL within** \\  & **Layers** & **Ports** & **(\(\lambda^{3}\))** & **(dB)** & **(GHz)** & **Bandwidth (\%)** & **HPBW (dB)** \\ \hline [2] & 7 & \(1\) & \(12.16\times 13.6\times 0.82\) & \(30.1\) & \(57.5\)–\(67\) & \(15.3\) & \(-50\) \\ \hline [4] & 2 & \(1\) & \(8.16\times 2.28\times 0.13\) & \(23.2\) & \(14.5\)–\(17.4\) & \(18.2\) & \(-30\) \\ \hline [21] & 3 & \(1\) & N. A. & \(19\) & \(23.2\)–\(24.8\) & \(6.67\) & \(-40\) \\ \hline [34] & 3 & 4 & N. A. & \(12.89\) & \(9.77\)–\(10.62\) & \(8.3\) & \(\sim\)\(-25\) \\ \hline [36] & 4 & \(1\) & \(2.49\times 2.29\times 0.24\) & \(17.1\) & \(11.2\)–\(13.6\) & \(19.36\) & \(-17\) \\ \hline [39] & 3 & \(1\) & \(1.36\times 1.36\times 0.32\) & \(11.1\) & \(9.35\)–\(10.25\) & \(9.18\) & \(-40\) \\ \hline [43] & 4 & \(1\) & \(2.25\times 2.25\times 0.48\) & \(16.3\) & \(9.4\)–\(10.7\) & \(12.9\) & \(-40\) \\ \hline [51] & 4 & \(1\) & \(2.95\times 3\times 0.37\) & \(18.6\) & \(56.3\)–\(66.3\) & \(11.7\) & \(\sim\)\(-25\) \\ \hline [58] & 2 & \(1\) & \(1.73\times 1.73\times 0.1\) & \(12.3\) & \(12.2\)–\(13.8\) & \(12.8\) & \(\sim\)\(-17\) \\ \hline
\begin{tabular}{c} This \\ Work \\ \end{tabular} & 3 & \(1\) & \(1.48\times 1.48\times 0.16\) & \(11\) & \(10.55\)–\(14.25\) & \(29.8\) & \(-30\) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Comparison between previously reported antennas and the proposed antenna. can be controlled to achieve a broader impedance matching. Applying the PF method has resulted in a wide operating bandwidth of about 19% for this structure. However, this structure suffers greatly from high cross-polarization levels. So, this structure has a serious limitation to use in some practical applications. In addition, this multi-layer structure has a number of pin couplings and blind screws for internal connection between layers. The fabrication process of this structure is complex, and its assembly requires high precision. In [39], the AP method is used to excite the patch elements. Applying the AP method causes a uniform distribution of EM fields on the radiation elements, and consequently, the level of cross-polarization is much lower than that of the PF method. Nevertheless, since MPAs are inherently narrowband, the AC method does not have a significant effect on increasing the impedance bandwidth. In [43], a two-layer power distribution network is utilized to suppress cross-polarization fields in a \(4\times 4\) patch array antenna. The feeding network of this array operates by changing the propagation mode from TE\({}_{20}\) to TE\({}_{10}\). The impedance bandwidth of this five-layer antenna is 12.9%. The used feeding network in this structure is complex. Moreover, the number of PCB layers will be increased by developing this structure to a large array. There are advantages of the proposed structure can overcome many of the challenges associated with previous designs.