## 3 Discussion of Experimental Result
### Antenna's Efficiency  
In reliable wireless communication systems, the antenna's efficiency is a significant feature. The impedance bandwidth and gain could be directly limited by this feature. Several single-feed MPA designs have already been proposed. Microstrip lines fed the majority of them. The designs based on microstrip lines present a feasible structure in which the feeding network production is easier while maintaining a low profile. However, as the antenna's operating frequency rises, the amount of unwanted energy transmission from these feeding lines rises dramatically. This unwanted energy transmission has a significant negative impact on the radiation pattern, resulting in a reduction in radiation efficiency. Antenna radiation efficiency is described by IEEE standards as \"the ratio of the total power radiated by an antenna to the net power accepted by the antenna from the connected transmitter\" [44]. Dielectric losses, conductor losses, and unwanted radiation from the feeding network structure are some important aspects that contribute to the reduction of radiation efficiency in a PCB-based propagating element. The effect of the dielectric losses can be diminished by the use of new low-loss tangent dielectrics [45]. This strategy can improve the antenna's radiation efficiency to some extent, but if the antenna's feeding network has unwanted radiation losses, the antenna's radiation efficiency will be significantly reduced. This unwanted energy transmission may have negative effects on the antenna's far-field pattern along with a growth in cross-polarization level, in addition to lowering antenna efficiency. The radiation efficiency of large arrays is primarily affected by unwanted radiation and losses from the combinatory feeding structure (such as in parallel feeding topology). It is possible to achieve higher radiation efficiency when minimizing the length of the feeding networks, such as those in series feeding [30; 46]. It does, however, usually have a narrow frequency bandwidth, and it can distract the main beam propagation features. The proposed array's maximum simulated radiation efficiency is 93%. The proposed design has an aperture dimension of 36 \(\times\) 36 mm\({}^{2}\), and it has a maximum realized gain of 11 dBi at 13.5 GHz frequency. Figure 5: _Cont._  
Figure 5: Simulated and measured radiation pattern of the proposed array for different frequencies of (**a**) 10.55, (**b**) 12.5, and (**c**) 14.25 GHz. (**d**) Simulated cross-polarization level of the proposed array versus \(\theta\) and \(\phi\) angles. In terms of frequency bandwidth, the proposed formation presents wideband radiation elements like the magneto-electric dipole [47], L-probe-fed patch array [48], U-shaped parasitic patches [49], and slot antenna [50]. Although using low temperature co-fired ceramic (LTCC) substrates to implement multi-layer broadband antennas is a viable option, the high cost of this technology keeps it from being widely adopted [51; 52]. A single-element X-band patch antenna is presented in [53] and fed by the SIW through a slot on its top wall. This structure has an impedance bandwidth of 8.8%. This single element antenna has a potential use in array applications. It appears that two approaches are available for designing a larger array from a single element in [53]: (1) using a series formation of the feeding structure, such as the one described in [38]. (2) Making use of a corporate feeding network like [54]. According to studies, a broad impedance bandwidth cannot be attained when a series feeding structure is used to design a larger array formation. Reference [38] shows a 6 \(\times\) 8 patch array fed by an SIW standing wave series feeding structure. The structure describes a proper radiation routine, but it only has a 1.8% operating bandwidth. A 2 \(\times\) 2 array of microstrip patch antennas is described in [54], which is fed by an SIW corporate feeding network. In fact, four single elements (like the formation presented in [53]) were used to create this array.