## 3 Discussion of Experimental Result
### Radiation Characteristic of the Proposed Structure  
The appearance of robust cross-polarized fields in the radiation characters of patch antennas is always a fundamental challenge for antenna designers. In the conventional patch antennas, excitation of higher modes with orthogonal polarization causes cross-polarized fields in the H-plane. A cross-polarized field in the H-plane is stronger than in the E-plane. Studies show that in the H-plane of patch antennas within the \(\pm 25^{\circ}\) beamwidth, the isolation between the co- and cross-polarization radiation is around 10-20 dB [37; 39].  
The EM radiation patterns of the proposed MPA are discussed in this section. The work is mainly focused on the study of cross-polarized fields. When investigating the cross-polarization behavior in the antennas, an important point must be considered. Only checking the isolation between co- and cross-polarization patterns in the two main cutting planes of E- and H- is not sufficient to confirm that an antenna has a low cross-polarization level. In antenna characteristics, the cross-polarization level could be extremely low in both the E- and H-planes, while on the other cutting planes, the cross-polarization level could be extremely high. This behavior can exist in various types of antennas. The distribution of the electric fields on the horn antenna's aperture clearly indicates that on the E- and H-planes, the cross-polarization level can be extremely high. This is a consequence of the fact that the cross-polarization level can be achieved by the high-gain antenna.  
Figure 4: Simulated and measured S\({}_{11}\) and the realized gain of the fabricated patch array.  
cutting plane, the cross-polarization level is minimal, while on the other orthogonal cutting planes, strong cross-polarization radiation is observed [40]. More rigorous studies on cross-polarization in microstrip patch antennas indicate that the isolation between co- and cross-polarizations in the E-plane is at its minimum, while its maximum occurs on cutting plane \(\phi=70^{\circ}\), somewhere between the E- and H-planes [41; 42; 43]. Therefore, investigating cross-polarization fields only on the E- and H-cutting planes can cause misjudgments concerning cross-polarization behavior on antennas.  
In this paper, in order to accurately investigate cross-polarization behavior, as well as ensure its low level on all the planes, contour plots are presented. These contour plots indicate the cross-polarization level on different cutting planes. Then, the simulated and measured radiation patterns of the proposed antenna on the cutting planes \(\phi=0^{\circ}\) (E-plane) and \(\phi=90^{\circ}\) (H-plane) are presented. Figure 5d displays the simulated cross-polarization levels of the proposed array versus \(\theta\) and \(\phi\) angles at 10.55, 12.5, 14.25 GHz. These charts are drawn for a \(120^{\circ}\) (\(-60^{\circ}\leq\theta\leq 60^{\circ}\)) beamwidth. In the contour plots, the area specified with a dashed line indicates half-power beamwidth (HPBW). The contour plots confirm that cross-polarization levels at HPBW in the whole operating bandwidth and in all \(\phi\) cutting planes are less than 30 dB. Figure 5 also shows the simulated and measured normalized radiation patterns of the proposed structure at 10.55, 12.5, and 14.25 GHz.