## 4 Conclusions  
We proposed a novel design of a two-element array antenna on an electrical thick substrate with an extremely narrow array distance, where the patch and monopole radiators had a modal difference in the radiation pattern, causing high isolation and a low correlation between the array elements. The proposed array was fabricated to measure the characteristics of each element. Both array elements provided low reflection coefficients of \(-10\) dB and \(-10.7\) dB for the monopole and patch elements, respectively, at 1.6 GHz; the measured mutual coupling between the two feeding ports was \(-20.7\) dB, and that of the simulation was \(-21.1\) dB. The measured maximum gains obtained in the upper hemisphere were 3.8 dBi and 6.1 dBi for the monopole and patch elements, respectively. The array radiation pattern was calculated by superposing the monopole and patch element patterns with the same weight. By measurement, the peak gains in \(zx\)- and \(zy\)- planes were 4.6 dBi and 2.9 dBi, respectively, while minimum cross-polarization levels of \(-31.62\) dBi and \(-19.3\) dBi in \(zx\)- and \(zy\)-planes were observed. In the spherical mode decomposition of the antenna pattern, the monopole element had the zero mode as the dominant mode, with the negative coefficient values after the second or higher order modes compared to the patch element mostly having the positive coefficients of the spherical harmonic modes. We also proposed the MDM to observe the modal difference between two antennas. MDMs of 0.704, 0.307, and 0.219 were obtained for the proposed array, the patch array,  
Figure 17: RMS null width in terms of JNR of the proposed array compared to the conventional patch and monopole arrays.  
Figure 18: RMSE in terms of the JNR of the proposed array compared to the conventional patch and monopole arrays.  
and the monopole array, respectively. In addition, the proposed array could maintain the low ECC levels under 16% despite decreasing the port spacing between the elements. To verify the proposed array in a practical pattern nulling application, we investigated the antijamming performance using a PI algorithm, and the null depth of \(-47.7\) dB with the null width of \(33.2^{\circ}\) was obtained when the interference signal arrived at the elevation angle of \(45^{\circ}\). In comparison with the conventional monopole and patch arrays, the proposed array also obtained the RMS null depth of \(-54.3\) dB, the RMS null width of \(27.6^{\circ}\), and the RMSE of \(0.8^{\circ}\). The results demonstrate that the proposed array antenna provided high nulling performances and maintained an extremely narrow array distance.  
Conceptualization, T.H.L., B.-J.J. and H.C.; methodology, T.H.L. and H.C.; software, T.H.L.; validation, T.H.L., B.-J.J.; and H.C.; formal analysis, T.H.L. and H.C.; investigation, T.H.L. and B.-J.J.; resources, T.H.L.; data curation, T.H.L.; writing--original draft preparation, T.H.L. and H.C.; writing--review and editing, T.H.L., B.-J.J., and H.C.; visualization, T.H.L.; supervision, H.C.; project administration, B.-J.J.; funding acquisition, B.-J.J. and H.C. All authors have read and agreed to the published version of the manuscript. This work was supported by the Samsung Research Funding Center of Samsung Electronics under project number SRFC-IT1801-06. The authors declare no conflict of interest.