## 2 Coding Metamaterials for RCS Reduction
### Comparison between Conventional and Unique Coding Metamaterial Structures  
As indicated in this section, the construction of coding metamaterials relies on the most important characteristics, known as digital characterizations of every unit-cell design by using binary codes. This study adopted one-bit coding metamaterials, and two types of unit cells were introduced to mimic the '0' and '1' elements, which possess opposite phase reflections of 0\({}^{\circ}\) and 180\({}^{\circ}\). The same elements were incorporated in a unique-structure coding metamaterial analysis. The arrangement of these coding particles in desired sequences on a two-dimensional plane could manifest certain functionalities. In this study, analytical investigations were performed to accurately estimate the radiation pattern of the proposed coding particles in N \(\times\) N equal-sized lattices. Besides conventional chessboard-like configurations, several unique configurations for coding metamaterial designs were also adopted in this work. The construction of these configurations with a 0 phase created a perfect magnetic conductor, while a 180\({}^{\circ}\) phase possessed a combination of perfect magnetic and electrical conductors. Hence, the reflections of any normally incident EM wave will typically cancel out, leading to a reduction in RCS values. For instance, the conventional coding metamaterial designs with six lattices were the first to produce the different basic functionalities of bistatic RCS scattering patterns with interesting coding sequences. Moreover, all the coding sequences with six lattices generated a normally incident plane wave that was diverted from a spherical shape to a several-directions scattering pattern, as illustrated in Figure 7. Furthermore, the coding patterns in the  
Figure 15: Bistatic scattering patterns at 8 GHz for (**a**) 4 lattices; (**b**) 6 lattices; (**c**) 8 lattices; and (**d**) 12 lattices.  
following number of lattices exhibited several scattering-pattern directions instead of spherical responses in the lower lattices. Therefore, the best reduction values can be gained by optimising the coding sequences in lattice forms.  
In general, the radiation pattern was always symmetrical in terms of the x-z or y-z axes for the one-bit coding metamaterial designs [17]. Hence, it was difficult to realize asymmetrical radiation patterns using one-bit coding metamaterials in a lattice form. Researchers opt for two-bit or higher bit design structures to gain unique scattering beam directions. Nevertheless, the proposed multi-layer cuboid metamaterial designs in this study were able to manifest asymmetrical radiation patterns, indicating an anomalous direction caused by the gradient phase distribution. This is because these unique structures have gradient phase changes and are based on [PERSON]'s law; normally incident waves are reflected at oblique angles, which was confirmed by using the simulation analysis of far-field scattering patterns, as shown in Figures 12 and 15. Hence, this unique phenomenon was a novelty of this research. Besides that, Coding Sequence 3 with 8 lattices manifested a slightly higher reduction value compared to the six-lattice design, as shown in Table 10. This clearly indicates that an increase in the number of lattices does not influence the RCS reduction value. A unique combination of coding sequences with specific lattices will greatly encourage the RCS value to reduce. However, the triple multi-layer coding metamaterial analysis on 12 lattices indicated a 23% higher reduction value when compared to conventional designs, and the value was further reduced by adopting cuboid coding metamaterials. As shown in Table 10, using from 4 to 12 lattices, the conventional and cuboid coding metamaterials enabled reduction values of approximately 70.1% and 74.5%, respectively. Hence, the results prove that the cuboid coding metamaterial designs exhibited better reduction values compared to the conventional or multi-layer coding metamaterial designs.