According to certain embodiments, the lower dielectric substrate material used for the corner elements 200a can be realized by an additive manufacturing (“3-D printing”) technique at precision. FIGS. 8A and 8B illustrate an example of the substrate 230 that can be used according to certain embodiments. FIG. 8A is a top view of the example of the substrate 230, and FIG. 8B is a corresponding side perspective view. According to certain embodiments, the density distribution of the substrate 230 can be manipulated by introducing air cavities 232 or voids to form a lattice-type structure, as shown in FIGS. 8A and 8B. Density variation, thus dielectric constant modulation, can be made quickly, conveniently and precisely by 3-D printing, whereas conventional manufacturing processes, such as milling, for example, are slow, waste material (and are therefore costly), may lack precision, and may be difficult or impractical to implement in certain circumstances. Using an additive manufacturing process, the substrate density can be manipulated by the cavity and/or voids formation via frame structuring, as illustrated in FIG. 8A and FIG. 8B, within the sample, for example. As a result, the process consumes the materials in a most effective way with minimum compromise of the mechanical rigidity of the substrate 230. In certain other examples, the density distribution of the substrate 230 can be manipulated by controlling the 3-D printing speed.