In view of the fact that the field distribution obtained at the top surface of the structure would not give any implication about the intensity enhancement, we further measured the electric field intensity spectrum at single points inside the structure (see Fig. 8(a)) to estimate the enhancement factors. Accordingly the normalized intensities were obtained at the center of the structural heights in the x- and y- directions and at a distance in the propagation z- direction equal to 5.15a = 9.25 cm and 9.55a = 17.15 cm with respect to the input side of the structure, and are superimposed with the regarding numerical calculations in Fig. 8(b,c), respectively. One can infer from these figures that the experimentally measured maximum intensities are around 72–74 for both cases, whereas the numerically calculated intensities are around 88–89. Moreover, one can observe that there are some spectral peak shifts and intensity differences between the numerical and experimental cases. We attributed these discrepancies to the structural fabrication errors, the non-uniform detection efficiency of the monopole antenna, the modified component of the detected electric field due to antenna tilting and the positional error of the detected area. Nevertheless, the experimental results verified that the wave can be enhanced and localized at various positions depending on its frequency.Figure 8Field enhancement measurements. (a) Schematic description of the measurement of the electric field intensity inside the structure. The numerically calculated and experimentally measured electric field intensity spectra, obtained at the center of the xy- plane and at a distance equal to (b) 9.25 cm and (c) 17.15 cm in the z- direction with respect to the input of the structure are superimposed.