Further numerical analyses were carried out to reveal the quantitative amount of propagation losses, which occur due to the finite transverse dimensions of the structure. The propagation losses were characterized by calculating the reflected power, by taking into consideration that in the case of a lossless propagation all incident power should be eventually reflected back after it is localized. The calculated reflected power is then normalized to the incident source power to give quantitative information about the loss spectra. Strictly speaking, since every frequency component will travel a different distance, it is more physically meaningful to normalize the loss to the distance travelled by the wave for every frequency component. The travelling distance has estimated as the twofold of the distance between the entrance of the structure and the turning point, by taking into account the forward and backward propagation distances. Therefore, for a more accurate analysis one must take this fact into account. However, in the present calculations we ignore this fact and define the travelling distance as an effective distance travelled by the wave, excluding the additional round trips. Following this direction, the loss spectra is calculated for 7 (original structure) and 12 (extended structure) transverse periods and are superimposed in Fig. 5. It can be observed from this figure that for the normalized frequency interval a/λ = {0.44, 0.48} the propagation losses are lower than the rest of the operational frequency spectrum, which is expected as the TE-like transverse PBG does not compromise outside this interval, as was discussed. Another conclusion that can be drawn from Fig. 5 is that increasing the number of the transverse periods reduces the losses, as expected. It is worth noticing that the narrowband enhancement peaks in Fig. 3(d) matches well with the loss peaks in Fig. 5. As above discussed, the narrowband enhancement peaks arises from multiple roundtrips inside the structure, which lies also in the origin of the enhanced losses.Figure 5Lateral loss analysis. Superimposed loss spectra of the PhC structure, with 7 and 12 transverse periods is shown. Note the oscillatory behavior, which is a direct consequence of the resonant propagation of the wave between the localization region and the input side of the structure.