(a,b) The 2D transmission spectra of the finite periodic microstrip calculated for different values of the bulkparameter: l/d. The solid black lines mark the edges of the bands for the infinite microstrip, corresponding to kz = 0 or kz = π/d. Two ratios l/d = 0.25 and 0.625 are indicated by vertical dashed lines. The symmetry of the Bloch function at the edges of the band is indicated by the letters S and A, respectively. We considered the system (a) composed of five centrosymmetric cells and (b) its modification, where we added cells of modified sizes at the beginning and end of the microstrip. The sizes of the edge cells and all other parameters are the same as those given in the System section. (c,d) The cross section of the 2D spectra (a,b) at l/d = 0.625 (solid black curves) is supplemented by the measured (red curve) transmission spectra for fabricated structures. For the microstrip with additional cells of modified sizes, we can identify the transmission peaks in the second frequency gap (gray area in (d)). This double peak is attributed to edge modes that decay exponentially in space. It is noteworthy that the edge modes do not exist in the second gap for smaller values of l/d. This is related to the qualitative change in the spectrum, where the order of the edges of the gap and their symmetry are swapped: from symmetric (antisymmetric), for small l/d, to antisymmetric (symmetric), for large l/d, at the lower (upper) edge of the gap. The frequency of the edge modes can be tuned by modifying the edge cells: d0 = 12.5 mm, l0 = 0.5 mm, w0 = 11 mm – see black dotted line. The induction of the edge modes is obtained at the expense of attenuation of the third band due to the strong impedance mismatch in this frequency range.