Typical dispersion diagram of a forward (a) and backward (b) transmission line model. The dependence of the normalized (/) Bloch impedance with frequency is shown in (c) and (d) for the forward and backward lines, respectively. is the maximum value of the impedance. Notice that for the backward transmission line the group velocity tends to infinity as frequency increases. Actually, the model is not valid at high frequencies since this implementation requires a host line that introduces parasitic elements to the model.