We present a model for periodic waveguides in terms of an equivalent periodic MTL[17, 6]. Let us consider the propagation in a periodically loaded waveguide, wherethe unit cell of such periodic waveguide is composed of two or more cascaded waveguides with cross sections. The waveguide is bounded in the transverse plane to the z -direction, and it is made of cascading waveguides regions (or segments) along z , as shown in Fig. 2.3.Each segment has transverse electric E t,n (r) and magnetic H t,n (r) components relative to the nth mode supported by that waveguide segment, similar to the case discussed above in the uniform waveguides, as