Selection of spatial harmonics in pseudoperiodic waveguides depending on frequency is studied. The selection method is based on coordinated variation of the step and phase distribution of elements along the waveguide, which provides constant phase velocity of one spatial harmonic and destroys other spatial harmonics. Moreover, such nonuniform waveguide system represents a microwave filter whose frequency characteristic is also determined by the choice of elements and the steps of their location. A combined application of the conditions of selecting the spatial harmonics and choosing the necessary filter characteristics enables to create new types of broadband one‐wave slow‐wave systems. For example, the possibility of application of pseudoperiodic folded waveguide for creation of powerful broadband travelling‐wave tubes is shown. © 2003 American Institute of Physics