A period-varying folded waveguide is formed by varying the period of a folded waveguide. It has the advantages of the space harmonic selectivity and the wide bandwidth. However, the regularities of the variety of these period-varying folded waveguides are unavailable from the published papers. In order to solve this problem, the principle of the space harmonic selectivity of a period-varying folded waveguide is analysed, and the conditions to select the space harmonic for this slow wave system are obtained. In addition, the space harmonic selectivities for a linear period-varying folded waveguide and a hyperbolic sine-varying period folded waveguide are also analysed as examples.
The periodic nonuniform folded waveguides are special structures, the physical dimension of which is between the periodic folded waveguide and the tapering period folded waveguide. Therefore, the synchronization between the microwave and the electron beam can be maintained in the whole interaction process and the periods are not tapered. In comparison with the tapering period folded waveguide, the theoretical analysis and the technological requirements for this structure are more convenient. In order to study this structure, the space harmonics are analysed, the conditions to make the rn-th space harmonic synchronizing with the electron beam in the whole interaction process are present, and the dispersion curve and the coupling impedance curve are obtained by the simulation software HFSS.
A method of designing a photonic crystal grating slow-wave circuit in which the cylinders of the 2D photonic crystals dot on a cross-sectional plane is established by calculating the band structures of the 2D photonic crystals, and the eigenfrequency of the equivalent waveguide grating. For calculating the band structures, the eigenvalue equations of the photonic crystals in the system of photonie crystal grating slow-wave circuit are derived in a special polarization mode. Two examples are taken to show the method. The design result is validated by the scattering parameters of the same circuit. The result indicates that there exists no photonic band gap if the metal gratings do not extend into the photonic crystals; the design of the circuit without the metal gratings extending into the photonie crystals is less flexible than that with the metal gratings extending into the photonic crystals.
