Forward and backward-wave propagation in “below cut-off ” waveguides loaded with dielectric resonators

Dispersion properties of a rectangular metallic waveguide periodically loaded with dielectric resonators (DRs) are investigated both theoretically and by full-wave simulations. Forward and backward wave propagation has been observed in the waveguide at below cut-off frequencies for the fundamental TE10 mode, when the DRs exhibited electric and magnetic resonances, respectively. It is demonstrated that the “below cutoff” forward wave propagation is related to shifting of the cut-off frequency of the waveguide filled with DRs with respect to that of an empty waveguide to lower frequencies in dependence on the effective permittivity component along the electric field direction. This effective permittivity increases substantially at the frequency just below that of the electric-type resonance in DRs.