Electric-field-coupled resonators as metamaterial loadings for waveguide miniaturization

We show that a rectangular metallic waveguide loaded with metamaterial elements consisting of electric-field-coupled (ELC) resonators placed at the sidewalls can operate well below the cutoff frequency of the respective unloaded waveguide. For the example considered of a waveguide with square cross-section and original TE-mode cut-off frequency at 16.67 GHz (under no loading), a TE-mode passband is obtained around 8.6 GHz once the waveguide is loaded with ELC resonators. Similarly, for TM-mode excitation with original cut-off at 23.57 GHZ, TM-mode passbands are obtained around 8.5 GHz and 11.3 GHz, for two different orientations of ELC resonators. The dispersion diagrams indicate that propagating modes in ELC-loaded waveguides are of forward-type for both TE and TM modes. We also study the dispersion diagram and transmission characteristics of rectangular metallic waveguides simultaneously loaded with both ELCs and split ring resonators. Such "doubly"-loaded waveguides can support both forward wave and backward waves and provide independent control of the propagation characteristics for the respective modes.