  We show that a rectangular metallic waveguide loaded with metamaterial
elements consisting of electric-field coupled (ELC) resonators placed at the
side walls can operate well below the cutoff frequency of the respective
unloaded waveguide. 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 ELCs and split ring resonators
(SRRs). Such doubly-loaded waveguides can support both forward wave and
backward waves, and provide independent control of the propagation
characteristics for the respective modes.
