  Following our recent interest in metamaterial-based devices supporting
resonant tunneling, energy squeezing and supercoupling through narrow waveguide
channels and bends, here we analyze the fundamental physical mechanisms behind
this phenomenon using a transmission-line model. These theoretical findings
extend our theory, allowing us to take fully into account frequency dispersion
and losses and revealing the substantial differences between this unique
tunneling phenomenon and higher-frequency Fabry-Perot resonances. Moreover,
they represent the foundations for other possibilities to realize tunneling
through arbitrary waveguide bends, both in E and H planes of polarization,
waveguide connections and sharp abruptions and to obtain analogous effects with
geometries arguably simpler to realize.
