  This paper proposes a novel technique for the design of miniaturized
waveguide filters based on locally resonant metamaterials (LRMs). We implement
ultra-small metamaterial filters (Meta-filters) by exploiting a subwavelength
(sub-lambda guiding mechanism in evanescent hollow waveguides, which are loaded
by small resonators. In particular, we use composite pin-pipe waveguides
(CPPWs) built from a hollow metallic pipe loaded by a set of resonant pins,
which are spaced by deep subwavelength distances. We demonstrate that in such
structures, multiple resonant scattering nucleates a sub-lambda mode with a
customizable bandwidth below the induced hybridization bandgap (HBG) of the
LRM. The sub-lambda guided mode and the HBG, respectively, induce pass- and
rejection- bands in a finite-length CPPW, creating a filter whose main
properties are largely decoupled from the specific arrangement of the resonant
inclusions. To guarantee compatibility with existing technologies, we propose a
unique subwavelength method to match the small CPPW filters to standard
waveguide interfaces, which we call a meta-port. Finally, we build and test a
family of low- and high-order ultra-compact aluminum CPPW filters in the
Ku-band (10-18GHz). Our measurements demonstrate the customizability of the
bandwidth and the robustness of the passband against geometrical scaling. The
3D-printed prototypes, which are one order of magnitude smaller and lighter
than traditional filters and are also compatible with standard waveguide
interfaces, may find applications in future satellite systems and 5G
infrastructures.
