Below-Cutoff Propagation in a Metasurface-Cladded Waveguide

Abstract

This work presents the design, fabrication, and experimental validation of a circular metasurface (MTS)-cladded waveguide that enables electromagnetic propagation below its natural cutoff frequency. A lumped-element model based on a coupled-resonator lattice is introduced to describe the cladding MTS behavior and to provide a geometry-dependent estimate of its cutoff frequency, thereby offering a practical design tool for initial parameter selection. Full-wave simulations and vector network analyzer measurements confirm the onset of below-cutoff propagation and reveal a cutoff-frequency reduction up to 72% compared to the unclad waveguide. This corresponds to a 74.5% reduction in radius compared to an equivalent hollow structure operating at the same frequency. The proposed architecture preserves a hollow core and enables straightforward integration of the feeding system, which is of fundamental importance in radio frequency plasma sources and lightweight aerospace applications. Finally, a tradeoff strategy between cutoff reduction and insertion loss (IL) is proposed.