Enhanced backward wave propagation in evanescent waveguides loaded with split ring resonators

Abstract

This paper reports on the design and experimental characterization of evanescent waveguides loaded with split ring resonators SRRs supporting enhanced backward wave propagation. With respect to previous negative refractive index devices based on metallic waveguides, the use of a direct coaxial probe excitation inside an evanescent waveguide drastically improves transmission and reflection characteristics of the periodic structure. Moreover, the addition of metallic windows interleaved between consecutive resonators offers an extra degree of freedom in order to control interresonator couplings, and hence, device parameters like overall transmission, reflection, and bandwidth of the backward wave pass band. A numerical and experimental analysis of different structures overcoming the high loss issue of previously published results is detailed. An experimental validation of the proposed enhancements is done for periodic structures, where no individual tuning of the resonator sections is performed. The proposed technology mainly offers the advantage of miniaturization, and it could be combined in a further step with standard design techniques to provide frequency selective devices.