Limitations and Solutions of Resonant-Type Metamaterial Transmission Lines for Filter Applications: the Hybrid Approach

In this work, the main limitations of resonant-type left handed transmission lines for filter applications are pointed out. Naturally, microstrip lines periodic loaded with series capacitive gaps and complementary split rings resonators (CSRRs) exhibit a band pass behavior with backward wave propagation in the allowed band. Cell dimensions and selectivity at the lower band edge are satisfactory. However, the upper transition band, in-band losses and bandwidth should be improved for the competitiveness and applicability of these devices. It is shown in this contribution that by introducing grounded inductive stubs to the basic cell, the previous limitations can be overcome. Actually the resulting structure is a combination of the CSRR-based (i.e. resonant-type) left handed line and the dual transmission line concept. For this reason this approach has been termed as the hybrid approach. Its advantages and potentialities for the design of small size high performance band pass filter are highlighted, and several prototype device examples are provided to illustrate the achievable performance