Resonant structures capable of metamaterial behavior can be quite beneficial when it comes to microwave filter designs. In particular, split-ring resonators (SRRs) and complementary SRRs (CSRRs) were found to enhance the performance of a conventional stepped-impedance microstrip lowpass filter (LPF) when etched into the filter’s ground plane. The metamaterial-like properties of CSRRs include the capabilities to achieve negative relative permeability and permittivity within the rejection band of the LPF while providing low insertion loss in the filter passband.
To explore the potential of SRRs and CSRRs in filter design, a conventional microstrip-based LPF was designed and simulated with the aid of a commercial electromagnetic (EM) simulator. Then variations were made with the addition of the CSRRs to understand their effects on performance. Filters were fabricated on commercial printed-circuit-board (PCB) material to validate the results of the simulation.
For electromagnetic (EM) applications, metamaterials refer to composite materials with EM properties that are not naturally occurring. Metamaterials typically feature periodic dielectric or metallic structures that act as homogeneous materials. These types of materials were first discovered by Veselago in 1968.1 Thirty years later, the properties were verified experimentally by Pendry.2,3 One year later, Smith and associates demonstrated a split ring resonator (SRR) using a left-handed metamaterial obtained by combining two periodic and homogeneous materials.4,5 It was capable of producing negative values of effective permeability and effective permittivity for metamaterials with negative refractive index.6,7
Some years later, Caloz and Itoh proposed another method to produce left-handed metamaterials by using a transmission-line format based on microstrip technology.8,9 Of the many EM metamaterial-type structures currently available, most are based on the SRR concept. These are resonators with magnetic responses well suited for microwave applications, including waveguide transmission lines, filters, and antennas.10,11