Passive Diplexers and Active Filters based on Metamaterial Particles

Composite transmission lines are one of the main developments in the increasingly popular field of electromagnetic metamaterials, artificial electromagnetic structures with both negative electric permittivity and magnetic permeability (Caloz & Itoh, 2005). These structures present a backward wave or left-handed (LH) propagation instead of the conventional right-handed one. The first experimental microwave structures that presented this behavior were the result of combining thin wires and split-ring resonators (Shelby et al, 2001). Soon it was evident that narrow bandwidth operation and high losses were inherent to the resonant nature of this kind of metamaterials. In order to overcome the previous problems, some authors proposed the so-called metamaterial transmission lines or lefthanded transmission lines LH TL (Sanada et al, 2004). The LH TL concept has been extended and generalized to the concept of composite right/left handed (CRLH) structures where mixed contributions of LH and RH cells occur in practice. More specifically, the CRLH transmission lines have become a very commonly used solution to obtain metamaterial properties with low losses and broader bandwidth. Then, below a certain frequency, a CRLH transmission line behaves as a left-handed transmission line while over higher frequencies it is basically a conventional right handed line. As a consequence of this combined behaviour, the phase response is not linear with respect to the frequency. From the circuit application point of view, the two main characteristics of the CRLH transmission lines consist on obtaining miniaturized and/or dual band circuits. Then, it can be mentioned that dual band hybrid couplers (Lin et al, 2004), branch-line couplers (Keung & Cheng, 2004), dual-performance rat-race couplers (Castro-Galan et al, 2009) and enhanced rat-race couplers (Okabe, Caloz & Itoh 2004). The most critical aspect in the design of combiners, filters or diplexers with conventional CRLH transmission lines is the losses associated to them. For the case of diplexers, using of the so called dual composite right left handed (D-CRLH) cells may overcome some of the previously stated problems. The D-CRLH transmission lines are the dual part of the CRLH transmission lines and were first proposed in (Caloz, 21