Many modern telecommunications systems, such as the embedded systems, systems of mobile phone and wireless communication, use filters; many conventional filter designs has reached their technologically defined limits. To satisfy the increasing demand for high performance, circuit integration and reduced size, alternative concepts have to be explored [1].

In recent years, left-handed Metamaterials have attracted considerable interest of scientists and engineers working in the field of microwave technology. These Metamaterials exhibit both a negative permittivity and permeability which result in a negative index of refraction, a property not available within any natural material [2-4]. From duality arguments, it has been shown that negative permittivity media can also be generated by means of a resonant element, namely the complementary split ring resonator (CSRR) introduced by Falcone et al. in 2004 [5]. These resonators can be considered as quasilumped elements and are, therefore, also very interesting for the miniaturization of planar microwave devices such as filters and diplexers, or for improving their performances.

In previous research of some of the authors, SRRs and CSRRs have been successfully applied to the design of microwave filters [6,7].

The aim of this paper is to apply a bandstop filter synthesis proposed in the literature [4] based on CSRRs, and two techniques of Metamaterials miniaturization [8-10], to design a very compact microwave bandstop filter.

In Section 2, we propose a design of conventional bandstop filter based on CSRRs etched on the center line of a microstrip. In Section 3, I will describe how to obtain a reduction of the filter size by the optimization of electrical and physical CSRR size. Finally, in Section 4, the obtained calculated results are discussed and compared with measurements.

A conventional bandstop filter based on a 50 Ω line and the CSRR etched in the conductor strip [4] is designed in the microstrip technology. This filter is considered as reference of comparison with the realised miniaturized filters.

CSRR is dual counterparts of SRR. Therefore a dual electromagnetic behaviour for them is expected according to the duality theorem. The incident electric field needs to be polarized in the axial direction of the resonator. In this way, CSRRs are etched on center line of the microstrip technology.

This arrangement makes sure that the CSRRs are properly exited by the electric field applied parallel to the ring axis.

The CSRR topology and equivalent circuit model are illustrated in Figure 1. The CSRR unit cell was designed to operate around 5.70 GHz. The geometry of the cell is as follows: c = d = 0.3 mm, g = 0.6 mm and the global size is 7.4 mm × 3 mm.

The substrate used is a RT/Duroid having the following characteristics (relative permittivity εr = 2.2, loss tangent tg(φ) = 0.0001 and thickness h = 0.8 mm).

The resonator is simulated by using a commercially available 3D full-wave solver (Ansoft HFSS).

Figure 2 shows the [S] parameters simulated results. It shows a rejected frequency band around the designed frequency of the CSRR resonator explained by a transmission of about −25 dB.