Unit Cells for Filter Design Based on CPW Metamaterial Transmission Line

Abstract

In the recent, metamaterials (MTMs) have concentrated extensive attention in many areas. This artificial material has simultaneously negative permittivity and permeability. Therefore, MTMs have special characteristics not occur in conventional materials. And the targets of miniaturization and low power loss can be possible if MTMs are used in microwave component or system design. In this thesis, the concepts and the fundamental properties of MTMs were illustrated. And several unit cells for filter design based on coplanar waveguide (CPW) MTM transmission line were proposed. First of all, a left-handed (LH) unit cell and a dual left-handed (DLH) unit cell with high-pass property were devised by the conventional composite right and left-handed (C-CRLH) theory based on folded coplanar waveguide (FCPW). Secondly, a right-handed (RH) unit cell with low-pass property was devised by the C-CRLH theory based on conductor-backed coplanar waveguide (CBCPW). Moreover, a unit cell with a conductor-backed defected ground (CBDG) structure based on dual composite right and left-handed (D-CRLH) CBCPW, and a unit cell with a groove line (GL) structure based on D-CRLH CPW showed band-rejection property. The characteristics of the proposed unit cells were analyzed by using high frequency structural simulator. By tuning the sizes of the unit cells, the parameters of the RH and LH immittance components were optimized and the desired characteristics of the unit cells were achieved. In addition, the equivalent circuits for the unit cells were extracted and analyzed by employing advanced design system. As a result, the S-parameter simulation results of the unit cells showed good agreements with those of the equivalent circuits. On the other hand, the dispersion characteristics and the energy distributions of the electromagnetic field were simulated to confirm the characteristics of C-CRLH and D-CRLH structures. These simulation results provided the design data of the proposed structures. Through the photosensitizing, etching, plating and assembling processes, the proposed unit cells were fabricated on the substrates made by Taconic with the relative dielectric constant of 2.5 and the substrate thickness of 0.787mm. And then, the fabricated unit cells were measured by the vector network analyzer of N5230A. As a result, the LH unit cell based on C-CRLH FCPW exhibited high-pass property with -9.88dB rejection in the range beyond 2.05GHz. And the DLH unit cell based on C-CRLH FCPW showed high-pass property that the return loss is more than 10dB in the range of 2.52 ~ 2.9GHz and the range beyond 4.23GHGHz of the pass-band. On the other hand, the RH unit cell based on C-CRLH CBCPW exhibited low-pass property with -16dB rejection and a 0.38dB insertion loss in the pass-band and a good skirt effect. Moreover, the CBDG unit cell based on D-CRLH CBCPW with band-rejection property showed a 0.6dB return loss and a 16dB insertion loss from 869MHz to 894MHz of the stop-band. And the GL unit cell based on D-CRLH CPW exhibited band-rejection property with an 11dB insertion loss and a 2dB return loss in the stop-band from 869MHz to 894MHz. In practical applications, since the proposed unit cells have good characteristics with compact structures, they can be used not only in filter design but also in other components of microwave systems, such as couplers and resonators. Especially, the compact unit cells are very useful for the designs of dual-band or multi-band duplexers, which have operational frequencies separated and combined by various frequency bands.