Jordi Bonache

Tunable metamaterial transmission lines based on varactor-loaded split-ring resonators

In this paper, it is demonstrated that varactor-loaded split-ring resonators (VLSRRs) coupled to microstrip lines can lead to metamaterial transmission lines with tuning capability. Both negative permeability (mu<0) and double (or left-handed) negative media have been designed and fabricated with tuning ranges as wide as 30%. The negative effective permeability is provided by the VLSRRs in a narrow band above their resonant frequency, which can be bias controlled by virtue of the presence of diode varactors. To achieve a negative effective permittivity in the left-handed structure, metallic vias emulating shunt inductances are periodically placed between the conductor strip and the ground plane. The lumped-element equivalent-circuit models of the designed structures have been derived. It has been found that these models provide a good qualitative description of device performance. Since the VLSRR microstrip line and the line loaded with both VLSRRs and vias exhibit stopband and bandpass behavior, respectively, the ideas presented in this study can be applied to the design of narrowband tunable frequency-selective structures with compact dimensions. This is the first time that a tunable left-handed transmission line, based on SRRs, is proposed

Electrically Small Resonators for Metamaterial and Microwave Circuit Design

A new branch in microwave engineering arose just few years ago with the emergence of metamaterials in 2000 (Smith et al., 2000). The implementation of the first artificial medium with negative effective dielectric permittivity and magnetic permeability opened the door to the experimental study of a new kind of media: left-handed media. The possibility of the artificial implementation of such media allowed the corroboration of many of their electromagnetic properties, predicted years before by Viktor Veselago (Veselago, 1968). Since the year 2000, the interest stirred up by these new materials has given rise to numerous works in a wide range of scientific branches. The possibilities that metamaterials offer to create artificial media with controllable characteristics has permitted the creation of a growing number of completely new applications. Undoubtedly, the most innovative and spectacular application of such artificial media is their use in the implementation of cloaking structures to achieve invisibility, which can be accomplished thanks to the engineering of the refraction index of the different layers of the cloaking shield (Schurig et al., 2006). Within the vast number of new applications of metamaterials, one of the most productive ones is the implementation of microwave devices by means of artificial transmission lines. The following sections will deal with one of the approaches devoted to this purpose: the resonant-type approach. Different subwavelength resonators employed in the design of metamaterial transmission lines based on the resonant-type approach will be studied. The equivalent circuit models of different kinds of metamaterial transmission lines, as well as the parameter extraction methods employed as design and corroboration tools will be also presented. In closing, a selection of application examples of resonant-type metamaterial transmission lines in the design of microwave devices will be presented. 19

Miniaturization and Dual-Band Operation in Planar Microwave Components by Using Resonant-Type Metamaterial Transmission Lines

In this work, it is shown that dual-band microwave components based on plusmn90deg metamaterial transmission lines can be miniaturized by implementing the artificial lines by means of complementary spiral resonators (CSRs). Such lines exhibit a composite right/left handed (CRLH) behavior. Dual-band operation is achieved by designing the lines to provide a phase of -90o at the lower operating frequency (within the left handed band) and +90o phase shift at the upper frequency (within the right handed band). Size reduction, as compared to conventional implementations, is achieved by implementing the lines with a single unit cell, and thanks to the small electrical size of CSRs. To illustrate the possibilities of the approach, a dual-band branch-line coupler based on metamaterial transmission lines implemented by means of CSRs is presented. The device has been designed to be functional at the mobile GSM bands (f1= 0.9 GHz, f2= 1.8 GHz). As compared to the conventional branch line, a 40% size reduction is obtained. The approach is fully compatible with planar technology.

Implementation of negative μ medium in coplanar waveguide technology

In this paper, a novel structure in coplanar waveguide (CPW) technology which exhibits an equivalent negative magnetic permeability is described. Such a structure consists in a conventional coplanar waveguide that is loaded with split ring resonator (SRR) cells. Due to the configuration of the magnetic field components in the coplanar waveguide, by adequately placing the SRR cells, quasi-static resonance occurs. In the vicinity of such resonance frequency, the magnetic permeability exhibits a negative value in a certain frequency range. Full wave simulation results as well as measurement from fabricated prototypes validate initial assumptions, providing a new method to implement band rejection filters with very small size.

Characterization of Metamaterial Transmission Lines with Coupled Resonators Through Parameter Extraction

Since resonant-type metamaterial transmission lines were proposed (Martin et al., 2003), this kind of transmission lines have been of significant importance in the development of new and innovative microwave devices. The small size and novel characteristics of these transmission lines based on sub-wavelength resonators allows the miniaturization and improvement of existing devices (Bonache et al., 2006a, 2006b; Gil et al., 2007a, 2007b), as well as the design of components with new functionalities (Siso et al., 2008, 2009). Due to the complicated layouts that these designs usually involve, having an accurate equivalent circuit model is an important assist during the design process. Besides, the application of parameter extraction methods (Bonache et al., 2006c) to obtain the values of the electrical parameters of the circuit model makes possible the characterization of both the transmission line and, therefore, the microwave device. The circuit models and parameter extraction methods presented in this chapter have been widely verified and their accuracy permits even their application for automatic layout generation based on space mapping techniques (Selga et al., 2010), which is a large and useful advance in the design of such structures.

Analysis of doubly periodic Electromagnetic Bandgap filters in coplanar waveguide technology

The use of Electromagnetic Bandgap (EBG) structures has proven to be effective in the implementation of many devices in planar circuit technology, such as filters, couplers and antenna design. In this paper, a low pass filter based in EBG structures in coplanar waveguide (CPW) technology is proposed. The device consists in a periodically loaded CPW with shunt capacitive elements. This way, a low pass frequency response is obtained. The capacitive elements are formed by t-shaped fingers that extend from the central conductor strip to both ground planes. Further enhancement is achieved by introducing a second periodicity in the central conductor strip, modulating the width of the strip. By doing so, effective rejection of undesired frequency harmonics is achieved. Full wave simulation results as well as measurement from fabricated prototypes confirm the performance of the proposed low pass filter, which exhibits low insertion losses in the passband, high rejection slope and effective rejection of undesired harmonics. Another advantage is the small footprint, due to the inherent slow-wave nature of the device.

Multiple-tuned continuous electromagnetic bandgap structures in coplanar waveguide technology

In this paper, electromagnetic bandgap structures are applied to a conventional coplanar waveguide. The EBG is obtained by etching a continuous sinusoidal perturbation pattern either on the central conductor strip or the both ground planes of the CPW. By doing so, a rejected frequency band appears. Since the frequency response of the device can be approximated by the Fourier Transform of the perturbation function, the application of a sinusoidal perturbation gives allows in principle a unique rejected band, without the presence of frequency harmonics. Full wave simulation results are presented for simple reflectors as well as for combination of several simultaneous perturbation functions, tailoring the desired frequency selective behavior.