Invited PaperMetamaterial filters: A review
Introduction
The main aim of this work is to review the work developed in this research group during the last years in the implementation of metamaterial filters based on resonant-type metamaterial transmission lines, which combine sub-wavelength resonators to, together with other elements like series capacitances or shunt inductances, load a host transmission line and obtain a propagating medium with controllable characteristics. These lines offer the possibility of tailoring their phase and impedance, what makes them suitable for, not only filter, but, in general, microwave device design. Power dividers, rat-race hybrid couplers, phase shifters, are some examples of the application possibilities of these resonant-type metamaterial transmission lines [1], [2], [3], which even offer the possibility of bandwidth enhancement [4] or dual-band operation [5].
One of the first issues to be discussed may be, perhaps, the use of the term “metamaterial” to name these devices based on such kind of lines. The authors do not apply the label metamaterial intending to main that such structures compose periodic, homogeneous and isotropic media with negative effective ɛ and μ parameters. Actually, most of our devices are based on only one unit cell and talking about effective parameters has no sense. What the authors try to point out calling these devices as “metadevices” is the fact that they are based on sub-wavelength resonators, like the SRRs [6], whose sub-wavelength characteristics opened the door to the synthesis of the first left-handed medium [7], and the fact that, indeed, most of these transmission lines exhibit backward propagation. In fact, the nature of the propagation is irrelevant in most of the devices based on resonant-type transmission lines. It is, on the contrary, the controllability of their electrical characteristics (beyond what is achievable in conventional lines) and the possibility that they offer to design compact devices what is exploited from these metamaterial resonator-based transmission lines.
As was mentioned above, in this article we will make a review on the different kinds of metamaterial filters based on resonant-type metamaterial transmission lines since their appearance in 2003 [8]. Their frequency response immediately suggested their application possibilities as filtering structures and, since that moment, an important amount of works has been devoted to the implementation and improvement of this kind of filters. One of the first strategies applying such metamaterial resonators consisted in their inclusion as additional elements in conventional filters for out-of-band rejection improvement [9], [10]. Without entailing a considerable area increment, SRRs can be printed close to the coupled-line sections in a conventional coupled-line bandpass filter and, when properly tuned, they can eliminate spurious bands and thus improve out-of-band rejection. In such filters, however, SRRs are merely added to a conventional distributed filter as signal rejecting elements, but they do not contribute as basic elements to generate a controllable transmission band. In the following sections, we will focus on filters in which metamaterial resonators are essential elements in the transmission lines forming the different stages of the filter. Diverse strategies and kinds of unit cells applied to design a big variety of filters with different performances are shown. Finally, several filters based on other approaches are also shown as examples of the application possibilities that metamaterial transmission lines offer in filter design.
Section snippets
Resonant-type metamaterial transmission lines
As has been previously mentioned, the resonant-type metamaterial transmission lines use different kinds of sub-wavelength resonators like, for example, split-ring resonators (SRRs) [6] or complementary split-ring resonators (CSRRs) [11] to load a host transmission line and, in combination with other elements, obtain a propagating media with controllable characteristics. These properties are mainly the phase shift and the characteristic impedance of the line [12], [13]. Due to the small
Metamaterial filters
The frequency selectivity of resonant-type metamaterial transmission lines suggests their application in filter design. Furthermore, the possibility of obtaining such broad responses by means of balanced lines opens the door to the application of these structures in the design of broad-band filters. As has been previously mentioned, the characteristics of the lines can be tailored to some extent in order to obtain the desired response. Additionally, the position of the transmission zeros can be
Conclusions
A review on the work developed during the last years in the field of filters using resonant-type metamaterial transmission lines has been carried out. Part of the work carried out by this research group has been exposed, showing different kinds of filters applying diverse structures and design strategies. The presented work shows the possibilities that this kind of metamaterial transmission lines offer in compact filter design. As the results show, different bandwidths, performances and
Acknowledgements
This work has been supported by MEC (Spain) by project contract TEC2007-68013-C02-02 METAINNOVA and a FPU Grant (Ref. AP2005-4523) awarded to Marta Gil and by the European Commission (VI Framework Program) contract no. 500252-2 METAMORPHOSE. Special thanks are also given to CIDEM (Generalitat de Catalunya) for funding CIMITEC.
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