engleski

Miniaturized Guiding Structures Based on Evanascent Wavguide Filled with Anisotropic Single-negative Meta-material

A rectangular waveguide has been used as a basic guiding structure in microwave technology for almost six decades. Well-known drawback of the waveguide is its relatively big physical size dictated by basic constraint [1] that transversal dimension must be at least one half of a wavelength in the filling material. In the last few years there have been several new ideas which may lead to the miniaturized waveguides. The artificial perfect magnetic conductors (complex surfaces) were used for waveguide walls in [2] providing TEM propagation. This phenomenon could be used for reduction of the waveguide width. In [3], the new type of dielectric loaded hard-walled waveguide that supports TEM propagation (‘ N waveguide’ ) was studied theoretically. On the other hand, there is neighboring field of meta-materials which has been growing rapidly since recent introduction of artificial materials with simultaneously negative permittivity and permeability [4] (‘ left-handed’ , ‘ backward-wave’ or ‘ double negative’ material). It is interesting that very few researchers have investigated possible application of these new materials for miniaturization of guiding structures. Very recently [5], the propagation below cut-off frequency of a waveguide loaded with split-ring resonators was demonstrated. This phenomenon was interpreted by thinking of the waveguide below cut-off frequency of dominant TE mode as the one-dimensional material with negative effective permittivity [2]. Therefore, the waveguide loaded with split-ring resonators was interpreted as a material with both negative permeability and negative permittivity. In this talk, we give different interpretation, based on uniaxial anisotropy of negative permeability meta-material [6]. If one allows usage of anisotropic material for waveguide filling it is not longer necessary to have both permittivity and permeability with same signs. Propagation is also possible if either permeability in transversal direction (for TE modes) or permittivity in transversal direction (for TM modes) has negative sign. In the case of TE modes, the waveguide supports propagation of backward waves below cut-off frequency of an empty waveguide There is no theoretical limit on the lowest frequency of propagation providing that one is able to fabricate uniaxial negative permeability material at given frequency. The transversal width of such a waveguide can be in principle arbitrarily small, which may be used for waveguide miniaturization. The lowest frequency of propagation, and therefore the size reduction is dictated by feasibility of fabrication of negative permeability uniaxial meta-material at the given frequency. The simple explanation this peculiar phenomenon is given by analysis of transmission line model of the waveguide. The series inductance per unit length and shunt inductance per unit length have the same sign if the waveguide is filled with isotropic magnetic material (either double positive material or double negative material). On contrary, if the waveguide is filled with uniaxial material with negative permeability in transversal direction, the series inductance will be negative. At one fixed frequency, one can think of negative series inductance as the ordinary capacitance. If the operating frequency is located below cut-off, the equivalent circuit now comprises series capacitance and shunt inductance similar to ‘ left-handed transmission line’ analyzed in [7]. Therefore, the propagation in the form of backward waves becomes possible below the cut-off frequency. Several miniaturized rectangular waveguides operating in 7 GHz and 350 MHz bands, filled with uniaxial negative permeability meta-material have been designed, fabricated and tested. The meta-material was based on double ring resonators or capacitively loaded loops. The measured results revealed backward-wave pass-band located below the cut off frequency. The transversal dimensions of fabricated miniaturized waveguides were only 35% and 4% of the half of o wavelength in 7 GHz and 350 MHz bands, respectively.

miniaturization ; guiding structure ; single-ngative ; meta-material

nije evidentirano