Silvio Hrabar

Waveguide miniaturization using uniaxial negative permeability metamaterial

A rectangular waveguide filled with anisotropic uniaxial metamaterial with transversal negative effective permeability is investigated both theoretically and experimentally. It is shown that such a waveguide supports propagation of the backward wave below the cutoff frequency, thus, it can be considered as a dual of the ordinary waveguide. The transversal dimension of this waveguide can be arbitrarily smaller than half of a wavelength in the filling material, provided that the transversal permeability is negative. This peculiar behavior may be used for fabrication of miniaturized rectangular waveguides. Several experimental miniaturized waveguides loaded with double ring resonators in 7 GHz frequency band have been designed, fabricated and tested. The measured results revealed backward-wave passband located below the cutoff frequency. Furthermore, it was experimentally shown that the increase of the physical length of the waveguide caused the decrease of the electrical length. This is a direct proof of the backward-wave propagation since the phase of the backward wave increases along the waveguide.

Negative capacitor paves the way to ultra-broadband metamaterials

Experimental demonstration of the overcoming of basic dispersion-energy constraints in metamaterials with the help of active non-Foster negative capacitors is reported. The experimental metamaterial operates in RF regime, and it is based on air transmission line loaded with negative capacitors. Measurement results clearly show almost dispersionless Epsilon-Near-Zero behavior, accompanied with superluminal both phase and group velocities, over a bandwidth of more than four octaves (2 MHz-40 MHz). The principle of periodic loading of transmission line with negative capacitors may find applications in ultra-broadband active metamaterials for antennas and cloaking technology.

Analysis of Uniaxial Multilayer Cylinders Used for Invisible Cloak Realization

Uniaxial cylindrical cloaks have recently been proposed to prevent scattering of electromagnetic waves, i.e. to render objects invisible. In this paper frequency and time domain analysis of such a cloak has been carried out. Furthermore, influence of number of cloak layers on obtained invisibility has been studied, with special attention given to the cloak with just three layers that should be relatively easy to practically realize.

Stability of Non-Foster Reactive Elements for Use in Active Metamaterials and Antennas

During the last couple years there have been lot of attempts to use negative non-Foster reactive elements for improving the bandwidth of electrically small antennas (ESAs) or metamaterial-based structures. However, negative non-Foster reactive elements are actually electronic circuits that are not absolutely stable, which could impose serious problems in practical applications. This communication studies the stability of several networks with non-Foster reactive elements by using a time domain approach. This will help to model more realistic situations for antenna and metamaterial technology.

Ultra-broadband simultaneous superluminal phase and group velocities in non-Foster epsilon-near-zero metamaterial

Experimental investigation of ultra-broadband simultaneous superluminal phase and group velocity in recently introduced epsilon-near-zero (ENZ) non-Foster metamaterial is reported. The phase and group velocities were extracted both from the measurements of scattering parameters in the frequency domain and the measurements of propagation time of broadband Gaussian pulse in time domain. Reported superluminal effects are extremely broadband (relative bandwidth of 180% (1:20)) and they are fundamentally different from widely reported narrowband experiments based on the anomalous dispersion. The phenomenon of simultaneous superluminal phase and group velocities may find applications in transformation electromagnetic devices and in communication systems.

Capacitively Loaded Loop as Basic Element of Negative Permeability Meta-material

A simple one-dimensional meta-material which exhibits negative permeability within a finite frequency band is proposed in this paper. Instead of recently introduced split-ring resonators, this structure comprises an array of the capacitively loaded small loop antennas. Due to resonant behaviour of the antenna current, intensity of the magnetic field of the incoming plane wave may be locally decreased yielding a stop-band with negative effective permeability. Theoretical analysis was verified by measurements of the transmission coefficient of experimental structures in free space and in rectangular waveguide, in 10 GHz frequency band. The developed structure was also combined with previously reported negative permittivity wire structure and left-handed (backward wave) properties were observed.

Numerical and Experimental Investigation of Field Distribution in Waveguide Filled with Anisotropic Single Negative Metamaterial

A field distribution in a rectangular waveguide filled with anisotropic single-negative (SNG) metamaterial is analyzed theoretically, numerically and experimentally. A highly simplified scenario with ideal lossless, continuous (either mu-negative (MNG) or epsilon-negative (ENG)) filling material is used in theoretical analysis. The results show that distributions of magnitudes of E and H fields are similar to the associated distributions in ordinary waveguide (filled with double-positive (DPS) material) that operates above cut-off frequency. However, the crucial difference, responsible for recently demonstrated peculiar phenomenon of backward wave propagation below cut-off is a pattern of field lines of either H field (a case with MNG filling) or E field (a case with ENG filling). Numerical approach that dealt with realistic structures (double-ring-resonator-based MNG metamaterial and thin-wire-based ENG metamaterial) confirmed existence of backward-wave propagation. Two experimental waveguides filled with double-ring resonators (an MNG filling) and thin wires (an ENG filling) were successfully designed, manufactured and tested in 8 GHz band. The backward-wave propagation was verified by measurement of a phase distribution along a waveguide with the help of a small E field probe

Application of Wire Media in Antenna Technology

This paper reviews the results of experimental investigation of radiating structures based on plasma-like wire media, undertaken at University of Zagreb. It is shown that all three regions of the dispersion curve of wire media, namely the Epsilon-NeGative (ENG) region, the Epsilon-Near-Zero (ENZ) region and the Epsilon-PoSitive (EPS) region, can be successfully utilized in antenna applica tions. The phenomenon of gain increase of an antenna embedded in wire medium, based on ultra-refraction in ENZ region, was investigated in 10 GHz band. The results revealed that the use of ultra-refraction may be a practical approach in the case of low-directivity radiators such as simple monopole antennas. Another example of the utilization of the ENZ region deals with the shortened horn antenna with embedded wire-medium-based ENZ slab operating in 10 GHz band. Two prototyped shortened horn antennas (labeled as horn I and horn II) had lengths of 52% and 33% of the length of the optimal horn, respectively. Measured gain was found to be very similar to the gain of the full length optimal horn (within 0.1 dB), but in a narrow band (12% for horn I and 8% for horn II). The last example deals with a scanning leaky-wave antenna operating at 10 GHz, based on a waveguide filled with double-wire medium operating in all three regions of the dispersion curve. These three regions correspond to three different modes of propagation in the waveguide: the backward-wave mode, the forward-wave mode and the mode with infinite wavelength. Experimental results revealed the possibility of main beam scanning within an angle of ±60° from broadside direction.

ENZ-based shortened horn antenna - an experimental study

Recent numerical study predicted phenomenon of a gain increase of a shortened horn antenna with the help of embedded double-thin-wire ENZ metamaterial slab. Here, we perform an experimental investigation of shortened horn antenna (length of which spans 52% of the length of optimal horn) with much simpler single-wire ENZ slab operated in 10 GHz band. Measurements revealed gain increase of more than 2 dB (comparing to the gain of a bare shortened horn) in 12% fractional bandwidth, with return loss better than -13 dB.

Simple Experimental Determination of Complex Permittivity or Complex Permeability of SNG Metamaterials

A very simple method of experimental determination of complex permittivity or complex permeability of single-negative (SNG) metamaterials in waveguide environment is proposed in this paper. The method uses only one measurement and it is based on the fact that any SNG material can be thought of an equivalent evanescent transmission line. The input impedance of this line is equal to the wave impedance, almost regardless of the line termination. The method was tested by measurements of complex parameters of SRR-based mu-negative (MNG) metamaterial and thin-wire-based epsilon-negative (ENG) metamaterial operating in 10 GHz frequency band and the results were found to be in good agreement with theoretical predictions