Juraj Bartolic

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.

Considerations on Embroidered Textile Antennas for Wearable Applications

Planar inverted-F antennas (PIFAs) made of woven conductive textiles and of patterns embroidered with conductive threads are investigated. The influence of conductive thread density on antenna properties is considered in both cases. The effect of lockstitch on antenna resonant frequency is quantified. A metallic button is proposed as the transition between textile antennas and classical circuits. A full-textile prototype of a PIFA is designed and shown to operate properly in the ISM 2.4 band.

Dual-Polarized Partially Reflective Surface Antenna With MEMS-Based Beamwidth Reconfiguration

A partially reflective surface (PRS) antenna design enabling 1-bit dynamic beamwidth control is presented. The antenna operates at X-band and is based on microelectromechanical systems (MEMS) technology. The reconfigurable PRS unit cell monolithically integrates MEMS elements, whose positions are chosen to reduce losses while allowing a considerable beamwidth variation. The combined use of the proposed PRS unit cell topology and MEMS technology allows achieving low loss in the reconfigurable PRS. In addition, the antenna operates in dual-linear polarization with independent beamwidth control of each polarization. An operative MEMS-based PRS unit cell is fabricated and measured upon reconfiguration, showing very good agreement with simulations. The complete antenna system performance is rigorously evaluated based on full-wave simulations and the unit cell measurements, demonstrating an 18° and 23° variation of the half-power beamwidth in the E-plane and the H-plane, respectively. The antenna radiation efficiency is better than 75% in all states of operation.

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.

Partially Reflective Surface Antenna With Dynamic Beamwidth Control

The dynamic control of the beamwidth of a partially reflective surface (PRS) antenna is reported for the first time. First, a design methodology is proposed by revisiting the PRS antenna theory from the perspective of its reconfigurable beamwidth operation, which is achieved by the control of the reflection magnitude of the PRS. The proposed PRS design only requires one varactor diode per PRS unit cell and a common control voltage for the entire structure. Finally, a fully operational reconfigurable antenna has been fabricated and measured, successfully demonstrating the targeted capability.

Modified rectangular patches for self-oscillating active-antenna applications

Two modifications to a rectangular-patch antenna, suitable for the integration of active devices, are presented. In the first modification, the impedance inverter was placed in the rectangular opening formed by removing the central part of the patch. This modification allows optimal matching of the active device by changing the position, width, and length of the impedance-line transformer. An oscillating antenna, using this modified patch and a Gunn diode, showed a high EIRP, higher spectral purity, and substantially lower cross-polarization levels, in comparison with the reference active antenna with an unmodified patch. Because of the higher overall Q, this modification is recommended for active-antenna applications with active devices that have a narrower negative-resistance bandwidth. In the second modification, an active device (Gunn diode) was integrated directly into a rectangular opening inside the patch, without the use of a line transformer. This reduced the overall Q of the antenna, thus allowing wide-band frequency tuning by changing the bias voltage. A clear spectrum, with no spurious components of the free-running oscillating antenna, was observed. Radiation patterns in the E and H planes were measured. Low levels of cross-polarization, as for the first modification, were obtained. Injection-locking properties were investigated throughout the tuning range. A relatively wide locking range, with a good locking gain, was achieved. Such a miniaturized wide-band VCO antenna is applicable for integration in injection-locked active arrays, and spatial power combiners.

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

Backward wave propagation in waveguide filled with negative permeability meta material

The waveguide filled with a lossless dispersionless material with negative permeability is analyzed. It is shown that such a waveguide supports the propagation of backward waves below the cut-off frequency. There is no propagation above the cut-off-frequency and the waveguide behaves as a low-pass filter; thus, it is a dual of the conventional waveguide. The theoretical analysis was verified by measurements of the scattering coefficients of a rectangular waveguide loaded with split ring resonators in the 10 GHz frequency band. As expected, the propagation band occurred below the cut-off frequency. It was observed that an increase of the frequency caused an increase of electrical length of the waveguide. Therefore, Fosters theorem is satisfied for this waveguide. It was also shown that an increase of the physical length of the waveguide causes a decrease of the electrical length. This unusual behavior is a consequence of the fact that the phase of the backward wave leads along the waveguide.

Absorber and resistor contribution in the GTEM-cell

The GTEM-cell is described. The influence of absorbers and resistors is shown. The SWR and return loss as well as time domain measurements were performed on the GTEM-cell as well as on GTEM-cell without resistors, absorbers, and with open and metal end. The obtained data was, compared and the conclusion drawn. The GTEM-cell was modeled with the FEM method as well.

Theoretical and experimental study of mutual coupling between microstrip stacked patch antennas

One of the major challenges that appears in the design of microstrip arrays is the impact of mutual coupling on radiation properties. The mutual coupling between array elements can be calculated using the standard moment method procedure in spectral domain. However, with increased distance between elements, the high-oscillating part of the sub-integral in method of moments (MoM) matrix elements makes standard integration procedures inapplicable. A convenient method for overcoming this problem is to change the integration path towards imaginary values in the complex plane, thus transforming the oscillatory behaviour to an exponentially decaying one. The challenge consists in careful management of the loci of the poles and branch cuts of the Greens function in the spectral domain (Garino, M.S. et al., 2000; Kitchener, D. et al., 1987), which generally differs for each MoM matrix element and must be tailored differently. A method for calculating these high-oscillating integrals is developed, implemented and compared with experimental results. The developed software can handle either single or stacked rectangular patch configurations with arbitrary number of dielectric layers.