Marko Bosiljevac

Metasurfing: Addressing Waves on Impenetrable Metasurfaces

Metasurfaces constitute a class of thin metamaterials, which are used from microwave to optical frequencies to create new antennas and microwave devices. Here, we propose the use of variable-impedance metasurfaces for transforming surface or guided waves into different wavefield configurations with desirable properties. We will shortly refer to this metasurface-driven wavefield transformation as “metasurfing.” Metasurfing can be obtained by an appropriate synthesis of inhomogeneous metasurface reactance that allows a local modification of the dispersion equation and, at constant operating frequency, of the local wave vector. The general effects of metasurface modulation are similar to those obtained in solid (volumetric) inhomogeneous metamaterial as predicted by the transformation optics-namely, readdressing the propagation path of an incident wave. However, significant technological simplicity is gained. Several examples are shown as a proof of concept.

Non-Uniform Metasurface Luneburg Lens Antenna Design

A metasurfing concept is demonstrated and applied in the design of Luneburg lens antennas. Using an array of size-varying circular patches on a dielectric substrate inside a parallel-plate waveguide (PPW) structure variable surface impedance is obtained, which realizes an equivalent refraction index as that of a Luneburg lens. The obtained lens has good bandwidth characteristics and significant fabrication advantages with respect to conventional dielectric lenses. Based on this PPW lens, an H-plane antenna has been designed and simulated.

Study of Mutual Coupling Between Circular Stacked-Patch Antennas on a Sphere

A rigorous mathematical analysis is given of spherical stacked-patch arrays with emphasis on the physical interpretation of mutual coupling mechanisms present in doubly- curved convex structures. The analysis method is based on electromagnetic field representation in terms of spherical harmonics where each harmonic has the same angular variation as the spectral source component. To obtain the spectral representation the vector-Legendre transformation is applied to currents and fields. A novel approach to the mutual coupling calculation within the method of moments analysis of spherical arrays is applied. By expressing the patch current in terms of two suitable potential-like auxiliary functions, it is possible to avoid the use of Eulers formulas for coordinate system rotation and the related lengthy integrations. Instead, the rotation of antenna elements and corresponding current distributions can be done in closed form with the help of Vilenkins addition theorem for associated Legendre functions. It is shown that the new approach results in significant acceleration and improved accuracy of the analysis of spherical patch antenna arrays. The algorithm is successfully tested against a commercially available electromagnetic software and measurements performed on the developed laboratory model, confirming its accuracy for both input impedance and mutual coupling calculation and with only a small difference between the predicted and measured resonant frequencies, due to limitations in the experimental model. The influence of the structure parameters on mutual coupling level is extensively investigated, including all coupling mechanisms and leakage of energy due to curvature of the structure. It is shown that stacked-patch antennas can have reduced coupling level comparing to single patch antennas with possible deep nulls above the antenna resonant frequency.

Efficient Analysis of Aperture Antennas on Generally Shaped Convex Multilayered Surfaces Using a Hybrid SD-UTD Method

A novel hybrid method is described for analyzing convex multilayered conformal array antennas. The hybrid method is based on the spectral domain approach in combination with the ray-based uniform theory of diffraction (UTD) method. The analysis is divided in two parts. First, the spectral domain approach is accelerated by using an asymptotic extraction technique where the extracted term of the Greens function is calculated using UTD. It is shown that this new approach results in significant acceleration of the existing spectral domain algorithm without losing accuracy. The modified spectral domain method is then used in the second part where generally shaped convex multilayered surfaces are analyzed by using sets of canonically shaped surfaces (spheres and/or circular cylinders). Their radii are obtained using the UTD formulation, which contains important information such as distance and curvature of the generally shaped surface along each geodesic. The results obtained using the new algorithm are compared to the available results (calculated and measured) for different conformal antennas, showing very good agreement.

New type of horn based on variable metasurface

This paper demonstrates the design and the analysis of a novel type of horn antennas with a potentially higher directivity. The idea is to replace one plate of an H-plane horn antenna with a plate that has specially designed surface impedance. In our case we generated the surface impedance variation that corresponds to a Luneburg law, thus obtaining the Luneburg lens effect and consequentially increasing the directivity and reducing the overall horn length.

Hybrid Spectral Domain - UTD method applied to conformal antenna analysis

A hybrid spectral domain - UTD method (Uniform Theory of Diffraction) developed for the analysis of conformal antennas is presented in this paper. The basic idea behind the hybrid method is to combine different analysis methods for conformal antennas and, at the same time, preserve the advantages of the considered methods. More specifically, the considered hybrid SD - UTD method joins the possibility of analyzing multilayer structures (spectral domain method) and the possibility of analyzing electrically large structures (UTD method). Hence, using the developed method it is possible to analyze large multilayered conformal antennas with high accuracy and speed. This work has been made possible by the ACE program for structuring the research on conformal antennas in Europe. The developed method is a result of joining research activities of University of Zagreb, Croatia and the Royal Institute of Technology, Sweden.

Acceleration of Series Summation Encountered in the Analysis of Conformal Antennas

Complexity and needed computation time of moment method analysis approach for conformal antennas is reduced by applying particular series acceleration techniques to the partial sums encountered in these problems. The Wynns ε-acceleration technique is investigated and modified in order to achieve better convergence. The results show excellent convergence properties when applying this method in the analysis of antennas printed on cylindrical and spherical substrates.

Cost effective FBG based optical sensor

Fiber Bragg grating (FBG) based sensors constitute majority of optical sensors for measuring physical values such as strain, temperature, or both. While properties of FBG alone are well known, interrogation techniques have not been standardized. Most sensors use expensive equipment, such as optical spectral analysers, for measuring Bragg wavelength changes as a result, of temperature, or strain change. Such method is reliable and gives good results, but cannot be used if we are to construct a cost effective sensor whose use could be widespread. In this paper we demonstrate an alternative way of measuring Bragg wavelength shift that could allow us to build cheap, reliable sensor. Furthermore, the proposed sensor can be thermally stabilised without additional resources.

Fiber optic vibration sensor for high-power electric machines realized using 3D printing technology

The objective of this work was to demonstrate a lightweight and inexpensive fiber-optic vibration sensor, built using 3D printing technology, for high-power electric machines and similar applications. The working principle is based on modulating the light intensity using a blade attached to a bendable membrane. The sensor prototype was manufactured using PolyJet Matrix technology with DM 8515 Grey 35 Polymer. The sensor shows linear response, expected bandwidth (< 150 Hz), and from our measurements we estimated the damping ratio for used polymer to be ζ ≈ 0.019. The developed prototype is simple to assemble, adjust, calibrate and repair.

Temperature-stable LED-based light source without temperature control

Many medical, environmental, and industrial sensing applications could take advantage of uncooled temperature-stable optical sources that are incoherent and un-polarized as such sources do not produce interference fringes, speckle patterns, or intensity variations due to polarization. For this purpose we propose an optical system for stabilization of light-emitting diodes over temperature exhibiting output power variation below 50 ppm/°C which does not employ any kind of TEC elements or even thermometers. This makes it especially suitable for handheld and battery operated instruments.