silvio hrabar

Non-Foster and Time-Varying Metamaterial-Inspired Antennas

While metamaterial-inspired electrically large antennas (such as aperture antennas with metasurfaces) are well documented in the literature, research in the field of metamaterial-based and metamaterial-inspired electrically small antennas (ES As) is still lagging behind. The most common application of ideas from the metamaterial field is broadband matching. It is well known that classical passive resonant matching of an ESA is inherently limited by a very narrow bandwidth caused by the inevitably high Q-factor. On the other hand, there are active approaches such as non-Foster-based matching and the recently intensively studied time­varying-based matching, which at least theoretically offer an operating bandwidth of several octaves. Unfortunately, these methods suffer from stability problems as well as the problem oflimited power-handling abilities when used in transmitting applications. In the first part of this talk, different approaches to overcome stability problems in general non­ Foster-based antennas are discussed. It will be shown that the main problem is not, as usually assumed, the limitations of the technology used (occurrence of "parasitics"). Surprisingly, the problems arise from the (still) insufficient understanding of the physical background and the "blind" use of commercial CAD tools for stability analysis. A simple design leading to stable systems is explained for different types of actively matched small dipole, small loop and small Huygens loop dipole radiators. These designs utilize stability-robust 'band-pass' negative "bandpass" capacitors, two-transmitter non-Foster-inspired matching, and self-oscillating, nearly perfectly matched and broadly tunable radiators. The second part deals with the concept of time-varying-based small antennas, which has recently been studied very intensively. It is shown that, in addition to the usual applications where attempts are made to exceed the data rate limits imposed by the high Q-factor of a small antenna, it is also possible to construct a time-varying-based non-Foster system for both the receiving and transmitting antennas. The third part shows that it is possible to combine both the non-Foster and the time-varying approach into novel active ESAs with both wide bandwidth and stable operation. The discussion of the above methods will be complemented by examples of practical experimental demonstrators operating in RF band (< 3GHz), developed at University of Zagreb. Finally, some unsolved problems and related future research directions will be highlighted.

Silvio Hrabar was born in Trogir, Croatia in 1962. He received Dipl. Ing. and M.S. degrees from University of Zagreb, Croatia and a Ph.D degree from Brunel University of West London, United Kingdom, all in electrical engineering. In the past, he was employed at various consulting, development, research and teaching positions both in industry and academia, in the fields of radio engineering, microwave electronics, antenna engineering, electromagnetic compatibility, electromagnetic metrology, computational electromagnetics and electrostatics. Currently, he is affiliated with University of Zagreb, Croatia, where he is a Professor of applied electromagnetics. His research interest includes applied electromagnetics, electromagnetic compatibility, antennas, microwave measurements and microwave engineering. He is the author and co-author of more than 200 technical papers in journals and conferences, many technical studies for government and industry, one textbook, and several book chapters. He also serves as a reviewer for a dozen of scientific journals in electrical engineering and applied physics. Professor Hrabar is a chair of Metamaterial group at Faculty of Electrical Engineering and Computing (FER), University of Zagreb. In 2012 he received 'Award for exceptional achievements in research and innovations' for 'Contribution to the understanding of basic physics of electromagnetic metamaterials and development of their engineering applications'. In 2023 and 2024, Prof. Hrabar was ranked among the 2% most cited scientists in the world in the field of Information and communication technologies (Stanford University list).