2020 Fourteenth International Congress on Artificial Materials for Novel Wave Phenomena (Metamaterials), 2020
Equivalent circuit of a non-Foster negative capacitor, based on an SCS voltage-inversion negative... more Equivalent circuit of a non-Foster negative capacitor, based on an SCS voltage-inversion negative converter with one-pole and two-pole amplifier model, has been developed. The circuit was loaded with an external RC network of series or parallel type, and the stability of the whole system is investigated. It was found that there is always a trade-off between operating bandwidth and the range of external capacities and resistances that assure stable operation.
In a last few years there has been a great interest in artificially made materials (metamaterials... more In a last few years there has been a great interest in artificially made materials (metamaterials), electromagnetic properties of which cannot be found in Nature. Probably mostly investigated metamaterial is an Double Negative metamaterial (DNG), which real parts of effective permeability and permittivity have negative values. Investigation of this type of material has revealed several new counter-intuitive physical phenomena such as negative refraction, growing evanescent waves and imaging with the subwavelength resolution. All these phenomena rely on the fact that DNG material supports propagation of the backward waves i.e. the waves whose phase velocity vector and Poynting vector point in the opposite directions. Although the backward-wave propagation has been used in microwave technology since fifties of previous century, the literature is sparse of its direct and intuitive demonstration in the metamaterials. This is an important issue because the metamaterials have already been...
Recently, it has been observed that a waveguide, transversal dimension of which is arbitrarily sm... more Recently, it has been observed that a waveguide, transversal dimension of which is arbitrarily smaller than a half of a wavelength supports the propagating modes if it is filled with an anisotropic single-negative (SNG) metamaterial. This counter-intuitive phenomenon is analyzed analytically, numerically and experimentally. An idealized scenario with a lossless, continuous (either mu-negative (MNG) or epsilon-negative (ENG)) filling material is used in the theoretical analysis. It is shown that the equivalent plane waves that make up the waveguide mode are inhomogeneous and this fact is responsible for the propagation below a cut-off. Numerical investigation was performed with the help of commercial FD code and it dealt with the waveguides filled with the realistic metamaterials. The simulation showed a backward-wave propagation band located below the cut-off as it was predicted by a simple homogenized model. Not surprisingly, it was also found that the microscopic (in-cell) field d...
So-called non-Foster elements (negative capacitors and negative inductors) are gaining popularity... more So-called non-Foster elements (negative capacitors and negative inductors) are gaining popularity in metamaterials and metasurfaces. However, there is a well-known ‘bottleneck’ of non-Foster technology: inherent proneness to instability. Recently, there have been several theoretical and experimental studies reporting novel ‘band-pass’ non-Foster elements with improved stability properties. Here, a connection between maximal achievable operation bandwidth and stability is investigated for the case of a band-pass non-Foster capacitor. To this end, a closed-form stability-bandwidth criterium was derived. Numerical and experimental verification of derived criterium proved its correctness and confirmed that maximal achievable bandwidth is inversely proportional to the overall system negative capacitance.
Skripta detaljno opisuje principe rada i građu vektorskog analizatora mreža s mjerenjem u frekven... more Skripta detaljno opisuje principe rada i građu vektorskog analizatora mreža s mjerenjem u frekvencijskoj i vremenskoj domeni te pripadne metode kalibracije. Također su prikazana osnovna mjerenja s vektorskim analizatorm koja se koriste u svakodnevnoj inženjerskoj praksi.
Design of high-power shortwave broadcast transmitters is one among the rare areas in the field of... more Design of high-power shortwave broadcast transmitters is one among the rare areas in the field of radiofrequency (RF) electronics, where the electron tube, as the main element for amplification of RF signal, is still in active use. The first successful attempts to replace the electron tube with solid-state components, primarily in the output stage amplifier modules for the Digital Radio Mondiale (DRM) shortwave broadcast transmitters have been published only recently. With the latest technology advancements in RF power transistors, it is now possible for commercially available RF power MOSFET transistors to be used for switching purposes at shortwave frequencies. Here, an architecture of a 10 kW shortwave broadcast transmitter with solid-state power amplifier modules in the RF output stage is proposed. Different implementations of the RF output stage that include power adding network from each amplifier module and the output matching network are analysed, both analytically and numerically. After choosing the optimal architecture, a prototype of a commercial shortwave DRM broadcast transmitter was designed and constructed, measured, tuned and tested at RIZ Transmitter factory Zagreb.
2015 9th International Congress on Advanced Electromagnetic Materials in Microwaves and Optics (METAMATERIALS), 2015
The feasibility of a D-dot wire/D-dot loop experiment in the optical regime is studied. Currently... more The feasibility of a D-dot wire/D-dot loop experiment in the optical regime is studied. Currently available materials, measurement techniques and fabrication methods, needed for the production of a sample with D-dot-based structure, are compared and discussed.
Metamaterials, Metadevices, and Metasystems 2015, 2015
In the present study, we theoretically investigate how shaping of the light distribution that ill... more In the present study, we theoretically investigate how shaping of the light distribution that illuminates a large numerical aperture lens can be used to efficiently couple light to a particular element of optical nano-circuits located at the focal region of the lens. In addition to standard plane waves, we consider illumination schemes that provide peculiar light distributions at focus, such as cylindrical vector beams. These results show that light excitation may be included as an additional degree of freedom in the optimization of optical and metatronic nano-circuitry and their coupling to conventional optical systems.
2020 Fourteenth International Congress on Artificial Materials for Novel Wave Phenomena (Metamaterials)
A recently proposed concept of a self-oscillating non-Foster antenna array is modified for the ca... more A recently proposed concept of a self-oscillating non-Foster antenna array is modified for the case of a single loop. An ordinary loop antenna is split into two smaller half-loops of unequal size, mutually connected via a voltage-inversion negative impedance converter (VNIC). This system effectively operates as a series connection of an ordinary positive-inductance half-loop and a slightly smaller negative-inductance half-loop. Insertion of additional series capacitor introduces instability and the system behaves as broadly tunable self-oscillating antenna. The idea was tested by simulations of a model of small $(\lambda/10)$ loop integrated with a Linvill floating NIC and operating in 50 MHz RF band. Obtained results revealed efficient self-oscillations with a 1:2 tuning bandwidth and the suppression of harmonics better than 20 dB. This antenna might find application in future active metasurfaces.
A concept of active ENZ non-Foster metamaterial, recently introduced at University of Zagreb is r... more A concept of active ENZ non-Foster metamaterial, recently introduced at University of Zagreb is reviewed. This approach goes around the basic dispersion constraints of a passive material and, therefore, enables ultra-broadband operation. The problem of achieving stable operation and associated technological constrains are discussed. Several state-of-the art examples of extremely broadband (a bandwidth of more than two octaves), 1D and 2D active RF ENZ metamaterials are presented.
A transformation electromagnetics (TO) cloak requires highly anisotropic metamaterial, permittivi... more A transformation electromagnetics (TO) cloak requires highly anisotropic metamaterial, permittivity and/or permeability of which may have values very close to zero (ENZ and/or MNZ metamaterials). Published experimental studies of SRR-based structure clearly proved the existence of cloaking phenomenon. Recently, there have been several attempts to design fully anisotropic cloak using transmission-line-based metamaterials. Here, we show that the bandwidth in both realizations (a volumetric SRR-based cloak or planar TL-based cloak) is constrained by the basic physics and not by applied technology. Briefly, one must use some kind of resonant circuit in order to synthesize either ENZ or MNZ behavior. The detailed circuit simulations showed that is indeed possible to construct a 2D TMz fully anisotropic cloak based on the uniaxial properties of the permeability tensor in cylindrical coordinates. The equivalent permeability and permittivity can be achieved by transmission lines loaded with...
2020 14th European Conference on Antennas and Propagation (EuCAP), 2020
Non-Foster elements are electronic circuits with inverse dispersion properties comparing to ordin... more Non-Foster elements are electronic circuits with inverse dispersion properties comparing to ordinary reactive elements. The evolution of their design is mostly related to antenna matching applications. Moreover, recent introduction of self-oscillating non-Foster antenna has motivated the negation of a complex impedance via negative impedance converter (NIC). This negation is affected by NIC conversion error, which is dependent on both load Q-factor and NIC properties. Here, we propose a novel method for design of Linvnill’s NIC, based on its decomposition into passive and active building blocks. Proposed approach is verified by analysis of Linvill’s NIC for a recently investigated self-oscillating non-Foster antenna.
The reflection coefficient measurement technique using a three-probe reflectometer was analyzed. ... more The reflection coefficient measurement technique using a three-probe reflectometer was analyzed. Based on the results of the theoretical analysis, a very simple and low cost automated stepped frequency six-port reflectometer in the 7 GHz band was designed and manufactured. A six-port network was fabricated using a simple three-probe hybrid waveguide-coaxial structure and "off the shelf" waveguide components. An automatic measuring system was implemented by adding sweep generator and commercial power meters controlled by a PC computer. Calibration was implemented using the modified Neumeyer method. Measurements were made using few unknown loads and the achieved accuracy was 3% in magnitude and 1 in the phase in the whole waveguide band. It was in good agreement with the theoretical prediction.
2018 IEEE International Symposium on Antennas and Propagation & USNC/URSI National Radio Science Meeting, 2018
The most challenging issue in implementation of negative capacitors and negative inductors (non-F... more The most challenging issue in implementation of negative capacitors and negative inductors (non-Foster elements) in active metamaterials and antennas is their proneness to instability. Recently, it was suggested that the ‘band-pass' design of a negative element may remove inherent unstable DC pole and, therefore, significantly improve stability properties. This paper reports analytical, numerical and experimental verification of aforementioned idea.
Recently introduced concept of a non-Foster antenna-transmitter that comprises a negative RC netw... more Recently introduced concept of a non-Foster antenna-transmitter that comprises a negative RC network and a short dipole was extended to an arrangement with two identical antennas. This improved system behaves as a perfectly matched, broadband antenna-transmitter array. It eliminates a need for specially designed NIC load that mimics antenna self-impedance, has a broader tuning bandwidth, and up to 3 dB higher radiated power.
It is well-known that the main drawback of nonFoster elements is their inherent proneness to inst... more It is well-known that the main drawback of nonFoster elements is their inherent proneness to instability. Although recent introduction of a band-pass non-Foster negative capacitor softened stability problem considerably, appropriate real-word applications are yet to be seen. To this end, a possible use of a band-pass non-Foster negative capacitor in impedance inverter is investigated. Preliminary analytical and numerical results show that is possible to construct a stable impedance inverter with a multi-octave bandwidth.
Although intriguing effects such as superluminal propagation in active and passive EM structures ... more Although intriguing effects such as superluminal propagation in active and passive EM structures have been experimentally confirmed and widely analyzed, there are still many open questions left. Particularly, there is no simple and physically sound method of analysis/synthesis of appropriate dispersion characteristic. Here, a novel approach that uses a complex trajectory for analysis of various complex dispersion parameters, is proposed. This method is demonstrated on causal active medium model that could support superluminal group velocity propagation with low dispersion and nearly zero losses. Presented approach could be applied to various narrowband or broadband causal active and passive structures.
2020 Fourteenth International Congress on Artificial Materials for Novel Wave Phenomena (Metamaterials), 2020
Equivalent circuit of a non-Foster negative capacitor, based on an SCS voltage-inversion negative... more Equivalent circuit of a non-Foster negative capacitor, based on an SCS voltage-inversion negative converter with one-pole and two-pole amplifier model, has been developed. The circuit was loaded with an external RC network of series or parallel type, and the stability of the whole system is investigated. It was found that there is always a trade-off between operating bandwidth and the range of external capacities and resistances that assure stable operation.
In a last few years there has been a great interest in artificially made materials (metamaterials... more In a last few years there has been a great interest in artificially made materials (metamaterials), electromagnetic properties of which cannot be found in Nature. Probably mostly investigated metamaterial is an Double Negative metamaterial (DNG), which real parts of effective permeability and permittivity have negative values. Investigation of this type of material has revealed several new counter-intuitive physical phenomena such as negative refraction, growing evanescent waves and imaging with the subwavelength resolution. All these phenomena rely on the fact that DNG material supports propagation of the backward waves i.e. the waves whose phase velocity vector and Poynting vector point in the opposite directions. Although the backward-wave propagation has been used in microwave technology since fifties of previous century, the literature is sparse of its direct and intuitive demonstration in the metamaterials. This is an important issue because the metamaterials have already been...
Recently, it has been observed that a waveguide, transversal dimension of which is arbitrarily sm... more Recently, it has been observed that a waveguide, transversal dimension of which is arbitrarily smaller than a half of a wavelength supports the propagating modes if it is filled with an anisotropic single-negative (SNG) metamaterial. This counter-intuitive phenomenon is analyzed analytically, numerically and experimentally. An idealized scenario with a lossless, continuous (either mu-negative (MNG) or epsilon-negative (ENG)) filling material is used in the theoretical analysis. It is shown that the equivalent plane waves that make up the waveguide mode are inhomogeneous and this fact is responsible for the propagation below a cut-off. Numerical investigation was performed with the help of commercial FD code and it dealt with the waveguides filled with the realistic metamaterials. The simulation showed a backward-wave propagation band located below the cut-off as it was predicted by a simple homogenized model. Not surprisingly, it was also found that the microscopic (in-cell) field d...
So-called non-Foster elements (negative capacitors and negative inductors) are gaining popularity... more So-called non-Foster elements (negative capacitors and negative inductors) are gaining popularity in metamaterials and metasurfaces. However, there is a well-known ‘bottleneck’ of non-Foster technology: inherent proneness to instability. Recently, there have been several theoretical and experimental studies reporting novel ‘band-pass’ non-Foster elements with improved stability properties. Here, a connection between maximal achievable operation bandwidth and stability is investigated for the case of a band-pass non-Foster capacitor. To this end, a closed-form stability-bandwidth criterium was derived. Numerical and experimental verification of derived criterium proved its correctness and confirmed that maximal achievable bandwidth is inversely proportional to the overall system negative capacitance.
Skripta detaljno opisuje principe rada i građu vektorskog analizatora mreža s mjerenjem u frekven... more Skripta detaljno opisuje principe rada i građu vektorskog analizatora mreža s mjerenjem u frekvencijskoj i vremenskoj domeni te pripadne metode kalibracije. Također su prikazana osnovna mjerenja s vektorskim analizatorm koja se koriste u svakodnevnoj inženjerskoj praksi.
Design of high-power shortwave broadcast transmitters is one among the rare areas in the field of... more Design of high-power shortwave broadcast transmitters is one among the rare areas in the field of radiofrequency (RF) electronics, where the electron tube, as the main element for amplification of RF signal, is still in active use. The first successful attempts to replace the electron tube with solid-state components, primarily in the output stage amplifier modules for the Digital Radio Mondiale (DRM) shortwave broadcast transmitters have been published only recently. With the latest technology advancements in RF power transistors, it is now possible for commercially available RF power MOSFET transistors to be used for switching purposes at shortwave frequencies. Here, an architecture of a 10 kW shortwave broadcast transmitter with solid-state power amplifier modules in the RF output stage is proposed. Different implementations of the RF output stage that include power adding network from each amplifier module and the output matching network are analysed, both analytically and numerically. After choosing the optimal architecture, a prototype of a commercial shortwave DRM broadcast transmitter was designed and constructed, measured, tuned and tested at RIZ Transmitter factory Zagreb.
2015 9th International Congress on Advanced Electromagnetic Materials in Microwaves and Optics (METAMATERIALS), 2015
The feasibility of a D-dot wire/D-dot loop experiment in the optical regime is studied. Currently... more The feasibility of a D-dot wire/D-dot loop experiment in the optical regime is studied. Currently available materials, measurement techniques and fabrication methods, needed for the production of a sample with D-dot-based structure, are compared and discussed.
Metamaterials, Metadevices, and Metasystems 2015, 2015
In the present study, we theoretically investigate how shaping of the light distribution that ill... more In the present study, we theoretically investigate how shaping of the light distribution that illuminates a large numerical aperture lens can be used to efficiently couple light to a particular element of optical nano-circuits located at the focal region of the lens. In addition to standard plane waves, we consider illumination schemes that provide peculiar light distributions at focus, such as cylindrical vector beams. These results show that light excitation may be included as an additional degree of freedom in the optimization of optical and metatronic nano-circuitry and their coupling to conventional optical systems.
2020 Fourteenth International Congress on Artificial Materials for Novel Wave Phenomena (Metamaterials)
A recently proposed concept of a self-oscillating non-Foster antenna array is modified for the ca... more A recently proposed concept of a self-oscillating non-Foster antenna array is modified for the case of a single loop. An ordinary loop antenna is split into two smaller half-loops of unequal size, mutually connected via a voltage-inversion negative impedance converter (VNIC). This system effectively operates as a series connection of an ordinary positive-inductance half-loop and a slightly smaller negative-inductance half-loop. Insertion of additional series capacitor introduces instability and the system behaves as broadly tunable self-oscillating antenna. The idea was tested by simulations of a model of small $(\lambda/10)$ loop integrated with a Linvill floating NIC and operating in 50 MHz RF band. Obtained results revealed efficient self-oscillations with a 1:2 tuning bandwidth and the suppression of harmonics better than 20 dB. This antenna might find application in future active metasurfaces.
A concept of active ENZ non-Foster metamaterial, recently introduced at University of Zagreb is r... more A concept of active ENZ non-Foster metamaterial, recently introduced at University of Zagreb is reviewed. This approach goes around the basic dispersion constraints of a passive material and, therefore, enables ultra-broadband operation. The problem of achieving stable operation and associated technological constrains are discussed. Several state-of-the art examples of extremely broadband (a bandwidth of more than two octaves), 1D and 2D active RF ENZ metamaterials are presented.
A transformation electromagnetics (TO) cloak requires highly anisotropic metamaterial, permittivi... more A transformation electromagnetics (TO) cloak requires highly anisotropic metamaterial, permittivity and/or permeability of which may have values very close to zero (ENZ and/or MNZ metamaterials). Published experimental studies of SRR-based structure clearly proved the existence of cloaking phenomenon. Recently, there have been several attempts to design fully anisotropic cloak using transmission-line-based metamaterials. Here, we show that the bandwidth in both realizations (a volumetric SRR-based cloak or planar TL-based cloak) is constrained by the basic physics and not by applied technology. Briefly, one must use some kind of resonant circuit in order to synthesize either ENZ or MNZ behavior. The detailed circuit simulations showed that is indeed possible to construct a 2D TMz fully anisotropic cloak based on the uniaxial properties of the permeability tensor in cylindrical coordinates. The equivalent permeability and permittivity can be achieved by transmission lines loaded with...
2020 14th European Conference on Antennas and Propagation (EuCAP), 2020
Non-Foster elements are electronic circuits with inverse dispersion properties comparing to ordin... more Non-Foster elements are electronic circuits with inverse dispersion properties comparing to ordinary reactive elements. The evolution of their design is mostly related to antenna matching applications. Moreover, recent introduction of self-oscillating non-Foster antenna has motivated the negation of a complex impedance via negative impedance converter (NIC). This negation is affected by NIC conversion error, which is dependent on both load Q-factor and NIC properties. Here, we propose a novel method for design of Linvnill’s NIC, based on its decomposition into passive and active building blocks. Proposed approach is verified by analysis of Linvill’s NIC for a recently investigated self-oscillating non-Foster antenna.
The reflection coefficient measurement technique using a three-probe reflectometer was analyzed. ... more The reflection coefficient measurement technique using a three-probe reflectometer was analyzed. Based on the results of the theoretical analysis, a very simple and low cost automated stepped frequency six-port reflectometer in the 7 GHz band was designed and manufactured. A six-port network was fabricated using a simple three-probe hybrid waveguide-coaxial structure and "off the shelf" waveguide components. An automatic measuring system was implemented by adding sweep generator and commercial power meters controlled by a PC computer. Calibration was implemented using the modified Neumeyer method. Measurements were made using few unknown loads and the achieved accuracy was 3% in magnitude and 1 in the phase in the whole waveguide band. It was in good agreement with the theoretical prediction.
2018 IEEE International Symposium on Antennas and Propagation & USNC/URSI National Radio Science Meeting, 2018
The most challenging issue in implementation of negative capacitors and negative inductors (non-F... more The most challenging issue in implementation of negative capacitors and negative inductors (non-Foster elements) in active metamaterials and antennas is their proneness to instability. Recently, it was suggested that the ‘band-pass' design of a negative element may remove inherent unstable DC pole and, therefore, significantly improve stability properties. This paper reports analytical, numerical and experimental verification of aforementioned idea.
Recently introduced concept of a non-Foster antenna-transmitter that comprises a negative RC netw... more Recently introduced concept of a non-Foster antenna-transmitter that comprises a negative RC network and a short dipole was extended to an arrangement with two identical antennas. This improved system behaves as a perfectly matched, broadband antenna-transmitter array. It eliminates a need for specially designed NIC load that mimics antenna self-impedance, has a broader tuning bandwidth, and up to 3 dB higher radiated power.
It is well-known that the main drawback of nonFoster elements is their inherent proneness to inst... more It is well-known that the main drawback of nonFoster elements is their inherent proneness to instability. Although recent introduction of a band-pass non-Foster negative capacitor softened stability problem considerably, appropriate real-word applications are yet to be seen. To this end, a possible use of a band-pass non-Foster negative capacitor in impedance inverter is investigated. Preliminary analytical and numerical results show that is possible to construct a stable impedance inverter with a multi-octave bandwidth.
Although intriguing effects such as superluminal propagation in active and passive EM structures ... more Although intriguing effects such as superluminal propagation in active and passive EM structures have been experimentally confirmed and widely analyzed, there are still many open questions left. Particularly, there is no simple and physically sound method of analysis/synthesis of appropriate dispersion characteristic. Here, a novel approach that uses a complex trajectory for analysis of various complex dispersion parameters, is proposed. This method is demonstrated on causal active medium model that could support superluminal group velocity propagation with low dispersion and nearly zero losses. Presented approach could be applied to various narrowband or broadband causal active and passive structures.
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Papers by Silvio Hrabar