Fast and Robust Characterization of Dielectric Slabs Using Rectangular Waveguides

Waveguide characterization of dielectric materials is a convenient and broadband approach for measuring dielectric constant. In conventional microwave measurements, material samples are usually mechanically shaped to fit the waveguide opening and measured in closed waveguides. This method is not practical for millimeter-wave and sub-millimeter-wave measurements where the waveguide openings become tiny, and it is rather difficult to shape the sample to exactly the same dimensions as the waveguide cross-section. In this paper, we present a method that allows one to measure arbitrarily shaped dielectric slabs that extend outside waveguides. In this method, the measured sample is placed between two waveguide flanges, creating a discontinuity. The measurement system is characterized as an equivalent Pi-circuit, and the circuit elements of the Pi-circuit are extracted from the scattering parameters. We have found that the equivalent shunt impedance of the measured sample is only determined by the material permittivity and is rather insensitive to the sample shape, position, sizes, and other structural details of the discontinuity. This feature can be leveraged for accurate measurements of permittivity. The proposed method is very useful for measuring the permittivity of medium-loss and high-loss dielectrics from microwave to sub-terahertz frequencies

Graphene-based Perfect Absorbers: Systematic Design and High Tunability

Experimental realization of efficient graphene-based absorbers is a challenging task due to the low carrier mobility in processed graphene. In this paper, we circumvent this problem by placing uniform graphene sheets on metallic metasurfaces designed for improving the absorption properties of low-mobility graphene. Complete absorption can be achieved for different frequencies with the proper metasurface design. In the THz band, we observe strong tunability of the absorption frequencies and magnitudes when modulating graphene Fermi level

Singular Vector Filtering Method for Disturbance Enhancement Mitigation in Active Noise Control Systems

In multichannel active noise control systems, when reference signals are correlated, the disturbance enhancement phenomenon is likely to occur, i.e., the resulting sound is enhanced instead of being reduced in some frequency bands, if the filter is designed to minimize the total energy for all frequencies. In previous works, a truncated singular value decomposition method was applied to the system autocorrelation matrix to mitigate the disturbance enhancement due to the correlation of reference signals. Some small singular values and the associated singular vectors are removed, if they are responsible for unwanted disturbance enhancement in some frequency bands. However, some of these removed singular vectors may still contribute to noise control performance in other frequency bands, thus a direct truncation will degrade the noise control performance. In the present work, through an additional filtering process, the set of singular vectors that cause the disturbance enhancement are replaced by a set of new singular vectors whose frequency responses are attenuated in the frequency band where disturbance enhancement occurs, while the frequency responses in other frequency bands are unchanged. Compared with truncation, the proposed method can maintain the performance in the noise reduction bands, while mitigating the influence in disturbance enhancement bands

Discrete Impedance Metasurfaces for 6G Wireless Communications in D-Band

Engineering and optimization of wireless propagation channels will be one of the key elements of future communication technologies. Metasurfaces may offer a wide spectrum of functionalities for passive and tunable reflecting devices, overcoming fundamental limits of commonly used conventional phase-gradient reflectarrays and metasurfaces. In this paper, we develop an efficient way for the design and implementation of metasurfaces with high-efficiency anomalous reflector functionalities. The developed numerical method provides accurate, fast, and simple metasurface designs, taking into account non-local near-field interactions between array elements. The design method is validated by manufacturing and experimental testing of highly efficient anomalous reflectors for the millimetre-wave band.Comment: 8 pages, 8 figure

Truncated Singular Value Decomposition Method for Mitigating Unwanted Enhancement in Active Noise Control Systems

It is well-known that good noise cancellation performance can only be realized by a multiple-input active noise control system when the primary noise sources are persistently exciting, and the reference signals are uncorrelated. Otherwise, the noise reduction performance will deteriorate and, quite possibly, the noise can be enhanced. In particular, when the reference signals are correlated in a certain frequency band, enhancement can occur in that band. In the present work, singular value decomposition was applied to the auto-correlation matrix of the reference signals to analyze this enhancement issue. It was found that the level of enhancement was associated with the small singular values. Also, the enhancement frequency bands were found to be associated with large values of the frequency response of the filters that correspond to the singular vectors associated with the small singular values. According to this analysis, a method that removes the small singular values and associated singular vectors of the auto-correlation matrix was proposed and applied to mitigate the noise enhancement. The designed controllers were experimentally implemented in real time and the experimental performance agreed well with off-line simulation results, in which the noise enhancement was reduced

Torque performance improvement on multi three-phase PMSM based on PWM drives for marine application

Multiphase drive systems have received a growing interest in recent decades for marine propulsion applications, due to their high power density, high reliability and good torque performance. Among the multiphase drives, the multi three-phase drive with independent neutral points is a popular option for this application, as it allows for the usage of standard control and standard power electronics for the individual three-phase systems. In high power systems the switching frequency of the power semiconductor is usually limited, which results in high frequency current ripple caused by the PWM of the DC/AC converter. The ripple affects the performance of the machine in terms of torque. This thesis presents a novel mathematical modelling of multi three-phase Permanent Magnet Synchronous Machines (PMSMs) fed by voltage source Pulse Width Modulation (PWM) converters. It is found that, based on the analytical models of the multi three-phase drive, the torque ripple introduced by PWM voltage excitation can be reduced by the shift of carrier phase angles among different three-phase inverters. For the torque ripple analyzed in this thesis, only the interaction between the armature field, resulting from the PWM voltage excitation, and the fundamental component of the permanent magnet field is considered. The proposed carrier phase shift angles are obtained for the case studies of a sectored triple three-phase PMSM and two dual three-phase PMSMs. Numerical and finite element analysis (FEA) and experimental results are presented to validate the analytical models of the multi three-phase drives. Additionally, the torque performance improvement and the effect on current ripple introduced by the proposed carrier phase shifts are presented and validated by means of both simulation and experimental results

Controlling surface waves with temporal discontinuities of metasurfaces

Static reactive metasurfaces allow excitation and propagation of surface waves. In this paper, we theoretically elucidate how surface-wave propagation along a reactive boundary is affected by temporal discontinuities of effective parameters characterizing the boundary. First, we show that by switching the value of the surface reactance, the velocity of surface waves is fully controlled, and the power of reflected and transmitted surface waves can be amplified. Second, we indicate that when a boundary supporting waves with transverse-electric polarization is switched to the one allowing only transverse-magnetic polarization, the propagating surface wave is “frozen” and converted to a static magnetic-field distribution. Moreover, efficiently, these fields can be “melted”, restoring propagating surface waves when the boundary is switched back to the initial state. Finally, we demonstrate that temporal jumps of the boundary reactance couple free-space propagating waves to the surface wave, in an analogy to a spatial prism. All these intriguing phenomena enabled by temporal discontinuities of effective properties of reactive metasurfaces open up interesting possibilities for the generation and control of surface waves

Controlling surface waves with temporal discontinuities of metasurfaces

In this paper, we investigate the scattering of surface waves on reactive impedance boundaries when the surface impedance undergoes a sudden change in time. We report three exotic wave phenomena. First, it is shown that by switching the value of the surface capacitance of the boundary, the velocity of surface waves can be fully controlled, and the power of reflected and transmitted surface waves are amplified. Second, we show that when a capacitive boundary is switched to an inductive one, the surface wave stops completely, with a "frozen" static magnetic field distribution. The static magnetic fields are "melt" and restore propagating surface waves when the boundary is switched back to a capacitive one. Third, we show that temporal jumps of the boundary impedance couple free-space propagating waves to the surface wave, which is an analog to a spatial prism. These interesting effects enabled by temporal jumps of metasurface properties open up new possibilities for the generation and control of surface waves.Comment: 19 pages, 10 figure