High Frequency Propagation Research Articles

Electrostatic drift-wave instability in a nonuniform quantum magnetoplasma with parallel velocity shear flows

The propagation of high and low frequency (in comparison with the cyclotron frequency) electrostatic drift-waves is investigated in a nonuniform, dense magnetoplasma (composed of electrons and ions), in the presence of parallel shear flow, by employing the quantum magnetohydrodynamic (QMHD) model. Using QMHD model, a new set of equations is presented in order to investigate linear properties of electrostatic drift-waves with sheared plasma flows for dense plasmas. In this regard, dispersion relations for coupled electron-thermal and drift-ion acoustic modes are derived and several interesting limiting cases are discussed. For instance, it is found that sheared ion flow parallel to the external magnetic field can drive the quantum drift-ion acoustic wave unstable, etc. The present investigation may have relevance in dense astrophysical environments where quantum effects are significant.

High-frequency propagation for the Schrödinger equation on the torus

The main objective of this paper is understanding the propagation laws obeyed by high-frequency limits of Wigner distributions associated to solutions to the Schrödinger equation on the standard d-dimensional torus T d . From the point of view of semiclassical analysis, our setting corresponds to performing the semiclassical limit at times of order 1 / h , as the characteristic wave-length h of the initial data tends to zero. It turns out that, in spite that for fixed h every Wigner distribution satisfies a Liouville equation, their limits are no longer uniquely determined by those of the Wigner distributions of the initial data. We characterize them in terms of a new object, the resonant Wigner distribution, which describes high-frequency effects associated to the fraction of the energy of the sequence of initial data that concentrates around the set of resonant frequencies in phase-space T * T d . This construction is related to that of the so-called two-microlocal semiclassical measures. We prove that any limit μ of the Wigner distributions corresponding to solutions to the Schrödinger equation on the torus is completely determined by the limits of both the Wigner distribution and the resonant Wigner distribution of the initial data; moreover, μ follows a propagation law described by a family of density-matrix Schrödinger equations on the periodic geodesics of T d . Finally, we present some connections with the study of the dispersive behavior of the Schrödinger flow (in particular, with Strichartz estimates). Among these, we show that the limits of sequences of position densities of solutions to the Schrödinger equation on T 2 are absolutely continuous with respect to the Lebesgue measure.

The Effects of Tropical Weather on Radio-Wave Propagation Over Foliage Channel

This paper investigates the dynamic property of a tropical forested channel due to the weather effect on very high frequency (VHF) and ultrahigh frequency (UHF) radio-wave propagation. In this paper, continuous-wave (CW) envelope fading waveforms are recorded over a period of 50 s with static antennas. This paper focuses on the analysis of the combined effect of wind and rain, which is often encountered in a tropical forest. The induced temporal effects are discussed and compared with theoretical models. It is found that the distribution of temporal fading components resembles a Rician distribution function. Its Rician K factor gradually decreases as the strength of either wind or rain increases due to the movement of the forest components.

High-Frequency Propagation on Printed Circuit Board Using a Material With a Low Dielectric Constant, a Low Dielectric Loss, and a Flat Surface

For the next-generation printed circuit boards with a fine pattern, low power consumption and high-speed propagation with low variation of signal propagation must be satisfied. In order to meet such requirements, we investigated a dielectric material for printed circuit boards which has three superior features: low dielectric constant (permittivity), low dielectric loss, and a flat surface with sufficient adhesion force for plating metal. The propagation loss of a microstrip line on the developed material is about 40% of the conventional material at 10 GHz. Significant reduction of the manufacturing fluctuation of the developed materials is achieved due to the flat surface. We adopted a distributed constant model to the propagation results and revealed that the significant reduction of the propagation loss is caused by the flat surface and low dielectric loss. This technology greatly contributes to the next generation of printed circuit boards.

PpTMDS as a new polymer technology for a high throughput bio-MEMS design

The development of more complex biochips in terms of functionality requires some technological evolutions. A high frequency biological micro-electro-mechanical-system dedicated to biological analysis is a typical case of this requirement for providing compatibility between high frequency propagation and microfluidic circulation. Mixed fabrication technologies using silicon and polymers appear to be a good alternative. We have developed a promising deposition process of an organosilicon polymer by remote afterglow plasma technology, also called plasma polymerized tetramethyldisiloxane (ppTMDS). This technique allows us to obtain high deposition rate values in the range of 160 Å s−1 and a thick layer up to 140 µm without any crack. Moreover, this process is compatible with high throughput microelectronic designs and it is done near room temperature. This last point is very interesting for further development of surface bio-functionalization, for example. We have characterized this siloxane polymer by physico-chemical analysis. The roughness has been optimized, thus allowing the realization of high frequency waveguides. The ppTMDS permittivity presents a low dispersive characteristic and constitutes one of the best low-loss polymers up to 1 THz.

Optimizing Learning Convergence Speed and Converged Error for Precision Motion Control

This brief paper considers iterative learning control (ILC) for precision motion control (PMC) applications. This work develops a methodology to design a low pass filter, called the Q-filter, that is used to limit the bandwidth of the ILC to prevent the propagation of high frequencies in the learning. A time-varying bandwidth Q-filter is considered because PMC reference trajectories can exhibit rapid changes in acceleration that may require high bandwidth for short periods of time. Time-frequency analysis of the initial error signal is used to generate a shape function for the bandwidth profile. Key parameters of the bandwidth profile are numerically optimized to obtain the best tradeoff in converged error and convergence speed. Simulation and experimental results for a permanent-magnet linear motor are included. Results show that the optimal time-varying Q-filter bandwidth provides faster convergence to lower error than the optimal time-invariant bandwidth.

High frequency propagation in and scattering from water‐saturated granular sediments: Laboratory study

Acoustic properties of water‐saturated granular sediments at frequencies from 150 kHz to 8 MHz were studied in controlled laboratory conditions using broadband transducers. Two samples of medium sand sediments, with the same mean grain size, but with different width of the size distribution, were taken for the study, degassed, and their surface was flattened. Another sample of sediments was composed of glass beads of the same grain size. The main difference of glass beads from sand grains was their shape. Backscattering strength at normal and oblique incidence and reflection coefficient at normal incidence were measured for the three samples. The reflection experiments were made for different thicknesses of the samples, so that reflections from both first and second interfaces of the sediment layer were measured. This allowed also estimating sound speed and attenuation in the sediments. The results obtained for the three chosen types of the sediments were compared to demonstrate effects of the grain size distribution width and the grain shape on acoustic properties of the sediment. [Work supported by ONR and CNRS].

High-Frequency Shear-Wave Propagation across the Hellenic Subduction Zone

Abstract This study investigates shear-wave propagation across the Hellenic subduction zone using high-quality waveforms of 71 intermediate-depth events (40–160-km depth). The data have been recorded by a permanent seismic network consisting of 22 broadband, three-component stations that cover most parts of Greece. Shear waves can be efficiently transmitted with low attenuation ( Q s ∼1100) through the descending slab, which extends laterally over much of the forearc area with a northwest–southeast strike. The slab also appears to be continuous downdip in the depth range 60–160 km, even though it was found that a gap smaller than 10 km would not affect shear-wave efficiency significantly. Two highly attenuating areas have been detected, one in the central/southeast Aegean corresponding to the mantle wedge and the other in the northern Aegean, probably caused by elevated upper mantle temperatures owing to the ongoing extension in that area. These results provide additional constraints for the geometry of the Hellenic subduction zone and the geodynamic setting of the Aegean region.

Emergent Activities in the Arctic: A Space Weather Opportunity

Emergent Activities in the Arctic: A Space Weather Opportunity

Time‐dependent characterization of an optically modulated CPW line characterized with a large signal network analyzer

AbstractTime‐dependent behavior of a CPW line under optical modulation is characterized using a large‐signal vector network analyzer. The influence of the optical modulation frequency on the propagation of high carrier frequencies along a CPW is discussed in this article. Different operation conditions in time are defined for a CPW line under illumination. © 2007 Wiley Periodicals, Inc. Microwave Opt Technol Lett 49: 3124–3128, 2007; Published online in Wiley Inter‐Science (www.interscience.wiley.com). DOI 10.1002/mop.22888

Laser induced short plane acoustic wave focusing in water

Laser induced high frequency acoustic wave generation, propagation, and focusing in water are studied. A large area, flat, and short duration acoustic wave was generated by the thermoelastic interaction of a homogenized short pulsed laser beam with the liquid-solid interface and propagated at the speed of sound. Laser flash Schlieren photography was used to visualize the transient interaction of the flat acoustic wave with a cylindrical concave lens and the subsequent acoustic wave focusing. Numerical simulations showed the acoustic wave could be focused to several tens of microns in size and 7bars in pressure.

High rate data transmission in the mid-latitude NVIS HF channel

The paper investigates the use of differential modulation techniques to achieve high rate data transmission by near vertical incidence sky wave high frequency propagation, using a 2.7 kHz voice channel bandwidth. Channel characteristics have been measured for a range of frequencies between 2.8 and 9.4 MHz, using a transmitter power of 100 W over a distance of 160 km. These measurements are used to characterise simulated channel conditions, and the simulator is then used to determine the performances of differential modulation methods. In particular, time-differential amplitude and phase shift keying and frequency-differential amplitude and phase shift keying modulation methods are investigated. Symbol error rates are presented for data rates ranging from 4.3 to 20.6 kbit/s, where the system performances are limited by time-varying channel dispersion rather than by additive noise.

Application of parabolic equation methods to HF propagation in an Arctic environment

This paper demonstrates the usefulness and flexibility of parabolic equation methods when applied to high frequency (HF) propagation calculations over cliffs and in realistic arctic environments, which are required for estimating the performance of High Frequency Surface Wave Radar (HFSWR). All calculations are performed using a modified version of the TERPEM (TERrain Parabolic Equation Model) software package. This software has been tested extensively through comparisons with other models. As an example, a comparison between data generated with TERPEM and data published in the literature is shown. Calculations performed for four cliff heights (0, 102, 300, 498 m) show that the cliff does not significantly affect the propagation of HFSWR signals at the ranges of interest. Site-specific calculations performed for realistic arctic conditions (cold, low-salinity water covered with broken sea ice and snow) show that HFSWR propagation can be equal to or better than that for ice-free conditions over a significant range for a significant portion of the year (/spl sim/3 or 4 months). Based on these results, and other considerations related to sea clutter, ionospheric clutter, and man-made noise, it is concluded that useful operation of HFSWR at specific sites in the arctic should be feasible for a significant portion of the year.

Simulation of Asymmetric Lamb Waves for Sensing and Actuation in Plates

Two approaches used for monitoring the health of thin aerospace structures are active interrogation and passive monitoring. The active interrogation approach generates and receives diagnostic Lamb waves to detect damage, while the passive monitoring technique listens for acoustic waves caused by damage growth. For the application of both methods, it is necessary to understand how Lamb waves propagate through a structure. In this paper, a Physics‐Based Model (PBM) using classical plate theory is developed to provide a basic understanding of the actual physical process of asymmetric Lamb mode wave generation and propagation in a plate. The closed‐form model uses modal superposition to simulate waves generated by piezoceramic patches and by simulated acoustic emissions. The generation, propagation, reflection, interference, and the sensing of the waves are represented in the model, but damage is not explicitly modeled. The developed model is expected to be a useful tool for the Structural Health Monitoring (SHM) community, particularly for studying high frequency acoustic wave generation and propagation in lieu of Finite Element models and other numerical models that require significant computational resources. The PBM is capable of simulating many possible scenarios including a variety of test cases, whereas experimental measurements of all of the cases can be costly and time consuming. The model also incorporates the sensor measurement effect, which is an important aspect in damage detection. Continuous and array sensors are modeled, which are efficient for measuring waves because of their distributed nature.

Can ion-neutral damping help to form spicules?

The possible mechanism of generation of spicules by Alfvénic waves is studied in dissipative MHD where dissipation is mainly caused by ion-neutral collision damping, as suggested by Haerendel ([CITE]). Ion-neutral damping becomes non-negligible at the high cyclic frequencies involved, typically greater than , and the potential role played by this effect in both forming and supporting solar spicules is investigated. The propagation of high frequency Alfvén waves on vertically open solar magnetic flux tubes is considered. The flux tubes are taken to be axisymmetric and initially untwisted with the field strength declining from in the photosphere to in the corona. Their propagation is investigated by numerically solving a set of fully nonlinear, dissipative 1.5D MHD equations with the waves being generated by a continuous sinusoidal driver introduced into the equation of angular momentum in the low atmosphere of the Sun. Spicule-like structures with heights of up to were formed. The formation was found to be caused by the impact of a series of slow shocks generated by the continuous interaction between the upward propagating driven wave train and the downward propagating train of waves created by reflection off the transition region and aided by the increased thermal pressure gradient caused by Joule heating due to ion-neutral collisions. The adiabatic results suggest that ion-neutral damping may not support spicules as described by Haerendel ([CITE]). However, the effect is highly sensitive to the level of ionisation and therefore to the energy balance. Including the effects of thermal conduction and radiation may well lead to different results and thus it would be premature to dismiss the mechanism completely at this point. In addition, the relatively high chromospheric temperatures obtained, even at frequencies for which ion-neutral damping and heating might be expected to be unimportant, suggest intriguing possibilities for combining the mechanism with others that are better able to recreate spicule dynamics but suffer from unrealistically low temperatures.

Analysis of very high frequency propagation in sediments: Experimental results and modeling

A current QinetiQ study is investigating the propagation of sound waves into sediment at frequencies higher than 300 kHz. Previous work has found notable discrepancies between model predictions and experimental results and comparisons are inconsistent and unreliable. This new work package investigates the development of new scattering theories for frequencies ranging from 300 kHz to 1 MHz. In particular, the application of a pseudospectral time difference approached is analyzed. The model originally developed for lower frequency applications, is set up in various geometrical scenarios and for varying very high frequencies. Results show the received simulated pulses obtained for hydrophones placed within the sediment and source colocated. Furthermore, simulations are compared in tank experimental data. The controlled tank experiments were conducted by Southampton University and data are analyzed and discussed for various conditions and frequencies.

Modeling High Frequency Propagation in Tunnel Environments by IterativePhysical Optics Method

A new formulation of the Iterative Physical Optics (IPO) method, including waveguide wall loss effects, has been developed to model high frequency electromagnetic wave propagation in tunnel environments. The lossy behavior of the tunnel walls has been considered throughout the inclusion of the Impedance Boundary Condition (IBC) model in the formulation of the IPO. Different arched tunnels have been studied, showing that this approach has a convergent behavior and providing accurate results for realistic tunnel models. Moreover, the computational cost associated with the analysis of long tunnels has been significantly reduced by using a connection scheme which splits the tunnel into a set of short sections that can be more easily analyzed. Some results are shown, illustrating the capability of the proposed method.

Gigahertz surface acoustic wave probe for chemical analysis

This paper considers the propagation of high frequency 0.1–2.6 GHz surface acoustic wave pulses in aqueous solutions of pure water, glycerol and protein. The GHz frequency components of the pulse are used to provide the highest operating frequencies so far reported and also to construct the first acoustic absorption spectrum associated with the evanescent field. Acoustic generation is sourced from a single non-linear SAW device that provides a series of harmonic frequencies, simultaneously. The received power level is determined from digital samples of the received pulse waveform. The power leaked into glycerol solutions at the fundamental frequency was found to be 50% smaller for pulses, than for continuous acoustic waves, an effect that could be related to the equilibration of the evanescent field. Increasing the concentration of the glycerol solutions or time exposed to the protein (IgG) solution, showed that the power losses from the surface acoustic wave pulse were broadly consistent with the behaviour of transverse shear mode sensors. Atomic force microscope measurements of the bare device revealed that the morphology of the silica overlayer was uniformly granular, whereas adsorbed protein films formed non-contiguous islands. Confirmation of the presence of the IgG film was obtained from quantitative X-ray photoelectron spectroscopy. An 8 gigasample per second digitising oscilloscope running a fast Fourier transform routine captured the acoustic absorption spectrum, and revealed a smooth characteristic for the glycerol and IgG, although for the latter, frequencies beyond 500 MHz were associated with an irregular spectrum. These multiple frequency measurements of the solid–liquid interface provide evidence that when the penetration depth and film thickness are similar, disruption of the predicted exponential form of the evanescent wave occurs, as indicated by the fluctuations seen in the absorption spectrum recorded. These preliminary results have shown that multiple frequency operation of single non-linear SH-SAW devices is possible, and an evanescent interfacial absorption spectrum can be obtained. By extending the measurement technique it may be possible to obtain additional information about the structure and composition of the solid–liquid interface.

The influence of bubble clouds on acoustic propagation in the surf zone

High-frequency propagation close to an active surf line is explored with 12and 100-kHz propagation paths together with measurements of bubble clouds, bubble size distributions, and waves. Breaking waves inject massive bubble plumes that are mixed downwards from the roller region by intense turbulence. If these injections follow one another at intervals less than the time taken for the bubbles to rise to the surface, acoustic signals will be continuously blocked, forming an acoustical barrier that effectively inhibits any propagation. Occasionally, waves break seaward of this barrier. In this case, dense bubble clouds are mixed down beneath the air entrainment zone, but there is sufficient time for them to disappear before succeeding breakers, allowing intermittent high-frequency propagation recharge the bubble field. The duration and shape of signal dropouts are then determined by the selective removal of bubbles by buoyancy and dissolution. In addition to turbulence created by the air entrainment process, a lower level of continuous background turbulence may be generated by interaction of residual currents with the wave boundary layer. Our observations illustrate the variable character of acoustic blocking by bubble clouds and serve as a basis for quantitative analysis of these effects with a 2D propagation model coupled to 2D models of bubble cloud evolution and background turbulence.

On Stability of Multitime Step Integration Procedures

Stability of multitime step integration methods for finite-element computations in structural dynamics is analyzed. Multitime step procedures based on the Newmark family of methods are described. The basic idea of multitime step methods is to utilize various time step sizes in different domains of an element mesh. Interpolated nodal values from the large time step domain are used in the computation of displacements, velocities, and accelerations in the small time step domain. An analytical study of the errors introduced by the interpolation is given. The analysis shows instability because of the resonance phenomena and because of the propagation of spurious high frequencies caused by unbalanced forces originating from interpolation errors. Numerical results exhibiting instability problems are presented. To complete the study, examples of solutions stabilized with numerical damping of high frequencies are included in the presentation. The conclusion from the present study is that multitime step methods are unstable in their nature. Numerical damping can stabilize the computations but alters the response of free vibrating undamped systems.