Low Frequency End Research Articles

Complex permittivity and clay mineralogy of grain-size fractions in a wet silt soil

We determined the complex permittivity and clay mineralogy of grain-size fractions in a wet silt soil. We used one clay-size fraction and three silt-size fractions, measured permittivity with low error from [Formula: see text] with time-domain spectroscopy, and estimated mineral weight percentages using X-ray diffraction (XRD). The volumetric water contents were near 30%, and the temperature was [Formula: see text]. For the whole soil, standard fractionation procedures yielded 2.4% clay-size particles by weight, but XRD showed that the phyllosilicate clay minerals kaolinite, illite, and smectite made up 17% and were significantly present in all fractions. Above approximately [Formula: see text], all real parts were similar. Below approximately [Formula: see text], the real and imaginary permittivities increased with decreasing grain size as frequency decreased, and the imaginary parts became dominated by direct-current conduction. Similarly, below approximately [Formula: see text], the measured permittivity of montmorillonite, a common smectite, dominated that of the other clay minerals. Total clay mineral and smectite mass fractions consistently increased with decreasing grain size. Below [Formula: see text], a model with progressively increasing amounts of water and parameters characteristic of montmorillonite matches the data well for all fractions, predicts permittivities characteristic of free water in smectite structural galleries, and shows that the similar real parts above [Formula: see text] are caused by a small suppression of the high-frequency static value of water permittivity by the smectite. We conclude that the clay mineral content, particularly smectite, appears to be responsible for permittivity variations between grain-size fractions. Small model mismatches in real permittivity near the low-frequency end and the greater fractions of kaolinite and illite suggest that the total clay mineral content might have been important for the coarser fractions.

Experimental investigation of turbulent density fluctuations and noise generation from heated jets

Low-frequency noise sources in heated single-stream jets were identified by cross-correlating turbulent density fluctuations ρ′ with the far-field sound pressure fluctuations p′. The turbulent density fluctuations were measured by a molecular Rayleigh-scattering technique. For a fixed jet velocity Uj, the normalized correlation coefficient 〈ρ′; p′〉/(ρ′rmsp′rms is found to increase progressively with an increase in the plume temperature (subscript rms stands for root-mean-square). The result indicates an improvement of the noise radiation efficiency with heating. Directly measured noise spectra from fixed velocity jets with increasing temperature ratio show confusing trends. However, if such spectra are normalized by theplume density, then a consistent trend of increasing noise level with increased plume temperature emerges. The increased noise is the most prominent at the low-frequency end, consistent with the correlation data. The effect of increasing jet velocity keeping the plume temperature constant was also studied. The correlation coefficients were found to improve significantly with velocity; a result consistent with prior observation from unheated jets. Additional findings on the time-averaged density variations and the changes in the air density fluctuations with increasing plume temperature are also discussed.

An Ultra-Wideband (UWB) Bandpass Filter Using Broadside-Coupled Structure and Lumped-Capacitor-Loaded Shunt Stub Resonators

This paper presents an ultra-wideband (UWB) bandpass filter using a combination of broadside-coupled structure and lumped-capacitor-loaded shunt stub resonator. The broadside-coupled microstrip-to-coplanar waveguide structure provides an ultra-wide bandpass filtering operation and keeps a good stopband at lower frequencies from DC at the same time. The lumped-capacitor-loaded shunt stub resonator creates two transmission zeros (attenuation poles which can be located at the outsides of the two bandedges of the UWB bandpass filter to improve the out-band performance by selecting a suitable combination of the length of the shunt stubs and the capacitance of the loaded chip capacitors. The filter was designed based on electromagnetic simulation for broadside-coupled structure, microwave circuit simulation and experiments for determining the transmission zeros. The filter was fabricated on a one-layer dielectric substrate. The measured results demonstrated that the developed UWB bandpass filter has good performance: low insertion loss about 0.46 dB and low group delay about 0.26 ns at the center of the passband and very flat over the whole passband, and less than -10 dB reflection over the passband. The implemented transmission zeros, particularly at the low frequency end, dramatically improved the out-band performance, leading the filter satisfy the FCC's spectrum mask not only for indoor but also for outdoor applications. These poles improved also the skirt performance at both bandedges of the filter. A lowpass filter has been also introduced and integrated with the proposed bandpass filter to have a further improvement of the out-band performance at the high frequency end. The filters integrated with lowpass section exhibit excellent filter performance: almost satisfying the FCC's spectrum mask from DC to 18 GHz. The developed UWB bandpass filter has a compact size of 4 cm x 1.5 cm, or 4.8 cm x 1.5 cm with lowpass section implemented.

The influence of the segregation of Cu-rich phase on the microstructural and impedance characteristics of CaCu3Ti4O12 ceramics

The influence of sintering conditions on the microstructural features and impedance characteristics of the giant dielectric constant material CaCu3Ti4O12 (CCTO) was investigated. The microstructure and impedance characteristics were found to be strongly dependent on the sintering conditions. Sintering of the CCTO ceramics at elevated temperatures (>1100 °C) for prolonged durations resulted in the segregation of Cu-rich phase, mostly confined to the surface, which was in concomitance with the appearance of the additional semicircle at the low frequency end in Impedance (Z*) plots. The absence of this additional semicircle in the Cu-deficient CCTO ceramics and the appearance of the same in Cu-rich CCTO ceramics that were deliberately fabricated corroborated the above observations. Also, La2/3Cu3Ti4O12 (LCTO), a low dielectric constant member of CCTO family, which consisted of small grains without the segregation of Cu-rich phase at the grain boundary, did not reveal the presence of additional semicircle in the Z*plots.

Analysis of the peculiar shift of the low-frequency end of a variable photonic band gap

The effect of the variation of a rod’s dielectric constant on the photonic band gap in a two-dimensional square lattice structure photonic crystal is studied in this paper. The effects were calculated by FDTD algorithm and some peculiar variation properties were obtained. By consecutively changing the dielectric constant of the rod over certain wide ranges and fixing that of the background, the low-frequency end of the TM band gap in low frequency regions reveals very tiny shifts which to some extent can be treated as fixed. This is primarily due to the fact that the high-e region in the crystal is nearly unchanged, hence, the concentrated energy of the low frequency mode changes only slightly. This effect does not exist for materials whose dielectrics are varied over wide ranges. We propose an alternative method using 2D core-shell photonic crystals. The final results of the band calculation show that the gap maps of the core-shell cases closely match those of the ideal case. Such peculiar changes in the photonic band gap have wide application prospects.

Wind-tunnel modelling of the Silsoe Cube

1:40 scale wind-tunnel modelling of the Silsoe 6 m Cube at the University of Auckland is reported. In such situations, it is very difficult to model the full turbulence spectra, and so only the high-frequency end of each spectrum was matched. It is this small-scale turbulence that can directly interact with the local flow field and modify flow behaviour. This is illustrated by studying data from tests conducted in a range of European wind tunnels. It is recommended that spectral comparisons should be carried out by using turbulence-independent normalising parameter, such as plotting fS( f)/ U 2 against reduced frequency f= nz/ U. Using parameters such as the variance and integral length scale can easily mask major differences. It is noted that it is the size of the tunnel that limits the low-frequency end of the spectra, and so the longitudinal and transverse turbulence intensities were lower than in full scale. In spite of this similar pressure distributions are obtained. Some differences are observed and these are partially attributed to the reduced standard deviation of wind directions, which affects both the observed mean and peak pressures by reducing the band of wind directions occurring during a run centred on a particular mean direction. The reduced turbulence intensities also affect the peak-to-mean dynamic pressure ratio. However, since the missing turbulence is at low frequencies, the peak pressures appear to reduce in proportion. By expressing the peak pressure coefficient as the ratio of the extreme surface pressures to the peak dynamic pressure observed during the run, reasonable agreement is obtained. It is argued that this peak–peak ratio is also less sensitive to measurement system characteristics or analysis method, provided the measurement and analysis of the reference dynamic pressure is comparable with that used for the surface pressures.

Impedance spectroscopic studies on congruentLiNbO3 single crystal

Electrical impedance measurements on a congruentLiNbO3 single crystal were performed as a function of both temperature andfrequency. The measurements were carried out in the directions along thec- anda-axes of the crystal. The temperature and frequency dependence of variousdielectric properties have been studied. The result has revealed two remarkabledynamic relaxations: dielectric dipolar relaxation and ionic conductivityrelaxation. The dipolar relaxation peaks were found at frequencies around4 × 106 and2 × 106 Hz forthe c-axisand a-axis, respectively, and they were only slightly temperature dependent. The ionicconductivity relaxation was found at the lower-frequency end but it was temperaturedependent. The temperature dependence of the dc electrical conductivityfollows the Arrhenius law. It corresponds to the long-range ionic motion ofLi+ ions which are thermally activated with activation energy of 0.90 and 0.87 eV along thec- anda-axis directions, respectively. The dc conductivities measured along thec- anda-axes are very close to each other, and the value increases from1.7 × 10−6 to1.9 × 10−3 Ω−1 cm−1 as the temperatureis raised from 300 to 700 °C. The sample crystal becomes an ionic conductor as the temperature is raised.

Unusual ionospheric effects observed during the intense 28 October 2003 solar flare in the Brazilian sector

Abstract. The 28 October 2003 solar flare (X-ray Class X17.2) was one of the most intense solar flares observed in the recent past. In the present investigation we show the unusual ionospheric effects observed in the Brazilian sector during this solar flare, using both the ionospheric sounding observations obtained at the UNIVAP stations: Palmas (7–10.2° S, 48.2° W, dip lat. 5.5° S) and Sao Jose dos Campos (23.2° S, 45.9° W, dip lat. 17.6° S), Brazil; and ground-based global positioning system (GPS) data obtained at the "Instituto Brasileiro de Geografia e Estatística" (IBGE) stations: Imperatriz (5.5° S, 47.5° W, dip lat. 2.9° S), Brasilia (15.9° S, 47.9° W, dip lat. 11.7° S), Presidente Prudente (22.3° S, 51.4° W, dip lat. 14.9° S), and Porto Alegre (30.1° S, 51.1° W, dip lat. 20.7° S), Brazil; on two consecutive days, viz., 27 (without solar flare) and 28 (with solar flare) October 2003. It should be mentioned that the vertical total electron content (VTEC) from the GPS observations obtained during the solar flare showed an unusual simultaneous increase in the VTEC values at about 11:00 UT at all four stations associated with the solar flare EUV enhancements and lasted for about 3 h. However, no ionograms were obtained at any of the two UNIVAP stations for a period of about 1 h between about 11:00 to 12:00 UT. Before 11:00 UT (from about 10:45 UT) and after 12:00 UT (to about 16:00 UT), the ionograms were only partial, with the low frequency end missing. During this intense solar flare, hard X-rays (1 to 10 A), as observed by the GOES 12 satellite, were ejected by the Sun during a long period (several hours), with peak radiation at about 11:10 UT. These hard X-ray radiations can penetrate further into the ionosphere, causing an increase in ionization in the lower part of ionosphere (D-region). In this way, the lack of ionograms or partial ionograms, which indicates no echoes or partial echoes of the transmitted digital ionosonde signals, are related to intense absorption, resulting in complete or partial fade-out of radio signals at the lower ionospheric heights. The partial and complete radio fade-out observed in the Brazilian sector for more than six hours must have resulted in considerable difficulties to the radio station operating agencies using ionospheric radio transmissions.

Energy correlations for a random matrix model of disordered bosons

Linearizing the Heisenberg equations of motion around the ground state of an interacting quantum many-body system, one gets a time-evolution generator in the positive cone of a real symplectic Lie algebra. The presence of disorder in the physical system determines a probability measure with support on this cone. The present paper analyzes a discrete family of such measures of exponential type, and does so in an attempt to capture, by a simple random matrix model, some generic statistical features of the characteristic frequencies of disordered bosonic quasiparticle systems. The level correlation functions of the said measures are shown to be those of a determinantal process, and the kernel of the process is expressed as a sum of biorthogonal polynomials. While the correlations in the bulk scaling limit are in accord with sine-kernel or Gaussian Unitary Ensemble universality, at the low-frequency end of the spectrum an unusual type of scaling behavior is found.

Electrical behaviour of LSGM–LSM composite cathode materials

Composite cathode materials were prepared by mixing La 0.83Sr 0.17Ga 0.83Mg 0.17O 2.83 (LSGM) and La 0.8Sr 0.2MnO 3 (LSM) powders fired at 1300 °C. Several compositions were set up containing 1, 5, 25, 50, 75 wt.% of LSM. Their microstructure and electrical behaviour were investigated by XRPD, SEM/EDS and EIS. In composites containing 50 and 75 wt.% of LSM, the electronic contribution to conductivity is predominant, then there is only a single point at the low frequency end of the Nyquist plot. On the contrary, in the composites with up to 25 wt.% of LSM, there is a significant amount of ionic transport, then the IS spectra show complex features: at least three different arcs can be devised and their interpretation depends upon temperature. LSGM bulk and grain boundary conductivity, as well as interface polarization between the ionic (LSGM) and electronic (LSM) phases can be separated at temperatures below 600 °C; total LSGM contribution, i.e. bulk plus grain boundary, LSGM–LSM interface and electrode polarizations are attributed above 600 °C.

Fixed lock-time relaxation dispersion in the rotating frame

The spin-lattice relaxation dispersion may be probed in the laboratory frame through field-cycling NMR relaxometry. The experiment, as usually done, has the basic weakness that the low frequency end of the measured dispersion can be blurred by the presence of local fields. An understanding of the nature of such local fields was found to be essential to the interpretation of the dispersion profile. In this work, an attempt was made to determine the extent to which specific information can be obtained from a rotating frame experiment. The technique consists in the study of the NMR signal dispersion at a fixed spin-lock time, as a function of the radio frequency field intensity. Within this scheme, a strong dispersion can be attributed to the presence of a non-zero magnetic field component along the laboratory-frame Zeeman-axis in the rotating-frame. At on-resonance condition, this component is exclusively due to the presence of local fields as projected on that axis.

Cochlear Preservation After Meningitis: An Animal Model Confirmation of Adjunctive Steroid Therapy

The objective of the present study was to determine whether treating pneumococcal meningitis with a combined antibiotic and steroid regime will prevent cochlear damage, a common pneumococcal meningitis side effect. This was a prospective animal study. Gerbils were randomly assigned to three experimental groups. Animals in group 1, the control animals, received intrathecal saline injections. Animals in groups 2 and 3 received intrathecal injections of Streptococcus pneumoniae to induce meningitis. Although group 2 solely was treated for 7 days with intraperitoneal penicillin injections (48,0000 units), group 3 received, in addition to the antibiotic for 4 days, 0.5 mg/kg intraperitoneal dexamethasone injections. Three months after the meningitis was induced, the animals' cochlear function was determined using auditory brainstem responses (ABRs). Fifteen frequencies were tested, five octaves at three steps per octave between 2 and 50 kHz. ABR thresholds were significantly elevated only in group 2. When compared with group 1, ABR thresholds were 19 dB higher (P<.05). Frequencies at the low-frequency end of the hearing range were affected more than the midfrequencies. Animals that received dexamethasone had 2-dB higher thresholds than the control group (P>.05). Dexamethasone therapy in conjunction with antibiotic therapy preserves cochlear function in cases of S. pneumoniae meningitis in the Mongolian gerbil model.

Investigation of flat spectrum radio sources by the interplanetary scintillation method at 111 MHz

Interplanetary scintillation observations of 48 of the 55 Augusto et al. (1998) flat spectrum radio sources were carried out at 111 MHz using the interplanetary scintillation method on the Large Phased Array (LPA) in Russia. Due to the large size of the LPA beam (1° x 0.5°) a careful inspection of all possible confusion sources was made using extant large radio surveys: 37 of the 48 sources are not confused. We were able to estimate the scintillating flux densities of 13 sources, getting upper limits for the remaining 35. Gathering more or improving extant VLBI data on these sources might significantly improve our results. This proof-of-concept project tells us that compact (<1) flat spectrum radio sources show strong enough scintillations at 11 MHz to establish/constrain their spectra (low-frequency end).

Analysis of a U-shaped vane-loaded helical slow-wave structure for wideband travelling-wave tubes

The dispersion equation and interaction impedance of a U-shaped vane-loaded helical slow-wave structure (SWS) is obtained by considering the azimuthal space harmonic in the non-vane region and the helical tape thickness, using the field theory matching method in the azimuthal field discontinuity. The theoretical calculation by electromagnetic analysis is consistent with the simulation result by MAFIA, which can explain some physical essence of the SWS. In the helical SWS, negative dispersion is reported to reduce the second harmonic content of a wideband travelling-wave tube at the low-frequency end of the band. When the SWS is assembled, the support rods are easily located in the groove of the vane. The solution provides a new instruction method for other types of vane-loaded helical SWS.

Improved noise analysis of distributed preamplifier with cascode FET cells

Improved noise analysis of a distributed amplifier (DA) with cascode unit cells is presented. The analysis is based on the lossy m-derived configuration, considering the effects of field-effect transistor resistances, as well as parasitic and intentional inductances, which have been ignored in previous research. Analytical expressions for the noise figure and equivalent input noise current density of the cascode DA are derived. The analysis also identifies the individual contributions from each noise source inside the cascode DA, providing insight into the noise of a DA. It is found from the analysis that the output noise current of cascode cell (~i/sub de//sup 2/) is the most significant contributor to the total noise of the DA over most of the operating frequency band. The gate termination noise (~i/sub Zg//sup 2/) has a large impact at the low-frequency end, while the input noise current of the cascode cell (~i/sub ge//sup 2/) degrades the high-frequency noise performance substantially. The resistive and inductive effects of the unit cells have been carefully studied by comparing the simulation results with those based on the lossless constant-k models, which shows that ignoring these effects may result in misleading results at high frequencies. To validate the noise analysis, the simulated noise figure of nine-section cascode DA monolithic microwave integrated circuit is compared with the measured data, yielding an excellent agreement over the entire operating frequency band.

Radio recombination lines

Over the past forty years, investigations of radio recombination lines have become an effective way of studying the interstellar medium. The electron temperatures, densities, emission measures, kinematics parameters, elemental abundances and other physical characteristics of emission processes in planetary nebulae, distributed ionized gas, stellar winds, the partially-ionized interstellar medium and external galaxies are determined with the lines of hydrogen, helium, carbon, etc. This astrophysical phenomenon is unique because of the large (>1000) number of possible quantum transitions and because of the wide observable frequency range (from ~150 GHz to ~10 MHz). In this short review the main results from recombination radio line investigations are discussed. Special attention is given to the areas which have been given least attention in the literature such as the radio recombination lines of very excited carbon atoms at the extremely low-frequency (v < 30 MHz) end of the decameter wavelength range. This is important to stress in the 70-th jubilee of radio astronomy because cosmic radio emission was discovered by Karl Jansky at decameter wavelengths. Investigations of the extremely low-frequency radio recombination lines are also very interesting for the prospective LOFAR system.

Investigation of propagation of partial discharges in power transformers and techniques for locating the discharge

The location of partial discharges in a power transformer can be determined based on the characteristics of the transfer function from the discharge source to the measuring terminal. Previous studies partially validated the technique using computer simulation and practical experiments based on a PD calibrator to represent a discharge signal. 'Real' discharges produced by insulation defect models were used to study how discharges propagate in an 11 kV plain-disc-type transformer winding: A corona model and a 'floating objects in oil' model produced discharge signals with different durations at various locations along the winding. Measurements were taken at the tap of the bushing capacitance through a conventional discharge detector. The signals were filtered, amplified and fed into a digital storage oscilloscope. The frequency spectra of the measured signals showed significant similarities irrespective of the type of discharge source. The characteristic of the transfer functions, i.e. the crests and troughs in the spectra, could be used for locating the source of the discharge. Energising the transformer increased the level of electric noise, which did affect the low frequency end of the spectra, but did not have any impact on the characteristics used for location.

Impedance measurements of the Spallation Neutron Source extraction kicker system

Transverse coupling impedance measurements of the Spallation Neutron Source (SNS) beam extraction system were performed and the results are here reported. The SNS beam extraction system is composed from 14 subsystems, each of which consists of a vertical kicker magnet plus a pulse forming network (PFN). Impedance bench measurements were performed on one large and one small aperture magnet, stand-alone as well as assembled with the first-article production PFN. The impedance measuring methods to cover the interesting frequency range from below 1 to 100 MHz are described in considerable detail. The upper frequency range is properly covered by the conventional twin-wire method but it had to be supplemented at the low-frequency end by a direct input impedance measurement at the magnet busbar. Required modifications of the PFN to maintain the impedance budget are discussed. The total impedance estimate was finally obtained by quadratic scaling with vertical aperture from the two tested kicker subsystems.

Choosing electrodes for deep brain stimulation experiments–electrochemical considerations

Deep brain stimulation (DBS) is a therapy of movement disorders including Parkinson's disease (PD). Commercially available electrodes for animal models of Parkinson's disease vary in geometry and material. We characterized such electrodes and found a drift in their properties within minutes and up to about 60 h after immersion in cell culture medium, both with and without a stimulation signal. Electrode properties could largely be restored by proteolytic treatment for platinum/iridium electrodes but not for stainless steel ones. Short-term drift and irreversible aging could be followed by impedance measurements. Aging was accompanied by metal corrosion and erosion of the plastic insulation. For both materials, the degradation rates depended on the current density at the electrode surfaces. Fourier analysis of the DBS pulse (60 μs, repetition rate 130 Hz) revealed harmonic frequencies spanning a band of more than three decades, with significant harmonics up to the MHz range. The band is located in a window imposed by electrode processes and capacitive cell membrane bridging at the low and high frequency ends, respectively. Even though electrode processes are reduced at higher frequencies they only vanish above 1 MHz and cannot be avoided. Therefore, the use of inert electrode materials is of special importance. The neurotoxicity of iron makes avoiding stainless steel electrodes imperative. Future developments need to avoid the use of corrosive materials and current density hot spots at the electrode surface, and to reduce low frequency components in the DBS pulses in order to diminish electrode processes.

AC-impedance spectroscopy of anodic reactions with adsorbed intermediates: electro-oxidations of 2-propanol and methanol on carbon-supported Pt catalyst

The ac-impedance spectroscopy of the electro-oxidation of 2-propanol and methanol was investigated in a H 2SO 4 solution using a carbon-supported platinum catalyst as a function of potential between 350 and 750 mV (vs. SHE) at a variety of temperatures between 303 and 353 K. It is found that the impedance spectrum of 2-propanol is very similar to that of methanol. Semicircles that have induction loops at the low frequency ends were observed in complex plane plots at high potentials between 600 and 750 mV in both the cases of 2-propanol and methanol, which suggests that the rate-determining steps of the electro-oxidation of intermediates adsorbed on platinum should be involved in their reaction processes. Assuming appropriate models involving the adsorbed intermediates for 2-propanol and methanol, the impedance spectra were explained successfully. The impedance analysis provides the values of reaction parameters concerning a coverage relaxation time for an intermediate and a charge-transfer resistance, which make it possible to compare the reaction rates of the intermediates between 2-propanol and methanol. The reaction parameters suggest that a consecutive reaction involving the intermediate for 2-propanol proceeds more slowly than that for methanol, and also suggest that a direct reaction pathway from 2-propanol to acetone, which does not go through the intermediate, exists in parallel. Additionally, the influences of acetone concentration on the coverage relaxation time and the charge-transfer resistance for the electro-oxidation of 2-propanol were also discussed.