Low Frequency Square Research Articles

Low-Base Frequency Helicopter AEM Data from a Square-Wave System - Helitem2

SummaryExploration for targets at depth or targets obscured by conductive overburden have historically been a challenge with airborne EM methods. Although modern systems have been improved with greater primary transmitter moments, noise from receiver coil motion in the Earth’s ambient field limits detection of secondary target signals, especially in late time. The new Helitem² system uses a patented low-noise receiver and a 50% duty cycle square pulse transmitter waveform to achieve increased signal detectability for deep and covered targets.A series of demonstration surveys were conducted by surveying a known target to compare several helicopterborne time-domain system configurations. Despite having a larger dipole moment, a half sine pulse at standard 30 Hz base frequency was predicted to have lower responses than low base frequency (15 Hz and 7.5 Hz) square pulse operation in a thin-plate nomogram over a wide range of target conductances. At early times, the sharper (quicker) turn off of the square wave results in much more highfrequency energy and therefore better signal for weakly conductive targets and better near-surface resolution. At the other extreme, the response from very conductive targets is determined by the area under the transmitter curve, so the low frequency square waves with 16 and 33 ms widths should produce more than twice the signal as the half sine. Survey line profiles and decay curves over the target and background locations confirmed these predictions for a 400 m deep target and variable overburden.The combination of pulse width, power, and low noise enabled the system to be effective at low base frequencies, where very late time data is beneficial for detecting strong and deep targets.

Constraints on dark matter annihilation in dwarf spheroidal galaxies from low frequency radio observations

We present the first observational limits on the predicted synchrotron signals from particle Dark Matter annihilation models in dwarf spheroidal galaxies at radio frequencies below 1 GHz. We use a combination of survey data from the Murchison Widefield Array (MWA) and the Giant Metre-wave Radio Telescope (GMRT) to search for diffuse radio emission from 14 dwarf spheroidal galaxies. For in-situ magnetic fields of 1 $\mu G$ and any plausible value for the diffusion coefficient, our limits do not constrain any Dark Matter models. However, for stronger magnetic fields our data might provide constraints comparable to existing limits from gamma-ray and cosmic ray observations. Predictions for the sensitivity of the upgraded MWA show that models with Dark Matter particle mass up to $\sim$ 1.6 TeV (1 TeV) may be constrained for magnetic field of 2 $\mu G$ (1 $\mu G$). While much deeper limits from the future low frequency Square Kilometre Array (SKA) will challenge the LHC in searches for Dark Matter particles, the MWA provides a valuable first step toward the SKA at low frequencies.

Increasing the Upper Mass/Charge Limit of a Quadrupole Ion Trap for Ion/Ion Reaction Product Analysis via Waveform Switching.

Quadrupole ion traps (QITs) are versatile platforms for performing experiments with gas-phase ions due to their abilities to store ions of both polarities and to conduct MSn experiments. The QIT is particularly useful as a reaction cell for ion/ion reactions. In the case of an ion/ion reaction experiment in a QIT, multiply charged reactant ions may initially be of relatively low m/z (e.g., m/z < 1000) whereas the product ions can be one or more orders of magnitude higher in m/z (e.g., m/z > 100,000). Several factors can limit the m/z range over which an ion/ion reaction experiment can be conducted. These include (1) the efficiency of the detector, (2) the m/z range over which oppositely charged ions can be mutually stored, and (3) the m/z range over which ions can be mass selectively ejected into an external detector. High-frequency waveforms provide larger m/z trapping ranges for mutual storage of oppositely charged ions whereas low-frequency waveforms provide better trapping for very high m/z product ions. Presented here is a method that switches from a high-frequency sine wave prior to and during an ion/ion reaction to a low-frequency square wave to eject low m/z reagent ions and improves confinement of the product ions before mass-selective ejection by scanning the frequency of the square wave. This approach addresses the third issue, which is the primary limiting factor with QITs operated at high RF (e.g., > 900MHz). Graphical Abstract.

MWA tied-array processing I: Calibration and beamformation

AbstractThe Murchison Widefield Array is a low-frequency Square Kilometre Array precursor located at the Murchison Radio-astronomy Observatory in Western Australia. Primarily designed as an imaging telescope, but with a flexible signal path, the capabilities of this telescope have recently been extended to include off-line incoherent and tied-array beam formation using recorded antenna voltages. This has provided the capability for high-time and frequency resolution observations, including a pulsar science program. This paper describes the algorithms and pipeline that we have developed to form the tied-array beam products from the summation of calibrated signals of the antenna elements, and presents example polarimetric profiles for PSRs J0437-4715 and J1900-2600 at 185 MHz.

Low-Cost Submersible Turbidity Sensors Using Low-Frequency Source Light Modulation

Submersible turbidimeter and fluorometer design generally involves tradeoffs between ambient light rejection, sensitivity, and cost. We introduce an optical backscatter transducer in which the light source is modulated with a low-frequency (tens of Hz) square wave, with a detection scheme that mitigates the effects of external interference from ambient light and from 50- and 60-Hz sources. With this design, high-pass filtering and subsequent amplification enable sensitive measurements using inexpensive circuitry. Additional accuracy-enhancing features include: a rail-splitting virtual ground, which eliminates the potential for clipping at low signal levels; a reference output, which provides correction for the intensity of the internal light source; and error-tracking provisions, which flag the output if circuit voltages reach saturation limits during measurement. We have designed, constructed, and evaluated submersible turbidimeters which incorporate this transducer. These sensors were inexpensive to fabricate (U.S. $70 of materials and 4 h of labor) and were proof tested to 7 atm of water pressure. They responded linearly and reproducibly to formazin turbidity standard, with negligible inherent offset. A calibrated sensor exhibited bias ${\le} 0.11$ NTU in suspensions ranging from 0 to 8 NTU. A similar sensor with reduced circuit gain exhibited a one-to-one response to increasing turbidity in suspensions ranging from 0 to 815 NTU.

Design and Characterization Methodology for Efficient Wide Range Tunable MEMS Filters.

Here, we demonstrate the advantages of the laser Doppler vibrometer (LDV) over conventional techniques (the network analyzer), as well as the techniques to create an application-based microelectromechanical systems (MEMS) filter and how to use it efficiently (i.e., tuning the tuning-capability and avoiding both failure and stiction). LDV enables crucial measurements that are impossible with the network analyzer, such as higher mode detection (highly sensitive biosensor application) and resonance measurement for very small devices (fast prototyping). Accordingly, LDV was used to characterize the frequency tuning range and resonance frequency at different modes of the MEMS filters built for this study. This wide range frequency tuning mechanism is based simply on Joule heating from the embedded heaters and relatively high thermal stress with respect to the temperature of a fixed-fixed beam. However, we demonstrate that another limitation of this method is the resulting high thermal stress, which can burn the devices. Further improvement was achieved and shown for the first time in this study, such that the tuning capability was increased by 32% through an increase in the applied DC bias voltage (25 V to 35 V) between the two adjacent beams. This important finding eliminates the need for the extra Joule heating at the wider frequency tuning range. Another possible failure is through stiction and requirement of structure optimization: We propose a simple and easy technique of low frequency square wave signal application that can successfully separate the beams and eliminates the need for the more sophisticated and complicated methods given in the literature. The above findings necessitate a design methodology, and so we also provide an application-based design.

Influence of Extremely Low frequency Magnetic Field on Proteus mirabilis bacteria

Urinary tract infections (UTI) due to Proteus mirabilis strains represented nearly 90% of Urinary tract infection cases. In this study the effect of applying extremely low frequency square magnetic pulses ELF-SMP on P. mirabilis was investigated. While, it interferes with the bioelectric signals generated during the cell division process. Also, the effect of exposure on bacterial antibiotic susceptibility and morphological cellular structure was studied. The results indicated that there was a significant inhibition effect for P. mirabilis exposed to 0.6 Hz SMP for one hour and significant increase in antibiotic susceptibility to DNA, proteins, β-lactamase enzyme and cell-wall synthesis inhibitors. Additionally, results of dielectric relaxation and TEM indicated molecular and morphological changes. According to these aforementioned results, it was concluded that treatment of P. mirabilis cells with SMP at mentioned frequency acts on its cellular activity and structure of the bacteria which can make this method is a promising one in further in vivo applications.

Effect of Temperature on Heart Rate Variability in Neonatal ICU Patients With Hypoxic-Ischemic Encephalopathy

To determine whether measures of heart rate variability are related to changes in temperature during rewarming after therapeutic hypothermia for hypoxic-ischemic encephalopathy. Prospective observational study. Level 4 neonatal ICU in a free-standing academic children's hospital. Forty-four infants with moderate to severe hypoxic-ischemic encephalopathy treated with therapeutic hypothermia. Continuous electrocardiogram data from 2 hours prior to rewarming through 2 hours after completion of rewarming (up to 10 hr) were analyzed. Median beat-to-beat interval and measures of heart rate variability were quantified including beat-to-beat interval SD, low and high frequency relative spectral power, detrended fluctuation analysis short and long α exponents (αS and αL), and root mean square short and long time scales. The relationships between heart rate variability measures and esophageal/axillary temperatures were evaluated. Heart rate variability measures low frequency, αS, and root mean square short and long time scales were negatively associated, whereas αL was positively associated, with temperature (p < 0.01). These findings signify an overall decrease in heart rate variability as temperature increased toward normothermia. Measures of heart rate variability are temperature dependent in the range of therapeutic hypothermia to normothermia. Core body temperature needs to be considered when evaluating heart rate variability metrics as potential physiologic biomarkers of illness severity in hypoxic-ischemic encephalopathy infants undergoing therapeutic hypothermia.

The Sterilization Effect of Solenoid Magnetic Field Direction on Heterotrophic Bacteria in Circulating Cooling Water

Compared to the chemical treatment of industrial circulating cooling water in the field of industrial water treatment, electromagnetic pulse sterilization technology, an advanced technology, is widely used because of its special characteristics--low energy consumption, nonpoisonous and environmentally friendly. In order to get a better cooling water sterilization effect under the premise of not polluting the environment, some experiments about sterilization of heterotrophic bacteria in industrial circulating cooling water by low frequency square wave pulsed magnetic field were carried out. The comparison experiment on the sterilization effect of low frequency square wave pulsed magnetic field on heterotrophic bacteria in industrial circulating cooling water was carried out by change relative flow direction of solenoid magnetic field and water. The bacterial inhibiting effect was 7.22%∼20.35% higher when the direction of flow of water was in parallel with the magnetic field than in anti-parallel. This study analyzed this phenomenon from the perspective of the diamagnetism of the bacteria cells and the electric dipole moment of proteins. Since the bacterium cell membrane contains diamagnetic substances, when a bacterium cell moves in parallel with the magnetic field, it would be slowed down and stay longer in the field than moving in anti-parallel, so the bacterial inhibiting effect would be stronger. Results of this study have important guiding significance for future application of magnetic field sterilization.

Combination of bacteriolytic therapy with magnetic field for Ehrlich tumour treatment.

Referable to the limited response of the current available cancer treatment modalities, new effective cancer fighting treatments are needed. This work investigates the efficiency of intratumoural injection of Pseudomonas aeruginosa bacteria combined with a local tumour exposure to extremely low frequency square pulsed magnetic field (ELF SPMF) in the mouse Ehrlich tumour. 64 Ehrlich ascites tumour-implanted female albino BALB/C mice were equally split up into 4 groups. Group 1 (GP1) was the positive control group. Group 2 (GP2) received a single intratumoural injection of P. aeruginosa bacteria. Group 3 (GP3) was exposed to ELF SPMF for tumour local exposure. Group 4 (GP4) was treated with P. aeruginosa intratumoural injection followed by local exposure of the tumour to ELF SPMF. Treatment monitoring was evaluated using ultrastructural examination, flow cytometry analysis in addition to the measurement of tumour dielectric properties. Tumour cell apoptosis was obvious in GP2 and GP4, but, with higher severity and percentages in GP2. Tumour biophysical properties revealed a significant increase in static conductivity σS of GP2, and decreases in dielectric increment Δɛ´of both GP2 and GP4 compared to the GP1. Unfortunately, GP2 mice showed severe signs of toxicity. We advocate the utilization of the combination of P. aeruginosa and SPMF to yield the most effective antitumour agent with less bacteria-related toxicity.

GaLactic and Extragalactic All-sky Murchison Widefield Array (GLEAM) survey – I. A low-frequency extragalactic catalogue

Abstract Using the Murchison Widefield Array (MWA), the low-frequency Square Kilometre Array precursor located in Western Australia, we have completed the GaLactic and Extragalactic All-sky MWA (GLEAM) survey, and present the resulting extragalactic catalogue, utilizing the first year of observations. The catalogue covers 24 831 square degrees, over declinations south of +30° and Galactic latitudes outside 10° of the Galactic plane, excluding some areas such as the Magellanic Clouds. It contains 307 455 radio sources with 20 separate flux density measurements across 72–231 MHz, selected from a time- and frequency-integrated image centred at 200 MHz, with a resolution of ≈2 arcmin. Over the catalogued region, we estimate that the catalogue is 90 per cent complete at 170 mJy, and 50 per cent complete at 55 mJy, and large areas are complete at even lower flux density levels. Its reliability is 99.97 per cent above the detection threshold of 5σ, which itself is typically 50 mJy. These observations constitute the widest fractional bandwidth and largest sky area survey at radio frequencies to date, and calibrate the low-frequency flux density scale of the southern sky to better than 10 per cent. This paper presents details of the flagging, imaging, mosaicking and source extraction/characterization, as well as estimates of the completeness and reliability. All source measurements and images are available online.1 This is the first in a series of publications describing the GLEAM survey results.

AN OPPORTUNISTIC SEARCH FOR EXTRATERRESTRIAL INTELLIGENCE (SETI) WITH THE MURCHISON WIDEFIELD ARRAY

ABSTRACT A spectral line image cube generated from 115 minutes of MWA data that covers a field of view of 400 sq, deg. around the Galactic Center is used to perform the first Search for ExtraTerrestrial Intelligence (SETI) with the Murchison Widefield Array (MWA). Our work constitutes the first modern SETI experiment at low radio frequencies, here between 103 and 133 MHz, paving the way for large-scale searches with the MWA and, in the future, the low-frequency Square Kilometre Array. Limits of a few hundred mJy beam−1 for narrowband emission (10 kHz) are derived from our data, across our 400 sq. deg. field of view. Within this field, 45 exoplanets in 38 planetary systems are known. We extract spectra at the locations of these systems from our image cube to place limits on the presence of narrow line emission from these systems. We then derive minimum isotropic transmitter powers for these exoplanets; a small handful of the closest objects (10 s of pc) yield our best limits of order 1014 W (Equivalent Isotropic Radiated Power). These limits lie above the highest power directional transmitters near these frequencies currently operational on Earth. A SETI experiment with the MWA covering the full accessible sky and its full frequency range would require approximately one month of observing time. The MWA frequency range, its southern hemisphere location on an extraordinarily radio quiet site, its very large field of view, and its high sensitivity make it a unique facility for SETI.

Structural characterization of Fe[sbnd]Pd nanowires grown by electrodeposition using an acid electrolyte

Fe70Pd30 nanostructures have potential application in actuators due to their conventional and magnetic shape memory. Here, we report the microstructure of electrodeposition grown FePd nanowires in which the process was confined to polycarbonate membranes with a nominal pore diameter of 200 nm. We used an acid electrolyte (pH ≈ 5) in which the solution was stabilized with sulfosalicylic acid. The average chemical concentration of the nanowires can be systematically shifted from rich palladium to rich iron by changing the growth potential. The study of the microstructure by transmission electron microscopy indicates high chemical inhomogeneities due to phase coexistence between rich palladium regions (with FCC structure) and rich iron regions. The latter present a combination of BCC and amorphous phases. The average chemical composition of the nanowires can be better adjusted by using a low frequency square wave voltage excitation (alternating rich Pd and rich Fe regions). However, independently of the growth process, the nanowires morphology collapses after thermal annealing. This could be ascribed to fragile grain boundaries due to the presence of amorphous hydroxides and chemical impurities produced during the electrochemical process.

HID lamps under low frequency square wave operation: Experimental Evaluation of Dimming effects

In this paper, the dimming effects on the photometric and electric characteristics of high intensity discharge lamps supplied by a low frequency square waveform are evaluated. For this evaluation an electronic ballast is presented. The ballast consists of a current source, AC/DC converter and a full-bridge inverter. The electronic supply provides to the lamp a different form of current excitation (square waves with variable short drops). Dimming control and the characteristics of low frequency operated high intensity discharge lamps are described. The lamp is represented in a simulation by its conductance model and coupled to its electronic power supply. Experimental results are shown and compared with the simulation. A dimming range from 100% to about 60% has been achieved.

Influence of Corrosive NaCl solution on Life times of 7075 Aluminum alloy under Combined Fatigue loading in the VHCF Regime

The loading histories of aircrafts and rockets are complex and the components are exposed to combined loading conditions with high numbers of high-frequency cycles superimposed to lower-frequency carrier vibrations. The amplitudes of both frequency ranges usually are variable. The small-amplitude cycles arise from several discrete sources such as structural vibrations which are especially critical at special components and places of an aircraft. The environment plays another important role and ambient air and sea-water have to be considered. Almost no knowledge exists of the material response in the VHCF regime, especially concerning the environmental influence. In this study, experiments were performed on aluminum alloy 7075-T651 in a 3.5%- sodium-chloride solution and in laboratory air of 22 °C of 50% relative humidity at high (105 – 108) and very high numbers of cycles (5 × 108 – 1010). The loading sequences consisted of a low-frequency square wave (0.4 to 1 Hz), being superimposed with a high frequency 20 kHz random vibration. The random vibrations were simulated by a Gauss distribution allowing stress ranges of 10 to 70 MPa or alternatively 50 to 90 MPa. Stress/strain vs. life time curves were measured. The results were correlated with in-situ microscopy observations of the specimen surfaces and with post-experimental fracture-surface images. This technology allowed identifying fatigue crack initiation and propagation stages being needed for an interpretation of the relevant fatigue-life mechanisms under environmental influences.

Sensitivity for 21 cm bispectrum from Epoch of Reionization

The 21cm line brightness temperature brings rich information about Epoch of Reionizaton (EoR) and high-$z$ universe (Cosmic Dawn and Dark Age). While the power spectrum is a useful tool to investigate the EoR signal statistically, higher-order statistics such as bispectrum are also valuable because the EoR signal is expected to be highly non-Gaussian. In this paper, we develop a formalism to calculate the bispectrum contributed from the thermal noise taking array configularion of telescopes into account, by extending a formalism for the power spectrum \cite{2006ApJ...653..815M}. We apply our formalism to the ongoing and future telescopes such as expanded Murchison Widefield Array (MWA), LOw Frequency ARray (LOFAR) and Square Kilometre Array (SKA). We find that expanded MWA does not have enough sensitivity to detect the bispectrum signal. On the other hand, LOFAR has better sensitivity and will be able to detect the peaks of the bispectrum as a function of redshift at large scales with comoving wavenumber $k \lesssim 0.03 {\rm Mpc}^{-1}$. The SKA has enough sensitivity to detect the bispectrum for much smaller scales $k \lesssim 0.3 {\rm Mpc}^{-1}$ and redshift $z \lesssim 20$

Antenna Rotation Error Tolerance for a Low-Frequency Aperture Array Polarimeter

We present antenna rotation error tolerance analysis for a polarimeter consisting of dual-linearly polarized dipole-like elements. Treating the elements as a phased array and expressing the measurement basis as circularly polarized (CP) results in a concise expression for the Jones matrix for the array. For the type of elements being considered, the matrix shows that the intrinsic cross-polarization ratio (IXR) of the array at the intended beam scanning direction is unaffected by small rotation errors. For random rotation error and very large number of elements, we further find that the relative Jones matrix estimation error converges to that of the error-free case at the intended beam scanning direction; however, the effect of element rotation error on array directivity and radiation pattern remains. Recasting the analysis with the array observing an unpolarized source, a relation between rotation error and cross-polarization “leakage” is obtained, wherein similar trends with very large number of elements hold true. Practical examples involving “large” number of elements such as the low frequency Square Kilometre Array are discussed.

Spatiotemporal character of the Bobylev-Pikin flexoelectric instability in a twisted nematic bent-core liquid crystal exposed to very low frequency fields.

The Bobylev-Pikin striped-pattern state induced by a homogeneous electric field is a volume flexoelectric instability, originating in the midregion of a planarly aligned nematic liquid crystal layer. We find that the instability acquires a spatiotemporal character upon excitation by a low frequency (0.5 Hz) square wave field. This is demonstrated using a bent-core liquid crystal, initially in the 90°-twisted planar configuration. The flexoelectric modulation appears close to the cathode at each polarity reversal and, at low voltage amplitudes, decays completely as the field becomes steady. Correspondingly, at successive polarity changes, the stripe direction switches between the alignment directions at the two substrates. For large voltages, the stripes formed nearly along the alignment direction at the cathode gradually reorient toward the midplane director. These observations are generally attributed to inhomogeneous and time-dependent field conditions that come to exist after each polarity reversal. Polarity dependence of the instability is attributed to the formation of intrinsic double layers that bring about an asymmetry in surface fields. Momentary field elevation near the cathode following a voltage sign reversal and concomitant gradient flexoelectric polarization are considered the key factors in accounting for the surfacelike modulation observed at low voltages.

Behaviour of a high pressure sodium lamp fed by a pulsed power supply

The aim of this paper is to present the colorimetric and photometric results for a pulsed power supply operated high pressure sodium (HPS) lamp. Especially, we are interested in controlling the spectral response radiation of the yellow D line. This sodium D line is composed of two spectral components, one at 589.6 nm and the other at 589.0 nm. The pulsed power supply is a driver for the HPS lamp based on the application of a low frequency (50 Hz) square current waveform through the lamp with one or more pulses on each half cycle. A description is given of the physical modeling of the arc’s behaviour. Experimental results obtained from our laboratory power supply for a 400 W HPS lamp supplied at 50 Hz and simulations are shown.

246 * A MONITORING AND PHYSIOLOGICAL CONTROL SYSTEM FOR DETERMINING AORTIC VALVE CLOSING

Objectives: Accurate monitoring and assessment of the residual cardiac function of a patient supported by a ventricular assist device (VAD) is critical for better patient management and potential weaning. Information regarding the VAD speed at which the aortic valve remains closed can assist clinicians to maintain appropriate unloading, reduce the risk of aortic insufficiency and potentially reduce the risk of aortic leaflet fusion. Methods: A pressure sensor implanted in the inlet cannula determines the left ventricular (LV) pressure. The area of the LV pressure and motor power is calculated for each heart beat. A feedback signal is monitored at high speed and low speed to determine the change in the system. Small amplitude (<300 rpm) and low frequency (<0.1 Hz) square wave changes to the VAD speed are made. Results: Ten ovine trials have been completed with the DP3 pump (LV to aortic cannulation). The feedback variable (pressure power area) proved to have a peak at the speed of aortic valve closure The gradient (Δ pressure power area) could be measured by using the square wave speed modulation. In both the baseline and beta blocked heart the zero-crossing of gradient occurred at the aortic closure speed. Conclusions: Feedback gives a definite peak around aortic valve closure point and uses pump speed modulation for more accuracy; there is no sensitivity to sensor drift or haematocrit changes, and it provides noninvasive, continuous measurement.