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Results for “RADIO WAVES”

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At least 433 records · Page 24Linked to original sources
PubMed2006

Minimization of finite beam effects in the determination of reflection and transmission coefficients of an elastic layer.

A special data acquisition technique was applied to determine the acoustic plane-wave reflection and transmission properties of a plane-parallel aluminum plate. In this technique, the reflected and transmitted wavefield along a plane or line normal to the reflected or transmitted wave vector is recorded at equidistant receiver positions. The obtained traces are subsequently added up in the temporal domain to satisfy plane-wave conditions, thus effectively removing the effect of the limited beam of commonly used transducers. The agreement between plane-wave theory and experiment was found to be excellent, both in the temporal and in the frequency domain.

Aluminum↗
PubMed1998

[Effects of millimeter wave irradiation with different frequency and power density on their offsprings in mice].

OBJECTIVE: To determine the threshold of millimeter wave irradiation leading to fetal damage in mice and to clarify the mechanism of decrease in their learning and memory function. METHODS: Pregnant mice were irradiated by millimeter wave with frequencies of 37.4, 42.2, 53.0 and 60.0 GHz at power densities of 1 to 8 mW/cm2 for two hours daily from the 6th to 15th day of their gestation. Learning and memory functions of their offsprings were tested by a Y-type electric maze. M-cholinergic receptor (M-R), monoamine transmitter and arginine vasopressor (AVP) in the brain were determined for their offsprings by receptor radiation binding analysis (RBA), high performance liquid chromatography-electrochemical detection (HPLC-ECD) and radioimmunoassay (RIA). RESULTS: The minimal power density of millimeter wave of 37.4, 42.2, and 53.0, 60.0 GHz causing the decrease in learning and memory function, increase of Bmax in the hippocampus and decrease in dopamine (DA) in the brain of the adult offspring mice was five and three mW/cm2 and that causing the decrease in AVP in the hypothalamus was eight and five mW/cm2. Severity of damage in fetal offspring mice caused by irradiation increased with the power density of millimeter wave. Millimeter wave could not cause the increase of their body temperature in pregnant mice. CONCLUSION: Threshold of millimeter wave with frequencies of 37.4, 42.2 and 53.0, 60.0 GHz causing fetal damage in mice was three to five mW/cm2 and one to three mW/cm2, respectively. The decrease in learning and memory functions in offspring mice related with lowering of AVP and DA in the brain and of activity in cholinergic nerve. All these were caused by the non-thermal effects of millimeter wave without a frequency-specific pattern.

Animals↗
PubMed1991

Experimental study about removal of the implanted tined polyurethane ventricular lead by radiofrequency waves through the lead.

Polyurethane pacemaker leads are widely used nowadays. However, only a few studies have been done to investigate the fixation mechanism of polyurethane leads. To elucidate how pacemaker leads are fixed at the early phase after implantation, polyurethane-insulated tined ventricular leads were implanted in seven mongrel dogs. One to 4 months later, tips of the leads were anchored among the trabeculae and the distal part of the leads were encapsulated by whitish fibrous tissue. It was found that not organized thrombi, but cell reaction with various stages of inflammatory cells was responsible for forming the fibrous tissue. We attempted to remove the lead by delivering radiofrequency wave through the lead. However, no lead could be removed.

Animals↗
PubMed2006

Double-layer PVDF transducer and V(z) measurement system for measuring leaky Lamb waves in a piezoelectric plate.

This paper presents a new experimental measurement method for leaky Lamb waves propagating in a piezoelectric plate immersed in a conductive fluid. The measurement system is a low-frequency version of lens-less acoustic microscopy which has been developed based on a line-focus double-layer PVDF transducer. The transducer and its defocusing measurement system can perform V(z) measurements on a sample plate immersed in a fluid, and therefore can obtain the leaky Lamb wave velocities with high accuracy. An X-cut LiNbO(3) plate is investigated with this experimental measurement system to find out its fluid-loading effects, especially the conductive loading effects by water of various conductivities. Angular dependence of this conductive loading effect along different propagating directions on the X-cut LiNbO(3) plate is measured. It is found out the conductive loading effects are strongly dependent on the piezoelectric coupling factor. Theoretical calculations based on partial wave theory have also been carried out and compared with experimental data. Good agreements have been observed.

Biomedical Engineering↗
PubMed2004

Inscribed matter as an energy-efficient means of communication with an extraterrestrial civilization.

It is well known that electromagnetic radiation-radio waves-can in principle be used to communicate over interstellar distances. By contrast, sending physical artefacts has seemed extravagantly wasteful of energy, and imagining human travel between the stars even more so. The key consideration in earlier work, however, was the perceived need for haste. If extraterrestrial civilizations existed within a few tens of light years, radio could be used for two-way communication on timescales comparable to human lifetimes (or at least the longevities of human institutions). Here we show that if haste is unimportant, sending messages inscribed on some material can be strikingly more energy efficient than communicating by electromagnetic waves. Because messages require protection from cosmic radiation and small messages could be difficult to find among the material clutter near a recipient, 'inscribed matter' is most effective for long archival messages (as opposed to potentially short "we exist" announcements). The results suggest that our initial contact with extraterrestrial civilizations may be more likely to occur through physical artefacts-essentially messages in a bottle-than via electromagnetic communication.

Journal Article↗
PubMed2006

Subcritical scattering from buried elastic shells.

Buried objects have been largely undetectable by traditional high-frequency sonars due to their insignificant bottom penetration. Further, even a high grazing angle sonar approach is vastly limited by the coverage rate dictated by the finite water depth, making the detection and classification of buried objects using low frequency, subcritical sonar an interesting alternative. On the other hand, such a concept would require classification clues different from the traditional high-resolution imaging and shadows to maintain low false alarm rates. A potential alternative, even for buried targets, is classification based on the acoustic signatures of man-made elastic targets. However, the elastic responses of buried and proud targets are significantly different. The objective of this work is to identify, analyze, and explain some of the effects of the sediment and the proximity of the seabed interface on the scattering of sound from completely and partially buried elastic shells. The analysis was performed using focused array processing of data from the GOATS98 experiment carried out jointly by MIT and SACLANTCEN, and a new hybrid modeling capability combining a virtual source-or wave-field superposition-approach with an exact spectral integral representation of the Green's functions for a stratified ocean waveguide, incorporating all multiple scattering between the object and the seabed. Among the principal results is the demonstration of the significant role of structural circumferential waves in converting incident, evanescent waves into backscattered body waves, emanating to the receivers at supercritical grazing angles, in effect making the target appear closer to the sonar than predicted by traditional ray theory.

Humans↗
PubMed2007

In-depth study of the electromagnetics of ultrahigh-field MRI.

In this work, numerical and experimental studies of the transverse electromagnetic (TEM) resonator modes at ultrahigh-field (UHF) MRI are performed using an in-house finite difference time domain package at 340 MHz and using an 8 T whole-body MRI system. The simulations utilized anatomically detailed human head mesh and a spherical head-sized phantom, while the experiments included an electromagnetically equivalent (to simulations) phantom and in vivo human head studies. An in-depth look at the homogeneity of the transmit-and-receive fields and local and global polarization of the electromagnetic waves inside the cavity of the head coil, and also the current distribution obtained on the resonator elements, is provided for several coil modes when the coil is empty and loaded. Based on the numerical and experimental results, which are in excellent agreement, an electromagnetic characterization of loading radio-frequency (RF) head coils during a UHF MRI experiment is provided. The possibility of using the aforementioned modes for specific types of imaging application is briefly reviewed.

Electromagnetic Fields↗
PubMedDate not supplied

Highly homogeneous circularly polarized RF field for whole body NMR imaging.

Conditions for generation of polarized RF fields in cylindrical symmetry by high order coils are presented. Whole body NMR transmitters utilizing this principle would require a current control on a certain number of loops. In the present design this is obtained by feeding each loop by an appropriate linear combination of two in quadrature waves. The development has been experimentally tested by three scaled down experimental prototypes operated at 500 kHz and a good agreement has been found.

Humans↗