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Sokolowski, M., N. D. R. Bhat, J. P. Macquart, R. M. Shannon, K. W. Bannister, R. D. Ekers, D. R. Scott et al. "No Low-frequency Emission from Extremely Bright Fast Radio Bursts". Astrophysical Journal 867, n. 1 (29 ottobre 2018): L12. http://dx.doi.org/10.3847/2041-8213/aae58d.
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Filonenko, A. D. "Radio Emission of Meteor Bodies in the Extremely Low Frequency Range". Solar System Research 54, n. 5 (settembre 2020): 442–48. http://dx.doi.org/10.1134/s003809462004005x.
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Zantis, Franz P., Ján Hribik e Daniela Ďuračková. "Extremely Low Frequency Signal Acquisition, Recording and Analysis". Journal of Electrical Engineering 67, n. 3 (1 maggio 2016): 185–91. http://dx.doi.org/10.1515/jee-2016-0026.
Testo completoAbstract (sommario):
Abstract Our environment is permeated by electrical and magnetic alternating waves in the frequency range above the AC voltage of 50 Hz and also in the radio frequency range. Much attention from the public is given to these waves. Through numerous studies and publications about this type of oscillations and waves it is largely known from which sources they occur and which impact they have. However, very little information could be found about electrical and magnetic alternating waves in the frequency range below 50 Hz. The aim of this research is to demonstrate that these signals exist and also to show how the signals look like and where and when they occur. This article gives an overview of the occurrence of these ELF (Extremely Low Frequencies) signals, their specific properties in view of the time domain and in view of the frequency domain and of the possible sources of these waves. Precise knowledge of the structures of the ELF signals allows conclusions about their potential to cause electromagnetic interference in electronic systems. Also other effects in our environment, eg on flora and fauna could be explained.
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Mlynarczyk, Janusz, Andrzej Kulak e Jacobo Salvador. "The Accuracy of Radio Direction Finding in the Extremely Low Frequency Range". Radio Science 52, n. 10 (ottobre 2017): 1245–52. http://dx.doi.org/10.1002/2017rs006370.
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Koemel, Walton C., e Philip S. Callahan. "Relationship of Extremely Low Frequency Radio Emission from Flying Insects to Semiochemical Communication". Annals of the Entomological Society of America 87, n. 5 (1 settembre 1994): 491–97. http://dx.doi.org/10.1093/aesa/87.5.491.
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Antonucci, R., e R. Barvainis. "Low Frequency Cutoffs in the Spectra of Radioquiet Quasars". Symposium - International Astronomical Union 121 (1987): 135–39. http://dx.doi.org/10.1017/s0074180900155032.
Testo completoAbstract (sommario):
Radio-quiet and normal radio-loud quasars have very similar spectral properties in the ultraviolet, optical and near infrared regions, but their radio powers differ by several orders of magnitude. Somewhere between the near infrared and the radio their spectra must diverge dramatically.The IRAS survey detected 17 radio quiet quasars and luminous Seyfert 1's with −29. 5 ≤ Mv ≤ −21.6 (for Ho = 75). By coadding the survey data and using pointed observations, we have detections of most of these objects in all four IRAS passbands. The spectra are all rising with wavelength all the way to 100μ. We are measuring fluxes in the centimeter, millimeter, and, together with R Cutri, the near infrared and optical regions for each of these objects. Our goal is to constrain the location, shape, and spectral context of the low frequency cutoffs. Here we present the IRAS, millimeter and centimeter data. Measurements at the other wavelengths are still in progress.Although the spectra are rising steeply between 60μ and 100μ, we find that all of our objects are undetectable at 1.3 mm with the NRAO 12-m telescope. Our limits are typically an order of magnitude below the 100μ fluxes. (Ennis et al (1982) and Robson et al (1985) have already shown that the 1 mm fluxes of some radio quiet quasars must be below the extrapolation of the near infrared continuum.) Our objects are all extremely weak or undetected with the VLA at 2 cm and 1.3 cm, at levels typically three orders of magnitude below the 100μ fluxes. The sharpness of the required cutoffs allows us to rule out the hypothesis that the infrared is synchrotron radiation with the cutoff due to an absence of low energy electrons. The high frequency of the cutoffs makes free-free absorption implausible, but not impossible. It is possible that synchrotron self-absorption is suppressing the radio. There is circumstantial evidence that the far infrared is thermal dust emission. This would require a lower cutoff in the distribution of dust temperatures, which we think we can explain.
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Rovenskaya, N. I. "The low-frequency radio recombination lines formed in the extremely extended C II regions". Astronomical & Astrophysical Transactions 26, n. 4-5 (ottobre 2007): 203–9. http://dx.doi.org/10.1080/10556790601135406.
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Ramasawmy, J., J. E. Geach, M. J. Hardcastle, P. N. Best, M. Bonato, M. Bondi, G. Calistro Rivera et al. "Low-frequency radio spectra of submillimetre galaxies in the Lockman Hole". Astronomy & Astrophysics 648 (aprile 2021): A14. http://dx.doi.org/10.1051/0004-6361/202039858.
Testo completoAbstract (sommario):
Aims. We investigate the radio properties of a sample of 850 μm-selected sources from the SCUBA-2 Cosmology Legacy Survey (S2CLS) using new deep, low-frequency radio imaging of the Lockman Hole field from the Low Frequency Array. This sample consists of 53 sources, 41 of which are detected at >5σ at 150 MHz. Methods. Combining these data with additional observations at 324 MHz, 610 MHz, and 1.4 GHz from the Giant Metrewave Radio Telescope and the Jansky Very Large Array, we find a variety of radio spectral shapes and luminosities (L1.4 GHz ranging from ~4 × 1023−1 × 1025) within our sample despite their similarly bright submillimetre flux densities (>4 mJy). We characterise their spectral shapes in terms of multi-band radio spectral indices. Finding strong spectral flattening at low frequencies in ~20% of sources, we investigate the differences between sources with extremely flat low-frequency spectra and those with ‘normal’ radio spectral indices (α > −0.25). Results. As there are no other statistically significant differences between the two subgroups of our sample as split by the radio spectral index, we suggest that any differences are undetectable in galaxy-averaged properties that we can observe with our unresolved images, and likely relate to galaxy properties that we cannot resolve, on scales ≲1 kpc. We attribute the observed spectral flattening in the radio to free–free absorption, proposing that those sources with significant low-frequency spectral flattening have a clumpy distribution of star-forming gas. We estimate an average spatial extent of absorbing material of at most several hundred parsecs to produce the levels of absorption observed in the radio spectra. This estimate is consistent with the highest-resolution observations of submillimetre galaxies in the literature, which find examples of non-uniform dust distributions on scales of ~100 pc, with evidence for clumps and knots in the interstellar medium. Additionally, we find two bright (>6 mJy) S2CLS sources undetected at all other wavelengths. We speculate that these objects may be very high redshift sources, likely residing at z > 4.
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Mandal, S., I. Prandoni, M. J. Hardcastle, T. W. Shimwell, H. T. Intema, C. Tasse, R. J. van Weeren et al. "Extremely deep 150 MHz source counts from the LoTSS Deep Fields". Astronomy & Astrophysics 648 (aprile 2021): A5. http://dx.doi.org/10.1051/0004-6361/202039998.
Testo completoAbstract (sommario):
With the advent of new generation low-frequency telescopes, such as the LOw Frequency ARray (LOFAR), and improved calibration techniques, we have now started to unveil the subgigahertz radio sky with unprecedented depth and sensitivity. The LOFAR Two Meter Sky Survey (LoTSS) is an ongoing project in which the whole northern radio sky will be observed at 150 MHz with a sensitivity better than 100 μJy beam−1 at a resolution of 6′′. Additionally, deeper observations are planned to cover smaller areas with higher sensitivity. The Lockman Hole, the Boötes, and the Elais-N1 regions are among the most well known northern extra-galactic fields and the deepest of the LoTSS Deep Fields so far. We exploited these deep observations to derive the deepest radio source counts at 150 MHz to date. Our counts are in broad agreement with those from the literature and show the well known upturn at ≤1 mJy, mainly associated with the emergence of the star-forming galaxy population. More interestingly, our counts show, for the first time a very pronounced drop around S ~ 2 mJy, which results in a prominent “bump” at sub-mJy flux densities. Such a feature was not observed in previous counts’ determinations (neither at 150 MHz nor at a higher frequency). While sample variance can play a role in explaining the observed discrepancies, we believe this is mostly the result of a careful analysis aimed at deblending confused sources and removing spurious sources and artifacts from the radio catalogs. This “drop and bump” feature cannot be reproduced by any of the existing state-of-the-art evolutionary models, and it appears to be associated with a deficiency of active galactic nuclei (AGN) at an intermediate redshift (1 < z < 2) and an excess of low-redshift (z < 1) galaxies and/or AGN.
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Jung, Ji S., Jung N. Lee, Joung M. Kim e Jong K. Park. "An UHF RFID Reader Antenna with Multitag Identification for Extremely Low-Temperature Medical Systems". International Journal of Antennas and Propagation 2020 (10 luglio 2020): 1–11. http://dx.doi.org/10.1155/2020/2482961.
Testo completoAbstract (sommario):
A radio frequency identification reader antenna having multitag identification for medical systems is presented, which consists of four PIFAs, two hybrid couplers, and four power dividers. The high isolation is achieved by the symmetric design of the antenna geometry and four power dividers, which are fed by two hybrid couplers. The experimental results show an isolation of more than 40 dB in the North American (902–928 MHz), Korean (917–923.5 MHz), and Japanese (916.7–923.5 MHz) RFID frequency bands.
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Urbinello*, Damiano, Benjamin Struchen e Martin Röösli. "Characterization of Radio- and Extremely Low-Frequency Electromagnetic Fields in Everyday Environments in Three European Cities". ISEE Conference Abstracts 2014, n. 1 (20 ottobre 2014): 1662. http://dx.doi.org/10.1289/isee.2014.p1-262.
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Kuzay, D., C. Ozer, B. Sirav, A. G. Canseven e N. Seyhan. "Oxidative effects of extremely low frequency magnetic field and radio frequency radiation on testes tissues of diabetic and healthy rats". Bratislava Medical Journal 118, n. 05 (2017): 278–82. http://dx.doi.org/10.4149/bll_2017_055.
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Selmaoui, Brahim. "Effects of extremely low frequency (ELF) and radio-frequency (RF) on melatonin and cortisol, two markers of the circadian rhythms". Toxicology Letters 280 (ottobre 2017): S31. http://dx.doi.org/10.1016/j.toxlet.2017.07.076.
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Tereshchenko, E. D., V. F. Grigor’ev, A. E. Sidorenko, A. N. Milichenko, A. V. Mol’kov, L. A. Sobchakov e A. V. Vasil’ev. "On the possibility of quasi-vertical radio sounding of the ionosphere in the extremely low frequency band". JETP Letters 85, n. 8 (giugno 2007): 386–88. http://dx.doi.org/10.1134/s0021364007080085.
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Havas, M. "Biological effects of non-ionizing electromagnetic energy: A critical review of the reports by the US National Research Council and the US National Institute of Environmental Health Sciences as they relate to the broad realm of EMF bioeffects". Environmental Reviews 8, n. 3 (1 marzo 2000): 173–253. http://dx.doi.org/10.1139/a00-004.
Testo completoAbstract (sommario):
Our dependence on electricity and our growing dependence on wireless telecommunication technology is causing this planet to be inundated with electromagnetic energy ranging in frequency from less than 60 Hz to greater than 2 GHz. Concerns expressed by the public, who live near power lines, cell phone antennas, or television and radio broadcast towers, have prompted two major reviews: one by the US National Research Council (NRC) and the other by the US National Institute of Environmental Health Science (NIEHS). Both of these documents deal with the biological and health effects primarily in a residential setting of extremely low frequency (ELF) or power frequency (50 and 60 Hz) fields. This paper critically evaluates the NRC and NIEHS documents. This evaluation includes both the content and the process leading to the final reports. It summarizes the information available on human exposure to electric and magnetic fields and identifies key biological markers and potential mechanisms that have been linked to electromagnetic exposure. It examines the conclusions of both documents in terms of the slightly broader realm associated with occupational exposure, non-power frequency fields, EMF hypersensitivity, and response of species other than humans. It presents some of the scientific controversy surrounding the question "Are low frequency electric and magnetic fields harmful?'' and examines the concepts of bias and consistency in data interpretation. This paper also attempts to place the discussions about technologically generated fields (technofields) into a much broader perspective, a perspective that includes naturally occurring geofields and biofields. Key words: leukemia, breast cancer, melatonin, calcium flux, extremely low frequency electromagnetic fields, radio frequency radiation.
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Vocks, C., G. Mann, F. Breitling, M. M. Bisi, B. Dąbrowski, R. Fallows, P. T. Gallagher et al. "LOFAR observations of the quiet solar corona". Astronomy & Astrophysics 614 (giugno 2018): A54. http://dx.doi.org/10.1051/0004-6361/201630067.
Testo completoAbstract (sommario):
Context. The quiet solar corona emits meter-wave thermal bremsstrahlung. Coronal radio emission can only propagate above that radius, Rω, where the local plasma frequency equals the observing frequency. The radio interferometer LOw Frequency ARray (LOFAR) observes in its low band (10–90 MHz) solar radio emission originating from the middle and upper corona. Aims. We present the first solar aperture synthesis imaging observations in the low band of LOFAR in 12 frequencies each separated by 5 MHz. From each of these radio maps we infer Rω, and a scale height temperature, T. These results can be combined into coronal density and temperature profiles. Methods. We derived radial intensity profiles from the radio images. We focus on polar directions with simpler, radial magnetic field structure. Intensity profiles were modeled by ray-tracing simulations, following wave paths through the refractive solar corona, and including free-free emission and absorption. We fitted model profiles to observations with Rω and T as fitting parameters. Results. In the low corona, Rω < 1.5 solar radii, we find high scale height temperatures up to 2.2 × 106 K, much more than the brightness temperatures usually found there. But if all Rω values are combined into a density profile, this profile can be fitted by a hydrostatic model with the same temperature, thereby confirming this with two independent methods. The density profile deviates from the hydrostatic model above 1.5 solar radii, indicating the transition into the solar wind. Conclusions. These results demonstrate what information can be gleaned from solar low-frequency radio images. The scale height temperatures we find are not only higher than brightness temperatures, but also than temperatures derived from coronograph or extreme ultraviolet (EUV) data. Future observations will provide continuous frequency coverage. This continuous coverage eliminates the need for local hydrostatic density models in the data analysis and enables the analysis of more complex coronal structures such as those with closed magnetic fields.
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Cios, Aleksandra, Martyna Ciepielak, Wanda Stankiewicz e Łukasz Szymański. "The Influence of the Extremely Low Frequency Electromagnetic Field on Clear Cell Renal Carcinoma". International Journal of Molecular Sciences 22, n. 3 (29 gennaio 2021): 1342. http://dx.doi.org/10.3390/ijms22031342.
Testo completoAbstract (sommario):
The development of new technologies and industry is conducive to the increase in the number and variety of electromagnetic field (EMF) sources in our environment. The main sources of EMF are high-voltage lines, household appliances, audio/video devices, mobile phones, radio stations, and radar devices. In the growing use of electronic devices, scientists are increasingly interested in the effects of EMF on human health. Even though many studies on the effects of EMF have already been carried out, none of them has shown a significant effect on mammals, including humans. Moreover, it is not entirely clear how EMF influences cell behavior. The International Agency for Research on Cancer on 31 May 2011, classified PEM as a possible carcinogenic factor. This study aimed to investigate the effect of the electromagnetic field on morphological and functional changes in clear cell renal carcinoma. The research was carried out on in vitro cultures of four cell lines: HEK293, 786-O 769-P, and Caki1. The results of the research showed that the EMF of low frequency had a slight effect on the viability of cells. EMF, which induced cell arrest in the G1 phase, increased the number of early apoptotic cells and decreased the number of viable cells in the 786-O line. EMF did not affect the proliferation and viability of HEK293 cells. Extreme low-frequency EMF (ELF-EMF) also showed an inhibitory effect on the migration and metastatic properties of clear cell kidney cancer cells. Moreover, shortly after the end of ELF-EMF exposure, significant increases in ROS levels were observed in all tested cell lines. As part of the work, it was shown that low-frequency EMF shows an inhibitory effect on the proliferation of primary cancer cells, diminishing their migratory, invasive, and metastatic abilities. It also increases the apoptosis of cancer cells and the amount of reactive oxygen species. Based on the results of our research, we want to point up that the effect of ELF-EMF depends on a specific metabolic state or at a specific stage in the cell cycle of the cells under study.
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Taylor, A. R. "Low Frequency Science with the Square Kilometre Array". Symposium - International Astronomical Union 199 (2002): 459–66. http://dx.doi.org/10.1017/s0074180900169578.
Testo completoAbstract (sommario):
Over the past several years an international community of scientists and engineers has emerged with a common goal to solve the technical challenge required to construct a giant radio telescope with a collecting area of one square kilometre. The Square Kilometre Array (SKA) will have a hundred times more collecting area than our most powerful existing radio telescopes, providing sensitivity of a few tens of nanoJy in the centimetre/decimetre wavelength continuum. With a spatial resolution better than the Hubble Space Telescope, a field of view larger than the full moon, and the ability to simultaneously image a wide range of red shift, the SKA will be the worlds premier spectroscopic imaging telescope at any waveband.At long wavelengths the SKA will be able to detect emission from atomic hydrogen gas at extreme redshifts, allowing study of the “Dark Ages” of the Universe, before, and during, the transition phase when the initial stars formed and reionization occurred. The combination of sensitivity, wide field of view and high angular resolution, will allow high resolution imaging of the interstellar media and magnetic field of a vast number of galaxies to high redshift. Measurements of atomic hydrogen emission and continuum emission will trace the star formation history of the Universe from primordial galaxies to the present.
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Shapirovskaya, N., O. B. Slee, P. Hughes e G. Tsarevsky. "Radio Source Variability as a Tool for Very High Resolution". Symposium - International Astronomical Union 158 (1994): 201–3. http://dx.doi.org/10.1017/s0074180900107582.
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If flux density variability is intrinsic to extragalactic sources, then the shortest time scale of the variability yields an extremely high angular resolution. There is, however, good evidence that some of the variability is due to refractive scintillation in our galaxy's interstellar plasma turbulence. The effect, initially discovered at low radio frequencies, is here shown to extend into the GHz band, which, up to now, has been assumed to display only the intrinsic variability. We conclude that further study in both the intermediate and high frequency bands needed to separate the intrinsic and extrinsic components of variability.
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Nickolaenko, A. P. "Parametric excitation of an extremely low-frequency radio pulse by an extragalactic gamma-ray burst on December 27, 2004". Radiophysics and Quantum Electronics 53, n. 4 (ottobre 2010): 215–24. http://dx.doi.org/10.1007/s11141-010-9219-7.
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Emig, K. L., P. Salas, F. de Gasperin, J. B. R. Oonk, M. C. Toribio, H. J. A. Röttgering e A. G. G. M. Tielens. "The first detection of radio recombination lines at cosmological distances". Astronomy & Astrophysics 622 (febbraio 2019): A7. http://dx.doi.org/10.1051/0004-6361/201834052.
Testo completoAbstract (sommario):
Context. Recombination lines involving high principal quantum numbers (n ∼ 50 − 1000) populate the radio spectrum in large numbers. Low-frequency (< 1 GHz) observations of radio recombination lines (RRLs) primarily from carbon and hydrogen offer a new, if not unique, way to probe cold, largely atomic gas and warm, ionised gas in other galaxies. Furthermore, RRLs can be used to determine the physical state of the emitting regions, such as temperature and density. These properties make RRLs, potentially, a powerful tool of extragalactic interstellar medium (ISM) physics. At low radio frequencies, it is conceivable to detect RRLs out to cosmological distances when illuminated by a strong radio continuum. However, they are extremely faint (τpeak ∼ 10−3 − 10−4) and have so far eluded detection outside of the local universe. Aims. With observations of the radio quasar 3C 190 (z = 1.1946), we aim to demonstrate that the ISM can be explored out to great distances through low-frequency RRLs. Methods. 3C 190 was observed with the LOw Frequency ARray (LOFAR) and processed using newly developed techniques for spectral analysis. Results. We report the detection of RRLs in the frequency range 112 MHz–163 MHz in the spectrum of 3C 190. Stacking 13 α-transitions with principal quantum numbers n = 266 − 301, a peak 6σ feature of optical depth τpeak = (1.0 ± 0.2)×10−3 and FWHM = 31.2 ± 8.3 km s−1 was found at z = 1.124. This corresponds to a velocity offset of −9965 km s−1 with respect to the systemic redshift of 3C 190. Conclusions. We consider three interpretations of the origin of the RRL emission: an intervening dwarf-like galaxy, an active galactic nucleus (AGN) driven outflow, and the inter-galactic medium. We argue that the recombination lines most likely originate in a dwarf-like galaxy (M ∼ 109 M⊙) along the line of sight, although we cannot rule out an AGN-driven outflow. We do find the RRLs to be inconsistent with an inter-galactic medium origin. With this detection, we have opened up a new way to study the physical properties of cool, diffuse gas out to cosmological distances.
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Sridhar, Sarrvesh S., Raffaella Morganti, Kristina Nyland, Bradley S. Frank, Jeremy Harwood e Tom Oosterloo. "LOFAR view of NGC 3998, a sputtering AGN". Astronomy & Astrophysics 634 (febbraio 2020): A108. http://dx.doi.org/10.1051/0004-6361/201936796.
Testo completoAbstract (sommario):
Low-power radio sources dominate the radio sky. They tend to be small in size and dominated by their cores, but the origin of their properties and the evolution of their radio plasma are not well constrained. Interestingly, there is mounting evidence that low-power radio sources can significantly affect their surrounding gaseous medium and may therefore be more relevant for galaxy evolution than previously thought. In this paper, we present low radio frequency observations obtained with LOFAR at 147 MHz of the radio source hosted by NGC 3998. This is a rare example of a low-power source that is extremely dominated by its core, but that has two large-scale lobes of low surface brightness. We combine the new 147 MHz image with available 1400 MHz data to derive the spectral index over the source. Despite the low surface brightness, reminiscent of remnant structures, the lobes show an optically thin synchrotron spectral index (∼0.6). We interpret this as being due to rapid decollimation of the jets close to the core, to high turbulence of the plasma flow, and to entrainment of thermal gas. This could be the result of intermittent activity of the central active galactic nucleus, or, more likely, temporary disruption of the jet due to the interaction of the jet with the rich circumnuclear interstellar matter. Both would result in sputtering energy injection from the core, which would keep the lobes fed, albeit at a low rate. We discuss these results in connection with the properties of low-power radio sources in general. Our findings show that amorphous low surface brightness lobes should not be interpreted by default as remnant structures. Large deep surveys (in particular the LOFAR 150 MHz LoTSS and the recently started 1400 MHz Apertif survey) will identify a growing number of objects similar to NGC 3998 where these ideas can be further tested.
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Timlin, John D., W. N. Brandt, S. Zhu, H. Liu, B. Luo e Q. Ni. "The frequency of extreme X-ray variability for radio-quiet quasars". Monthly Notices of the Royal Astronomical Society 498, n. 3 (2 settembre 2020): 4033–50. http://dx.doi.org/10.1093/mnras/staa2661.
Testo completoAbstract (sommario):
ABSTRACT We analyse 1598 serendipitous Chandra X-ray observations of 462 radio-quiet quasars to constrain the frequency of extreme amplitude X-ray variability that is intrinsic to the quasar corona and innermost accretion flow. The quasars in this investigation are all spectroscopically confirmed, optically bright (mi ≤ 20.2), and contain no identifiable broad absorption lines in their optical/ultraviolet spectra. This sample includes quasars spanning z ≈ 0.1–4 and probes X-ray variability on time-scales of up to ≈12 rest-frame years. Variability amplitudes are computed between every epoch of observation for each quasar and are analysed as a function of time-scale and luminosity. The tail-heavy distributions of variability amplitudes at all time-scales indicate that extreme X-ray variations are driven by an additional physical mechanism and not just typical random fluctuations of the coronal emission. Similarly, extreme X-ray variations of low-luminosity quasars seem to be driven by an additional physical mechanism, whereas high-luminosity quasars seem more consistent with random fluctuations. The amplitude at which an X-ray variability event can be considered extreme is quantified for different time-scales and luminosities. Extreme X-ray variations occur more frequently at long time-scales (Δt ≳ 300 d) than at shorter time-scales and in low-luminosity quasars compared to high-luminosity quasars over a similar time-scale. A binomial analysis indicates that extreme intrinsic X-ray variations are rare, with a maximum occurrence rate of $\lt 2.4{{\ \rm per\ cent}}$ of observations. Finally, we present X-ray variability and basic optical emission-line properties of three archival quasars that have been newly discovered to exhibit extreme X-ray variability.
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Lerkbangplad, Chawanat, Alongkorn Namahoot, Prayoot Akkaraekthalin e Suramate Chalermwisutkul. "A compact wideband circularly polarized quadrifilar antenna with PIFA elements for UHF RFID readers". International Journal of Microwave and Wireless Technologies 12, n. 10 (8 aprile 2020): 1020–28. http://dx.doi.org/10.1017/s1759078720000276.
Testo completoAbstract (sommario):
AbstractIn this paper, a compact circularly polarized quadrifilar antenna with planar inverted-F antenna (PIFA) elements is presented. The proposed antenna consists of four PIFA elements and a Wilkinson divider-based feed network fabricated on FR-4 substrate (ɛr = 4.4, loss tangent = 0.02, thickness = 1.6 mm). The total size of the antenna is 120 × 120 × 13.2 mm3. Impedance matching with a reflection coefficient <−15 dB and an axial ratio (AR) <3 dB are achieved over the global ultra-high frequency (UHF) radio frequency identification (RFID) frequency band and beyond. The realized gain ranges from 2.25 to 3.75 dBic within the frequency band of interest from 860 to 960 MHz with a directional radiation pattern. The proposed antenna is compact, low-cost and extremely wideband in terms of matching and AR compared to state-of-the-art UHF RFID reader antennas.
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Giglio, Jeffrey J., e Joseph A. Caruso. "Evaluation of Alternative Plasma Gases for Radio-Frequency Glow Discharge Mass Spectrometry (rf GD-MS)". Applied Spectroscopy 49, n. 7 (luglio 1995): 900–906. http://dx.doi.org/10.1366/0003702953964796.
Testo completoAbstract (sommario):
Interferences are a major consideration in all forms of plasma mass spectrometry. Investigations into the use of He, Ne, N2, and Kr in radio-frequence glow discharge mass spectrometry (rf GD-MS) have been undertaken to study their potential as alternative plasma gases. All gases exhibited expected behavior with respect to pressure and power. Background species are discussed for each gas, along with the effect of each gas on analyte intensities in Standard Reference Material (SRM) copper 500. A helium plasma yields the highest intensities for analytes (Cr, Fe, Ni, Co, Zn, As, Se, Ag, Au, Pb, and Bi) of all gases investigated. Kr was not a practical alternative gas since analyte intensities were extremely low, severe memory effects were present, and the UHP Kr is very expensive. Relative sensitivity factors are presented for each of the gases for the same copper SRM 500. Argon has the highest sputter rate, with Ne and He being similar and N2 being the lowest. Isotope abundances are given for Se, Fe, Br, and Cl in a He plasma.
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Li, Jingtao, Yang Liu, Wengang Pei e Zhen Cao. "Behaviors of High-Frequency Subscribers in Cellular Data Networks". Security and Communication Networks 2018 (6 novembre 2018): 1–12. http://dx.doi.org/10.1155/2018/2506319.
Testo completoAbstract (sommario):
Cellular networks operate under restrictive constraints of resources including radio channel capacity and network processing capability. The tremendous growth in the cellular data network usage brings operators with unprecedented signaling overloads and threatens the stability of the network. High-frequency subscribers, who produce low data volume but cause high signaling overhead, are extremely resource-inefficient. For subscribers who activate more than 5 sessions per hour, they only account for 1.19% of the total subscribers and generate about 3.81% data traffic but consume roughly 19.46% of the signaling resources, resulting in the inconsistent signaling-data bandwidth consumptions. Understanding the characteristics of those users has an important significance of capacity design and optimal allocation of resources. A lack of understanding of this active group potentially leads to low network performance and security threats. In this paper, we perform the first city-wide, large-scale investigation of high-frequency subscribers. By applying a set of novel approaches, such as pattern extraction and user behavior rebuilding, we observed that high-frequency subscribers correspond to a lower percentage of none-pattern traffic, showing positive correlation between access regularity and session activation frequency. Besides, we found that amount of high-frequency subscribers has abnormal behaviors, resulting in unwanted signaling loads. We demonstrate that our findings have significant implications on network optimization.
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Aller, Daniel G., Diego G. Lamar, Juan Rodriguez, Pablo F. Miaja, Valentin Francisco Romero, Jose Mendiolagoitia e Javier Sebastian. "Adapting Techniques to Improve Efficiency in Radio Frequency Power Amplifiers for Visible Light Communications". Electronics 9, n. 1 (10 gennaio 2020): 131. http://dx.doi.org/10.3390/electronics9010131.
Testo completoAbstract (sommario):
It is well known that modern wireless communications systems need linear, wide bandwidth, efficient Radio Frequency Power Amplifiers (RFPAs). However, conventional configurations of RFPAs based on Class A, Class B, and Class AB exhibit extremely low efficiencies when they manage signals with a high Peak-to-Average Power Ratio (PAPR). Traditionally, a number of techniques have been proposed either to achieve linearity in the case of efficient Switching-Mode RFPAs or to improve the efficiency of linear RFPAs. There are two categories in the application of aforementioned techniques. First, techniques based on the use of Switching-Mode DC–DC converters with a very-fast-output response (faster than 1 µs). Second, techniques based on the interaction of several RFPAs. The current expansion of these techniques is mainly due to their application in cellphone networks, but they can also be applied in other promising wireless communications systems such as Visible Light Communication (VLC). The main contribution of this paper is to show how Envelope Tracking (ET), Envelope and Elimination (EER), Outphasing, and Doherty techniques can be helpful in developing more efficient VLC transmitters capable of reaching high bit-rates (higher than 1 Mbps) by using advance modulation schemes. Finally, two examples based on the application of the Outphasing technique and the use of a Linear-Assisted Envelope Amplifier (EA) to VLC are presented and experimentally verified.
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He, Xiao-Ming, S.-T. Lee, I. Bello, A. C. Cheung, W. Z. Li, D. S. Chiu, Y. W. Lam, C. S. Lee, K. M. Leung e X. T. Zhou. "Characterization and Optical Investigation of Diamondlike Carbon Prepared by Electron Cyclotron Resonance Plasma". Journal of Materials Research 14, n. 4 (aprile 1999): 1617–25. http://dx.doi.org/10.1557/jmr.1999.0217.
Testo completoAbstract (sommario):
Diamondlike carbon (DLC) films have been prepared on radio-frequency (rf) biased substrates maintained at low temperature (∼60 °C) using electron cyclotron resonance CH4 –Ar plasma. The structures of the resultant films were characterized by Fourier transform infrared (FTIR), Raman, and ultraviolet/visible (UV/VIS) spectrometry. The studies revealed that the deposited structures were DLC films with sp3/sp2 bond hybridization, extremely high hardness (>3000 kgf/mm2), and high electrical resistivity (up to 1014 Ω cm). The DLC films deposited on colorless (transparent) polymer plastics were examined to determine visible light transparencies and optical bandgaps. The results indicate that electron cyclotron resonance (ECR) plasma processing with low negative rf bias, low deposition temperature, and suitable CH4/Ar gas composition can form optically visible light transparent and hard protective DLC films on polymer plastic surfaces.
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Teitelbaum, Lawrence, Walid Majid, Manuel M. Franco, Daniel J. Hoppe, Shinji Horiuchi e T. Joseph W. Lazio. "Precision Pulsar Timing with NASA's Deep Space Network". Proceedings of the International Astronomical Union 11, A29B (agosto 2015): 367–69. http://dx.doi.org/10.1017/s174392131600555x.
Testo completoAbstract (sommario):
AbstractMillisecond pulsars (MSPs) are a class of radio pulsars with extremely stable rotation. Their excellent timing stability can be used to study a wide variety of astrophysical phenomena. In particular, a large sample of these pulsars can be used to detect low-frequency gravitational waves. We have developed a precision pulsar timing backend for the NASA Deep Space Network (DSN), which will allow the use of short gaps in tracking schedules to time pulses from an ensemble of MSPs. The DSN operates clusters of large dish antennas (up to 70-m in diameter), located roughly equidistant around the Earth, for communication and tracking of deep-space spacecraft. The backend system will be capable of removing entirely the dispersive effects of propagation of radio waves through the interstellar medium in real-time. We will describe our development work, initial results, and prospects for future observations over the next few years.
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Tokarsky, Peter, Alexander Konovalenko, Mykola Kalinichenko e Serge Yerin. "Impact of Ground Parameters on Performances of an Active Phased Array Element of the Low-Frequency Radio Telescope GURT". Journal of Astronomical Instrumentation 09, n. 04 (12 ottobre 2020): 2050015. http://dx.doi.org/10.1142/s2251171720500154.
Testo completoAbstract (sommario):
This paper presents the technique of numerical estimation of ground parameters impact on the performances of an active antenna used as an element of a phased array antenna of a modern low-frequency radio telescope. Three ground conditions were considered, two of them wet and dry, which correspond to the extreme values of seasonal deviations of its parameters, as well as its median state (“normal ground”). The results of computer simulation are given for the active antenna of the GURT radio telescope with a ground screen in the form of a square wire grid of [Formula: see text][Formula: see text]m and without it. It is shown that the addition of the ground screen markedly reduces the influence of the ground conditions on some parameters of the antenna, in particular, the dipole impedance and radiation efficiency. At the same time, the ground screen does not protect the most important antenna parameters for radio astronomy, such as sensitivity and absorption area, from the impact of the ground conditions changes. Variations of these parameters for an active antenna with a screen remain approximately the same as without it.
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Kollár, Zsolt, Lajos Varga, Bálint Horváth, Péter Bakki e János Bitó. "Evaluation of Clipping Based Iterative PAPR Reduction Techniques for FBMC Systems". Scientific World Journal 2014 (2014): 1–12. http://dx.doi.org/10.1155/2014/841680.
Testo completoAbstract (sommario):
This paper investigates filter bankmulticarrier (FBMC), a multicarrier modulation technique exhibiting an extremely low adjacent channel leakage ratio (ACLR) compared to conventional orthogonal frequency division multiplexing (OFDM) technique. The low ACLR of the transmitted FBMC signal makes it especially favorable in cognitive radio applications, where strict requirements are posed on out-of-band radiation. Large dynamic range resulting in high peak-to-average power ratio (PAPR) is characteristic of all sorts of multicarrier signals. The advantageous spectral properties of the high-PAPR FBMC signal are significantly degraded if nonlinearities are present in the transceiver chain. Spectral regrowth may appear, causing harmful interference in the neighboring frequency bands. This paper presents novel clipping based PAPR reduction techniques, evaluated and compared by simulations and measurements, with an emphasis on spectral aspects. The paper gives an overall comparison of PAPR reduction techniques, focusing on the reduction of the dynamic range of FBMC signals without increasing out-of-band radiation. An overview is presented on transmitter oriented techniques employing baseband clipping, which can maintain the system performance with a desired bit error rate (BER).
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Capetti, A., M. Brienza, R. D. Baldi, G. Giovannini, R. Morganti, M. J. Hardcastle, H. J. A. Rottgering, G. F. Brunetti, P. N. Best e G. Miley. "The LOFAR view of FR 0 radio galaxies". Astronomy & Astrophysics 642 (ottobre 2020): A107. http://dx.doi.org/10.1051/0004-6361/202038671.
Testo completoAbstract (sommario):
We explore the low-frequency radio properties of the sources in the Fanaroff-Riley class 0 catalog (FR0CAT) as seen by the Low-Frequency ARray (LOFAR) observations at 150 MHz. This sample includes 104 compact radio active galactic nuclei (AGN) associated with nearby (z < 0.05) massive early-type galaxies. Sixty-six FR0CAT sources are in the sky regions observed by LOFAR and all of them are detected, usually showing point-like structures with sizes of ≲3–6 kpc. However, 12 FR 0s present resolved emission of low surface brightness, which contributes between 5% and 40% of the total radio power at 150 MHz, usually with a jetted morphology extending between 15 and 50 kpc. No extended emission is detected around the other FR 0s, with a typical luminosity limit of ≲5 × 1022 W Hz−1 over an area of 100 kpc × 100 kpc. The spectral slopes of FR 0s between 150 MHz and 1.4 GHz span a broad range (−0.7 ≲ α ≲ 0.8) with a median value of ᾱ ∼ 0.1; 20% of them have a steep spectrum (α ≳ 0.5), which is an indication of the presence of substantial extended emission confined within the spatial resolution limit. The fraction of FR 0s showing evidence for the presence of jets, by including both spectral and morphological information, is at least ∼40%. This study confirms that FR 0s and FR Is can be interpreted as two extremes of a continuous population of jetted sources, with the FR 0s representing the low end in size and radio power.
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Ahmad, Sheikh Faisal, Young Cheol Kim, Ick Chang Choi e Hyun Deok Kim. "Recent Progress in Birdcage RF Coil Technology for MRI System". Diagnostics 10, n. 12 (27 novembre 2020): 1017. http://dx.doi.org/10.3390/diagnostics10121017.
Testo completoAbstract (sommario):
The radio frequency (RF) coil is one of the key components of the magnetic resonance imaging (MRI) system. It has a significant impact on the performance of the nuclear magnetic resonance (NMR) detection. Among numerous practical designs of RF coils for NMR imaging, the birdcage RF coil is the most popular choice from low field to ultra-high field MRI systems. In the transmission mode, it can establish a strong and homogeneous transverse magnetic field B1 for any element at its Larmor frequency. Similarly, in the reception mode, it exhibits extremely high sensitivity for the detection of even faint NMR signals from the volume of interest. Despite the sophisticated 3D structure of the birdcage coil, the developments in the design, analysis, and implementation technologies during the past decade have rendered the development of the birdcage coils quite reasonable. This article provides a detailed review of the recent progress in the birdcage RF coil technology for the MRI system.
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García-Minguillán López, Olga, Ana Jiménez Valbuena e Ceferino Maestú Unturbe. "Significant Cellular Viability Dependence on Time Exposition at ELF-EMF and RF-EMF In Vitro Studies". International Journal of Environmental Research and Public Health 16, n. 12 (13 giugno 2019): 2085. http://dx.doi.org/10.3390/ijerph16122085.
Testo completoAbstract (sommario):
The human concern about the effect of electromagnetic fields (EMFs) has changed over time from the effects produced by EMFs of extremely low frequencies (ELFs) to the effects produced by exposure to a radio frequency (RF), with concerns shifting toward EMFs due to the development of new technologies and forms of communication. Previous studies have analysed the effects produced at different frequencies without considering in detail the effect of the time of exposure. Therefore, in the present study, we analysed in vitro the effect produced by a 100 µT EMF at different ELFs and exposure times in glioblastomas, as well as the effect produced in a fibroblast by an RF-EMF of 2.54 GHz. Our results indicate a significant time dependence in cell viability of fibroblasts exposed to an RF-EMF of 2.54 GHz and a non-time-dependent effect in cell viability of glioblastomas exposed to an ELF-EMF, highlighting the possible relation between frequency and time of exposure.
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Kim, Kyungki, Sining Li, Milad Heydariaan, Nour Smaoui, Omprakash Gnawali, Wonho Suh, Min Jae Suh e Jung In Kim. "Feasibility of LoRa for Smart Home Indoor Localization". Applied Sciences 11, n. 1 (4 gennaio 2021): 415. http://dx.doi.org/10.3390/app11010415.
Testo completoAbstract (sommario):
With the advancement of low-power and low-cost wireless technologies in the past few years, the Internet of Things (IoT) has been growing rapidly in numerous areas of Industry 4.0 and smart homes. With the development of many applications for the IoT, indoor localization, i.e., the capability to determine the physical location of people or devices, has become an important component of smart homes. Various wireless technologies have been used for indoor localization including WiFi, ultra-wideband (UWB), Bluetooth low energy (BLE), radio-frequency identification (RFID), and LoRa. The ability of low-cost long range (LoRa) radios for low-power and long-range communication has made this radio technology a suitable candidate for many indoor and outdoor IoT applications. Additionally, research studies have shown the feasibility of localization with LoRa radios. However, indoor localization with LoRa is not adequately explored at the home level, where the localization area is relatively smaller than offices and corporate buildings. In this study, we first explore the feasibility of ranging with LoRa. Then, we conduct experiments to demonstrate the capability of LoRa for accurate and precise indoor localization in a typical apartment setting. Our experimental results show that LoRa-based indoor localization has an accuracy better than 1.6 m in line-of-sight scenario and 3.2 m in extreme non-line-of-sight scenario with a precision better than 25 cm in all cases, without using any data filtering on the location estimates.
36
El-Absi, Mohammed, Feng Zheng, Ashraf Abuelhaija, Ali Al-haj Abbas, Klaus Solbach e Thomas Kaiser. "Indoor Large-Scale MIMO-Based RSSI Localization with Low-Complexity RFID Infrastructure". Sensors 20, n. 14 (15 luglio 2020): 3933. http://dx.doi.org/10.3390/s20143933.
Testo completoAbstract (sommario):
Indoor localization based on unsynchronized, low-complexity, passive radio frequency identification (RFID) using the received signal strength indicator (RSSI) has a wide potential for a variety of internet of things (IoTs) applications due to their energy-harvesting capabilities and low complexity. However, conventional RSSI-based algorithms present inaccurate ranging, especially in indoor environments, mainly because of the multipath randomness effect. In this work, we propose RSSI-based localization with low-complexity, passive RFID infrastructure utilizing the potential benefits of large-scale MIMO technology operated in the millimeter-wave band, which offers channel hardening, in order to alleviate the effect of small-scale fading. Particularly, by investigating an indoor environment equipped with extremely simple dielectric resonator (DR) tags, we propose an efficient localization algorithm that enables a smart object equipped with large-scale MIMO exploiting the RSSI measurements obtained from the reference DR tags in order to improve the localization accuracy. In this context, we also derive Cramer–Rao lower bound of the proposed technique. Numerical results evidence the effectiveness of the proposed algorithms considering various arbitrary network topologies, and results are compared with an existing algorithm, where the proposed algorithms not only produce higher localization accuracy but also achieve a greater robustness against inaccuracies in channel modeling.
37
Karamanavis, V., E. Angelakis, S. Komossa, I. Myserlis, D. Blinov e J. A. Zensus. "Radio and γ-ray loud narrow-line Seyfert 1 galaxies in the spotlight". Proceedings of the International Astronomical Union 12, S324 (settembre 2016): 184–87. http://dx.doi.org/10.1017/s1743921317001065.
Testo completoAbstract (sommario):
AbstractNarrow-line Seyfert 1 (NLS1) galaxies provide us with unique insights into the drivers of AGN activity under extreme conditions. Given their low black hole (BH) masses and near-Eddington accretion rates, they represent a class of galaxies with rapidly growing supermassive BHs in the local universe. Here, we present the results from our multi-frequency radio monitoring of a sample of γ-ray loud NLS1 galaxies (γNLS1s), including systems discovered only recently, and featuring both the nearest and the most distant γNLS1s known to date. We also present high-resolution radio imaging of 1H 0323+342, which is remarkable for its spiral or ring-like host. Finally, we present new radio data of the candidate γ-emitting NLS1 galaxy RX J2314.9+2243, characterized by a very steep radio spectrum, unlike other γNLS1s.
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Gosar, Žiga, Denis Đonlagić, Simon Pevec, Janez Kovač, Miran Mozetič, Gregor Primc, Alenka Vesel e Rok Zaplotnik. "Deposition Kinetics of Thin Silica-Like Coatings in a Large Plasma Reactor". Materials 12, n. 19 (3 ottobre 2019): 3238. http://dx.doi.org/10.3390/ma12193238.
Testo completoAbstract (sommario):
An industrial size plasma reactor of 5 m3 volume was used to study the deposition of silica-like coatings by the plasma-enhanced chemical vapor deposition (PECVD) method. The plasma was sustained by an asymmetrical capacitively coupled radio-frequency discharge at a frequency of 40 kHz and power up to 7 kW. Hexamethyldisilioxane (HMDSO) was introduced continuously at different flows of up to 200 sccm upon pumping with a combination of roots and rotary pumps at an effective pumping speed between 25 and 70 L/s to enable suitable gas residence time in the plasma reactor. The deposition rate and ion density were measured continuously during the plasma process. Both parameters were almost perfectly constant with time, and the deposition rate increased linearly in the range of HMDSO flows from 25 to 160 sccm. The plasma density was of the order of 1014 m−3, indicating an extremely low ionization fraction which decreased with increasing flow from approximately 2 × 10−7 to 6 × 10−8. The correlations between the processing parameters and the properties of deposited films are drawn and discussed.
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Hamidi, Zety Sharizat, N. N. M. Shariff e C. Monstein. "First Light Detection of a Single Solar Radio Burst Type III due to Solar Flare Event". International Letters of Chemistry, Physics and Astronomy 30 (marzo 2014): 51–58. http://dx.doi.org/10.18052/www.scipress.com/ilcpa.30.51.
Testo completoAbstract (sommario):
The eruption mechanism of solar flares and type III are currently an extremely active area of research, especially during the solar cycle is towards maximum. In this case, the total energy of solar burst type III is of the order of solar flare with the explosion of the energy can up to 1015 ergs. The solar flare event is one of the most spectacular explosions that still be on-going study in the solar physics world. This event occurred at 2:000 UT on 15th April 2012 is due to the explosion of the magnetic energy in from the chromosphere and converted into the heating, mass motion and particle acceleration which can be detected by solar radio burst type III. In this work, we will highlight our first light detection of very tiny solar radio burst type III, which has been observed at the National Space Centre, Banting, Selangor detected by the Compound Low Cost Low Frequency Transportable Observatories (CALLISTO) system at 5:53:23. The region of the data is from 150 − 400 MHz in radio region. This burst is drifted from 150 MHz till 260 MHz. It represents a total energy of 6.2035 × 10-7 eV − 1.0753 × 10-6 eV. This fast drift burst is a continuity of the acceleration of the particles which is intermittent, and can be observed since the explosion of the solar flare. Although the burst is very tiny, it is still significant because this burst is the first detection of a single type III burst from our site. Still, the acceleration of the particles can be detected from Earth in the radio region within 3 hours period of observation at the post stage of solar flare.
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BACELAR, J., O. SCHOLTEN, A. G. de BRUYN e H. FALCKE. "USING THE WESTERBORK RADIO OBSERVATORY TO DETECT UHE COSMIC PARTICLES INTERACTING ON THE MOON". International Journal of Modern Physics A 21, supp01 (luglio 2006): 147–52. http://dx.doi.org/10.1142/s0217751x06033532.
Testo completoAbstract (sommario):
Ultra-High-Energy (UHE) particles of cosmological origin (cosmic-rays and neutrinos), carry information on the most spectacular events known. These extremely energetic (energies larger than 1 ZeV= 1021 eV) cosmic-rays or neutrinos initiate in the lunar regolith a cascade of charged particles which acts as a radio pulse emitter. The instantaneous power produced can be detected here at the Earth, with a radio telescope operating at the optimal frequency window around 150 MHz. Using 12 telescopes of the Westerbork Synthesis Radio Telescope, WSRT, with a field of view covering the whole lunar surface, our calculations show that one should identify 10 UHE events within an observation time of 500 hours, assuming an extrapolated power law dependence of the highest ever measured cosmic-ray events, around an energy of 1020 eV. A null result will determine unambiguously the GKZ effect for the cosmic-ray flux and improve the present world upper limit on the neutrino flux above 1 ZeV, by three orders of magnitude, allowing for the first time to test the Waxman-Bahcall neutrino flux limit.
41
Kuiack, Mark, Ralph A. M. J. Wijers, Antonia Rowlinson, Aleksandar Shulevski, Folkert Huizinga, Gijs Molenaar e Peeyush Prasad. "Long-term study of extreme giant pulses from PSR B0950+08 with AARTFAAC". Monthly Notices of the Royal Astronomical Society 497, n. 1 (13 luglio 2020): 846–54. http://dx.doi.org/10.1093/mnras/staa1996.
Testo completoAbstract (sommario):
ABSTRACT We report on the detection of extreme giant pulses (GPs) from one of the oldest known pulsars, the highly variable PSR B0950+08, with the Amsterdam-ASTRON Radio Transient Facility And Analysis Centre (AARTFAAC), a parallel transient detection instrument operating as a subsystem of the LOw Frequency ARray (LOFAR). During processing of our Northern Hemisphere survey for low-frequency radio transients, a sample of 275 pulses with fluences ranging from 42 to 177 kJy ms were detected in one-second snapshot images. The brightest pulses are an order of magnitude brighter than those previously reported at 42 and 74 MHz, on par with the levels observed in a previous long-term study at 103 MHz. Both their rate and fluence distribution differ between and within the various studies done to date. The GP rate is highly variable, from 0 to 30 per hour, with only two 3-h observations accounting for nearly half of the pulses detected in the 96 h surveyed. It does not vary significantly within a few-hour observation, but can vary strongly one from day to the next. The spectra appear strongly and variably structured, with emission sometimes confined to a single 195.3 kHz subband, and the pulse spectra changing on a time-scale of order 10 min.
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Künkel, Lars, Rajat M. Thomas e Joris P. W. Verbiest. "Detecting pulsars with neural networks: a proof of concept". Monthly Notices of the Royal Astronomical Society 506, n. 1 (9 giugno 2021): 1111–27. http://dx.doi.org/10.1093/mnras/stab1628.
Testo completoAbstract (sommario):
ABSTRACT Pulsar searches are computationally demanding efforts to discover dispersed periodic signals in time- and frequency-resolved data from radio telescopes. The complexity and computational expense of simultaneously determining the frequency-dependent delay (dispersion) and the periodicity of the signal is further exacerbated by the presence of various types of radio-frequency interference (RFI) and observing-system effects. New observing systems with wider bandwidths, higher bit rates, and greater overall sensitivity (also to RFI) further enhance these challenges. We present a novel approach to the analysis of pulsar search data. Specifically, we present a neural-network-based pipeline that efficiently suppresses a wide range of RFI signals and instrumental instabilities and furthermore corrects for (a priori unknown) interstellar dispersion. After initial training of the network, our analysis can be run in real time on a standard desktop computer with a commonly available, consumer-grade graphics processing unit (GPU) . We complement our neural network with standard algorithms for periodicity searches. In particular with the Fast Fourier Transform and the Fast Folding Algorithm and demonstrate that with these straightforward extensions, our method is capable of identifying even faint pulsars while maintaining an extremely low number of false positives. We furthermore apply our analysis to a subset of the PALFA survey and demonstrate that in most cases the automated dispersion removal of our network produces a time series of similar quality as dedispersing, using the actual dispersion measure of the pulsar in question. On our test data, we are able to make predictions whether a pulsar is present in the data or not 200 times faster than real time.
43
Jiang, Pei-Cheng, Yu-Ting Chow, Chi-Wei Chien, Cheng-Hsun-Tony Chang e Chii-Ruey Lin. "Silica Layer Used in Sensor Fabrication from a Low-Temperature Silane-Free Procedure". Chemosensors 9, n. 2 (4 febbraio 2021): 32. http://dx.doi.org/10.3390/chemosensors9020032.
Testo completoAbstract (sommario):
Silica (SiO2, silicon dioxide—a dielectric layer commonly used in electronic devices) is widely used in many types of sensors, such as gas, molecular, and biogenic polyamines. To form silica films, core shell or an encapsulated layer, silane has been used as a precursor in recent decades. However, there are many hazards caused by using silane, such as its being extremely flammable, the explosive air, and skin and eye pain. To avoid these hazards, it is necessary to spend many resources on industrial safety design. Thus, the silica synthesized without silane gas which can be determined as a silane-free procedure presents a clean and safe solution to manufactures. In this report, we used the radio frequency (rf = 13.56 MHz) plasma-enhanced chemical vapor deposition technique (PECVD) to form a silica layer at room temperature. The silica layer is formed in hydrogen-based plasma at room temperature and silane gas is not used in this process. The substrate temperature dominates the silica formation, but the distance between the substrate and electrode (DSTE) and the methane additive can enhance the formation of a silica layer on the Si wafer. This silane-free procedure, at room temperature, is not only safer and friendlier to the environment but is also useful in the fabrication of many types of sensors.
44
Dea, Jack Y., Charles I. Richman e Wolfgang-M. Boerner. "Observations of seismo-electromagnetic earthquake precursor radiation signatures along Southern Californian fault zones: Evidence of long-distance precursor ultra-low frequency signals observed before a moderate Southern California earthquake episode". Canadian Journal of Physics 69, n. 8-9 (1 agosto 1991): 1138–45. http://dx.doi.org/10.1139/p91-173.
Testo completoAbstract (sommario):
Although questioned for a long time, there is accumulating growing evidence for the existence of detectable seismo-electromagnetic phenomena worldwide. California is geologically as well as seismically a unique region for studying these phenomena in depth; and, in particular, the southern California geologic province with a multitude of offshore and inland fault zones with San Diego in its center. At the Naval Ocean Systems Center Low Frequency Noise Laboratory, San Diego CA., we monitor 0.1–10 Hz ultra-low frequency (ULF) and 10–40 Hz extremely low-frequency (ELF) signals using mu-metal loaded multitum search coil sensors, as well as 10–100 kHz very low-frequency signals using large 1 m diameter loop antennas. We have observed precursor seismo-electromagnetic emissions of several earthquake events. In this paper, we report on observations of broadband ULF signals before and during the Upland quake of April 17, 1990 (Ms = 4.6), centered 200 km N of San Diego. The signals were detected with the vertically oriented search coil sensor and not with the horizontally oriented sensors, which suggests a disturbed ionosphere as the most likely source of these signals. The large prequake ULF activity, the rapid decay of ULF activity after the quake, and the absence of any geomagnetic storms indicate a good correlation of the ULF activity with the Upland quake. Although the exact mechanisms for coupling geologic activity to the ionosphere is not known, we cite a number of hypotheses concerning these mechanisms. Based on this succinct overview, an interpretation of our radio observations of seismic activity is presented and extended to earthquake precursor or predictor studies. We are in the process of expanding this research with the building of more monitoring stations and the improvement of our measurement, data collection, formatting, and data processing capabilities.
45
Tian, Tian, Cai Xiaojing e Zhu Huang. "Puerarin, an isoflavone compound extracted from Gegen (Radix Puerariae Lobatae), modulates sclera remodeling caused by extremely low frequency electromagnetic fields". Journal of Traditional Chinese Medicine 36, n. 5 (ottobre 2016): 678–82. http://dx.doi.org/10.1016/s0254-6272(16)30089-9.
Testo completo
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Hessels, J. W. T. "Charting the Transient Radio Sky on Sub-Second Time-Scales with LOFAR". Proceedings of the International Astronomical Union 7, S285 (settembre 2011): 104. http://dx.doi.org/10.1017/s174392131200035x.
Testo completoAbstract (sommario):
SummaryThe LOw Frequency ARray (LOFAR) is a radio interferometric telescope that promises to open a largely unexplored window on transient sources in the “radio sky”, from time-scales of nanoseconds to years. An important aspect of this will be the study of radio-emitting neutron stars in their various incarnations: slow pulsars, young pulsars, millisecond pulsars, magnetars, rotating radio transients, intermittent pulsars, et cetera. Pulsars and their brethren are the prototype of the more general “fast transients”: sub-second, dispersed radio bursts which point the way to extreme, and potentially still unknown, phenomena. For instance, prompt radio bursts from supernovæ and other extra-galactic bursts have been hypothesized; these could prove to be powerful cosmological probes.This talk discussed LOFAR's impressive ability to observe pulsars and to enlarge greatly the discovery space for (even rarer) fast transients. It also presented the latest pulsar observations made during LOFAR's commissioning period. These are demonstrating powerful observing techniques that will be crucial for the next generation of radio telescopes as well as the effort to increase our understanding of the dynamic nature of the Universe.An expanded version of the talk can be found at http://adsabs.harvard.edu/abs/2011A
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Vincenti Gatti, Roberto, Luca Marcaccioli, Elisa Sbarra e Roberto Sorrentino. "Flat Array Antennas for Ku-Band Mobile Satellite Terminals". International Journal of Antennas and Propagation 2009 (2009): 1–5. http://dx.doi.org/10.1155/2009/836074.
Testo completoAbstract (sommario):
This work presents the advances in the development of two innovative flat array antennas for Ku-band mobile satellite terminals. The first antenna is specifically conceived for double-deck trains to allow a bi-directional high data rate satellite link. The available circular surface (diameter 80 cm) integrates both a transmitting and a receiving section, operating in orthogonal linear polarizations. The TX frequency range is fully covered while the RX bandwidth is around 1 GHz arbitrarily allocated on the DVB range depending on requirements. The beam is steered in elevation through a phased array architecture not employing costly phase shifters, while the steering in azimuth is mechanical. Active BFNs allow excellent performance in terms of EIRP and G/T, maintaining extremely low profile. High antenna efficiency and low fabrication cost are ensured by the employment of innovative SIW (Substrate Integrated Waveguide) structures. The second antenna, receiving-only, is designed for radio/video streaming services in mobile environment. Full DVB coverage is achieved thanks to cavity-backed patches operating in double linear polarization. Two independent broadband active BFNs allow simultaneous reception of both polarizations with full tracking capabilities and a squintless beam steering from to in elevation. A minimum gain of 20 dBi and G/T > dB/ are achieved, while maintaining extremely compact size and flat profile. In the design of both antennas fabrication cost is considered as a driving factor, yet providing high performance with a flat profile and thus resulting in a great commercial potentiality.
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Qiu, Chencheng, Liu Liu, Botao Han, Jiachi Zhang, Zheng Li e Tao Zhou. "Broadband Wireless Communication Systems for Vacuum Tube High-Speed Flying Train". Applied Sciences 10, n. 4 (18 febbraio 2020): 1379. http://dx.doi.org/10.3390/app10041379.
Testo completoAbstract (sommario):
A vactrain (or vacuum tube high-speed flying train) is considered as a novel proposed rail transportation approach in the ultra-high-speed scenario. The maglev train can run with low mechanical friction, low air resistance, and low noise mode at a speed exceeding 1000 km/h inside the vacuum tube regardless of weather conditions. Currently, there is no research on train-to-ground wireless communication system for vactrain. In this paper, we first summarize a list of the unique challenges and opportunities associated with the wireless communication for vactrain, then analyze the bandwidth and Quality of Service (QoS) requirements of vactrain’s train-to-ground communication services quantitatively. To address these challenges and utilize the unique opportunities, a leaky waveguide solution with simple architecture but excellent performance is proposed for wireless coverage for vactrains. The simulation of the leaky waveguide is conducted, and the results show the uniform phase distribution along the horizontal direction of the tube, but also the smooth field distribution at the point far away from the leaky waveguide, which can suppress Doppler frequency shift, indicating that the time-varying frequency-selective fading channel could be approximated as a stationary channel. Furthermore, the train-to-ground wireless access architectures based on leaky waveguide are studied and analyzed. Finally, the moving scheme is adopted based on centralized, cooperative, cloud Radio Access Network (C-RAN), so as to deal with the extremely frequent handoff issue.
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Pérez, I. A., M. L. Sánchez, M. Á. García e B. de Torre. "Boundary layer structure and stability classification validated with CO<sub>2</sub> concentrations over the Northern Spanish Plateau". Annales Geophysicae 27, n. 1 (21 gennaio 2009): 339–49. http://dx.doi.org/10.5194/angeo-27-339-2009.
Testo completoAbstract (sommario):
Abstract. A description of the lower boundary layer is vital to enhance our understanding of dispersion processes. In this paper, Radio Acoustic Sounding System sodar measurements obtained over three years were used to calculate the Brunt-Väisälä frequency and the Monin-Obukhov length. The Brunt-Väisälä frequency enabled investigation of the structure of this layer. At night, several layers were noticeable and the maximum was observed at the first level, 40 m, whereas during the day, it was present at about 320 m. The Monin-Obukhov length was calculated with the four first levels measured, 40–100 m, by an original iterative method and used to establish four stability classes: drainage, extremely stable, stable and unstable. Wind speed and temperature median profiles linked to these classes were also presented. Wind speeds were the lowest, but temperatures were the highest and inversions were intense at night in drainage situations. However, unstable situations were linked to high wind speeds and superadiabatic temperature profiles. Detrended CO2 concentrations were used to determine the goodness of the classification proposed evidencing values which under drainage at night in spring were nearly 28 ppm higher than those corresponding to unstable situations. Finally, atmosphere structure was presented for the proposed stability classes and related with wind speed profiles. Under extremely stable situations, low level jets were coupled to the surface, with median wind speeds below 8 m s−1 and cores occasionally at 120 m. However, jets were uncoupled in stable situations, wind speed medians were higher than 11 m s−1 and their core heights were around 200 m.
50
Kuiack, Mark J., Ralph A. M. J. Wijers, Aleksandar Shulevski e Antonia Rowlinson. "Apparent radio transients mapping the near-Earth plasma environment". Monthly Notices of the Royal Astronomical Society 504, n. 4 (26 aprile 2021): 4706–15. http://dx.doi.org/10.1093/mnras/stab1156.
Testo completoAbstract (sommario):
ABSTRACT We report the discovery of bright, fast, radio flashes lasting tens of seconds with the AARTFAAC high-cadence all-sky survey at 60 MHz. The vast majority of these coincide with known, bright radio sources that brighten by factors of up to 100 during such an event. We attribute them to magnification events induced by plasma near the Earth, most likely in the densest parts of the ionosphere. They can occur both in relative isolation, during otherwise quiescent ionospheric conditions, and in large clusters during more turbulent ionospheric conditions. Using a toy model, we show that the likely origin of the more extreme (up to a factor of 100 or so) magnification events likely originate in the region of peak electron density in the ionosphere, at an altitude of 300–400 km. Distinguishing these events from genuine astrophysical transients is imperative for future surveys searching for low frequency radio transient at time-scales below a minute.