6,667 research outputs found
Radio Recombination Lines at Decametre Wavelengths: Prospects for the Future
This paper considers the suitability of a number of emerging and future
instruments for the study of radio recombination lines (RRLs) at frequencies
below 200 MHz. These lines arise only in low-density regions of the ionized
interstellar medium, and they may represent a frequency-dependent foreground
for next-generation experiments trying to detect H I signals from the Epoch of
Reionization and Dark Ages ("21-cm cosmology"). We summarize existing
decametre-wavelength observations of RRLs, which have detected only carbon
RRLs. We then show that, for an interferometric array, the primary instrumental
factor limiting detection and study of the RRLs is the areal filling factor of
the array. We consider the Long Wavelength Array (LWA-1), the LOw Frequency
ARray (LOFAR), the low-frequency component of the Square Kilometre Array
(SKA-lo), and a future Lunar Radio Array (LRA), all of which will operate at
decametre wavelengths. These arrays offer digital signal processing, which
should produce more stable and better defined spectral bandpasses; larger
frequency tuning ranges; and better angular resolution than that of the
previous generation of instruments that have been used in the past for RRL
observations. Detecting Galactic carbon RRLs, with optical depths at the level
of 10^-3, appears feasible for all of these arrays, with integration times of
no more than 100 hr. The SKA-lo and LRA, and the LWA-1 and LOFAR at the lowest
frequencies, should have a high enough filling factor to detect lines with much
lower optical depths, of order 10^-4 in a few hundred hours. The amount of
RRL-hosting gas present in the Galaxy at the high Galactic latitudes likely to
be targeted in 21-cm cosmology studies is currently unknown. If present,
however, the spectral fluctuations from RRLs could be comparable to or exceed
the anticipated H I signals.Comment: 9 pages; Astron. & Astrophys., in pres
The Radio Sky at Meter Wavelengths: m-Mode Analysis Imaging with the Owens Valley Long Wavelength Array
A host of new low-frequency radio telescopes seek to measure the 21-cm
transition of neutral hydrogen from the early universe. These telescopes have
the potential to directly probe star and galaxy formation at redshifts , but are limited by the dynamic range they can achieve
against foreground sources of low-frequency radio emission. Consequently, there
is a growing demand for modern, high-fidelity maps of the sky at frequencies
below 200 MHz for use in foreground modeling and removal. We describe a new
widefield imaging technique for drift-scanning interferometers,
Tikhonov-regularized -mode analysis imaging. This technique constructs
images of the entire sky in a single synthesis imaging step with exact
treatment of widefield effects. We describe how the CLEAN algorithm can be
adapted to deconvolve maps generated by -mode analysis imaging. We
demonstrate Tikhonov-regularized -mode analysis imaging using the Owens
Valley Long Wavelength Array (OVRO-LWA) by generating 8 new maps of the sky
north of with 15 arcmin angular resolution, at frequencies
evenly spaced between 36.528 MHz and 73.152 MHz, and 800 mJy/beam thermal
noise. These maps are a 10-fold improvement in angular resolution over existing
full-sky maps at comparable frequencies, which have angular resolutions . Each map is constructed exclusively from interferometric observations
and does not represent the globally averaged sky brightness. Future
improvements will incorporate total power radiometry, improved thermal noise,
and improved angular resolution -- due to the planned expansion of the OVRO-LWA
to 2.6 km baselines. These maps serve as a first step on the path to the use of
more sophisticated foreground filters in 21-cm cosmology incorporating the
measured angular and frequency structure of all foreground contaminants.Comment: 27 pages, 18 figure
Machine Learning with Abstention for Automated Liver Disease Diagnosis
This paper presents a novel approach for detection of liver abnormalities in
an automated manner using ultrasound images. For this purpose, we have
implemented a machine learning model that can not only generate labels (normal
and abnormal) for a given ultrasound image but it can also detect when its
prediction is likely to be incorrect. The proposed model abstains from
generating the label of a test example if it is not confident about its
prediction. Such behavior is commonly practiced by medical doctors who, when
given insufficient information or a difficult case, can chose to carry out
further clinical or diagnostic tests before generating a diagnosis. However,
existing machine learning models are designed in a way to always generate a
label for a given example even when the confidence of their prediction is low.
We have proposed a novel stochastic gradient based solver for the learning with
abstention paradigm and use it to make a practical, state of the art method for
liver disease classification. The proposed method has been benchmarked on a
data set of approximately 100 patients from MINAR, Multan, Pakistan and our
results show that the proposed scheme offers state of the art classification
performance.Comment: Preprint version before submission for publication. complete version
published in proc. 15th International Conference on Frontiers of Information
Technology (FIT 2017), December 18-20, 2017, Islamabad, Pakistan.
http://ieeexplore.ieee.org/document/8261064
Efeito da pré-molhagem do agregado nas propriedades das argamassas com agregado reciclado deconcreto e agregado leve
This paper examines the suitability of partially replacing natural aggregate, sand, (NA) with recycled concrete aggregate (RCA) or lightweight aggregate (LWA) in mortars, under the hypothesis that pre-wetting aggregates would produce improvement in mortar properties. Fresh mortar properties such as density, entrained air content, consistency and heat of hydration, as well as hardened mortar properties such as dry density, compressive and flexural strength, and dimensional instability at 0% and 100% saturation were determined. The results show that mortars made with natural aggregate (75%) and recycled concrete aggregate (25%) have similar properties to mortars made with only natural aggregate (100%) and that pre-wetting the aggregates does not influence the properties of mortars significantly. Therefore, partial replacement with recycled concrete aggregate is a viable alternative for producing mortar.Peer ReviewedPostprint (published version
Broadside Dual-channel Orthogonal-Polarization Radiation using a Double-Asymmetric Periodic Leaky-Wave Antenna
The paper demonstrates that double unit-cell asymmetry in periodic leaky-wave
antennas (P-LWAs), i.e. asymmetry with respect to both the longitudinal and
transversal axes of the structure -- or longitudinal asymmetry (LA) and
transversal asymmetry (TA) -- allows for the simultaneous broadside radiation
of two orthogonal modes excited at the two ports of the antenna. This means
that the antenna may simultaneously support two orthogonal channels, which
represents an interesting polarization diversity characteristics for wireless
communications. The double asymmetric (DA) unit cell combines a circularly
polarized LA unit cell and a coupled mode TA unit cell, where the former
provides equal radiation in the series and shunt modes while the latter
separates these two modes in terms of their excitation ports. It is also shown
that the degree of TA in the DA unit cell controls the cross-polarization
discrimination level. The DA P-LWA concept is illustrated by two examples, a
series-fed line-connected patch (SF-LCP) P-LWA and a series-fed
capacitively-coupled patch (SF-CCP) P-LWA, via full-wave simulation and also
experiment for the SF-LCP P-LWA case
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