69,726 research outputs found
Phased array-fed antenna configuration study: Technology assessment
Spacecraft array fed reflector antenna systems were assessed for particular application to a multiple fixed spot beam/multiple scanning spot beam system. Reflector optics systems are reviewed in addition to an investigation of the feasibility of the use of monolithic microwave integrated circuit power amplifiers and phase shifters in each element of the array feed
Phased array-fed antenna configuration study
The scope of this contract entails a configuration study for a phased array fed transmit antenna operating in the frequency band of 17.7 to 20.2 GHz. This initial contract provides a basis for understanding the design limitations and advantages of advanced phased array and cluster feeds (both utilizing intergral MMIC modules) illuminating folded reflector optics (both near field and focused types). Design parametric analyses are performed utilizing as constraints the objective secondary performance requirements of the Advanced Communications Technology Satellite (Table 1.0). The output of the study provides design information which serves as a data base for future active phased array fed antenna studies such as detailed designs required to support the development of a ground tested breadboard. In general, this study is significant because it provides the antenna community with an understanding of the basic principles which govern near field phased scanned feed effects on secondary reflector system performance. Although several articles have been written on analysis procedures and results for these systems, the authors of this report have observed phenomenon of near field antenna systems not previously documented. Because the physical justification for the exhibited performance is provided herein, the findings of this study add a new dimension to the available knowledge of the subject matter
Development of a HgCdTe photomixer and impedance matched GaAs FET amplifier
A research program for the development of a 10.6 micron HgCdTe photodiode/GaAs field effect transistor amplifier package for use at cryogenic temperatures (77k). The photodiode/amplifier module achieved a noise equivalent power per unit bandwidth of 5.7 times 10 to the 20th power W/Hz at 2.0 GHz. The heterodyne sensitivity of the HgCdTe photodiode was improved by designing and building a low noise GaAs field effect transistor amplifier operating at 77K. The Johnson noise of the amplifier was reduced at 77K, and thus resulted in an increased photodiode heterodyne sensitivity
A Statistical Description of AGN Jet Evolution from the VLBA Imaging and Polarimetry Survey (VIPS)
A detailed analysis of the evolution of the properties of core-jet systems
within the VLBA Imaging and Polarimetry Survey (VIPS) is presented. We find a
power-law relationship between jet intensity and width that suggests for the
typical jet, little if any energy is lost as it moves away from its core. Using
VLA images at 1.5 GHz, we have found evidence that parsec-scale jets tend to be
aligned with the the direction of emission on kiloparsec scales. We also found
that this alignment improves as the jets move farther from their cores on
projected scales as small as ~50-100 pc. This suggests that realignment of jets
on these projected scales is relatively common. We typically find a modest
amount of bending (a change in jet position angle of ~5 deg.) on these scales,
suggesting that this realignment may typically occur relatively gradually.Comment: Accepted to ApJ, 20 pages, 8 figure
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Healthy shopper? Blood pressure testing in a shopping centre Pop-Up in England
Background: Improving detection of elevated blood pressure (BP) remains a public health need. We present results from a Pop-Up health check stationed in shopping centres in England. We hypothesise the rate of case detection is related to measurable ‘unhealthiness’ of the shopping centres.
Methods: A Pop-Up health check was sited in four and three shopping centres sampled from the top ten unhealthiest and top 15 healthiest shopping regions respectively, following a report ranking towns/cities based on their unhealthy and healthy retail outlets. On one day in each shopping centre, people were approached and consented to BP testing. Outcome measure was people flagged with BP ≥ 140/90 mmHg (cases).
Results: We detected 45 (22.6%) and 20 (13.1%) cases from testing 199 and 152 adults in the unhealthy and healthy locations respectively (relative risk 1.72; 95% confidence interval: 1.06 to 2.78). A measure of unhealthy retail outlets (e.g. fast-food outlets) within each shopping centre was associated with detection rate (R2 = 0.61; p = 0.04).
Conclusion: An association exists between cases of suspect hypertension found in a health check Pop-Up and measured ‘unhealthiness’ of the shopping centre site. Results hint at strategies for public testing of BP, potentially in the context of reducing health inequalities
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Feeling the pressure: a cross-sectional study exploring feasibility of a healthcare Pop-Up for intraocular pressure measurements in shopping centres in England
OBJECTIVES: To test the hypothesis that a shopping centre Pop-Up health check combining an intraocular pressure (IOP) check with a general health check (blood pressure (BP)) is more readily accepted by the general public than an IOP check only. We investigate public awareness of IOP compared with BP and the feasibility of measuring IOP in large numbers in a Pop-Up. DESIGN: A cross-sectional study using a tailor-made healthcare Pop-Up. SETTING: The 'Feeling the Pressure' Pop-Up was sited in eight regionally-different shopping centres in England. PARTICIPANTS: Adult members of the public in shopping centres. METHODS: On one day we measured IOP only and on another measured BP and IOP. IOP was measured by Icare IC100 tonometer (Helsinki, Finland). Potential participants were asked about their awareness of IOP and BP and when they last visited their optometrist. RESULTS: More people attended the combined BP + IOP days (461; 60%; 95% CI 56% to 64%) than IOP-only days (307; 40%, 95% CI 37% to 43%) over 16 days of testing. We recorded IOP in 652 participants (median (IQR) age and IOP of 54 (42 to 68) years and 13 (11 to 15) mm Hg, respectively). Fewer people reported awareness about IOP (19%, 95% CI 16% to 23%) compared with BP (71%, 95% CI 66% to 75%). Of 768 participants, 60 (8%) reported no previous optometric eye examination and 185 (24%) reported >2 years since their most recent examination. CONCLUSIONS: Measuring IOP in large numbers of the public via a shopping centre Pop-Up is feasible. Public engagement was greater when a BP check was offered alongside an IOP check, suggesting unfamiliar health checks can be promoted by aligning them with a more familiar check. Our findings hint at strategies for public health schemes that engage the public with their eye health
Energy benchmarks for water clusters and ice structures from an embedded many-body expansion
We show how an embedded many-body expansion (EMBE) can be used to calculate
accurate \emph{ab initio} energies of water clusters and ice structures using
wavefunction-based methods. We use the EMBE described recently by Bygrave
\emph{et al.} (J. Chem. Phys. \textbf{137}, 164102 (2012)), in which the terms
in the expansion are obtained from calculations on monomers, dimers, etc. acted
on by an approximate representation of the embedding field due to all other
molecules in the system, this field being a sum of Coulomb and
exchange-repulsion fields. Our strategy is to separate the total energy of the
system into Hartree-Fock and correlation parts, using the EMBE only for the
correlation energy, with the Hartree-Fock energy calculated using standard
molecular quantum chemistry for clusters and plane-wave methods for crystals.
Our tests on a range of different water clusters up to the 16-mer show that for
the second-order M\o{}ller-Plesset (MP2) method the EMBE truncated at 2-body
level reproduces to better than 0.1 m/monomer the correlation energy
from standard methods. The use of EMBE for computing coupled-cluster energies
of clusters is also discussed. For the ice structures Ih, II and VIII, we find
that MP2 energies near the complete basis-set limit reproduce very well the
experimental values of the absolute and relative binding energies, but that the
use of coupled-cluster methods for many-body correlation (non-additive
dispersion) is essential for a full description. Possible future applications
of the EMBE approach are suggested
Density functional theory embedding for correlated wavefunctions: Improved methods for open-shell systems and transition metal complexes
Density functional theory (DFT) embedding provides a formally exact framework
for interfacing correlated wave-function theory (WFT) methods with lower-level
descriptions of electronic structure. Here, we report techniques to improve the
accuracy and stability of WFT-in-DFT embedding calculations. In particular, we
develop spin-dependent embedding potentials in both restricted and unrestricted
orbital formulations to enable WFT-in-DFT embedding for open-shell systems, and
we develop an orbital-occupation-freezing technique to improve the convergence
of optimized effective potential (OEP) calculations that arise in the
evaluation of the embedding potential. The new techniques are demonstrated in
applications to the van-der-Waals-bound ethylene-propylene dimer and to the
hexaaquairon(II) transition-metal cation. Calculation of the dissociation curve
for the ethylene-propylene dimer reveals that WFT-in-DFT embedding reproduces
full CCSD(T) energies to within 0.1 kcal/mol at all distances, eliminating
errors in the dispersion interactions due to conventional exchange-correlation
(XC) functionals while simultaneously avoiding errors due to subsystem
partitioning across covalent bonds. Application of WFT-in-DFT embedding to the
calculation of the low-spin/high-spin splitting energy in the hexaaquairon(II)
cation reveals that the majority of the dependence on the DFT XC functional can
be eliminated by treating only the single transition-metal atom at the WFT
level; furthermore, these calculations demonstrate the substantial effects of
open-shell contributions to the embedding potential, and they suggest that
restricted open-shell WFT-in-DFT embedding provides better accuracy than
unrestricted open-shell WFT-in-DFT embedding due to the removal of spin
contamination.Comment: 11 pages, 5 figures, 2 table
Transport of heat and mass in a two-phase mixture. From a continuous to a discontinuous description
We present a theory which describes the transport properties of the
interfacial region with respect to heat and mass transfer. Postulating the
local Gibbs relation for a continuous description inside the interfacial
region, we derive the description of the Gibbs surface in terms of excess
densities and fluxes along the surface. We introduce overall interfacial
resistances and conductances as the coefficients in the force-flux relations
for the Gibbs surface. We derive relations between the local resistivities for
the continuous description inside the interfacial region and the overall
resistances of the surface for transport between the two phases for a mixture.
It is shown that interfacial resistances depend among other things on the
enthalpy profile across the interface. Since this variation is substantial the
coupling between heat and mass flow across the surface are also substantial. In
particular, the surface puts up much more resistance to the heat and mass
transfer then the homogeneous phases over a distance comparable to the
thickness of the surface. This is the case not only for the pure heat
conduction and diffusion but also for the cross effects like thermal diffusion.
For the excess fluxes along the surface and the corresponding thermodynamic
forces we derive expressions for excess conductances as integrals over the
local conductivities along the surface. We also show that the curvature of the
surface affects only the overall resistances for transport across the surface
and not the excess conductivities along the surface.Comment: 25 pages, 2 figure
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